// SPDX-License-Identifier: GPL-2.0+ /* * RCU CPU stall warnings for normal RCU grace periods * * Copyright IBM Corporation, 2019 * * Author: Paul E. McKenney */ #include ////////////////////////////////////////////////////////////////////////////// // // Controlling CPU stall warnings, including delay calculation. /* panic() on RCU Stall sysctl. */ int sysctl_panic_on_rcu_stall __read_mostly; int sysctl_max_rcu_stall_to_panic __read_mostly; #ifdef CONFIG_PROVE_RCU #define RCU_STALL_DELAY_DELTA (5 * HZ) #else #define RCU_STALL_DELAY_DELTA 0 #endif #define RCU_STALL_MIGHT_DIV 8 #define RCU_STALL_MIGHT_MIN (2 * HZ) /* Limit-check stall timeouts specified at boottime and runtime. */ int rcu_jiffies_till_stall_check(void) { int till_stall_check = READ_ONCE(rcu_cpu_stall_timeout); /* * Limit check must be consistent with the Kconfig limits * for CONFIG_RCU_CPU_STALL_TIMEOUT. */ if (till_stall_check < 3) { WRITE_ONCE(rcu_cpu_stall_timeout, 3); till_stall_check = 3; } else if (till_stall_check > 300) { WRITE_ONCE(rcu_cpu_stall_timeout, 300); till_stall_check = 300; } return till_stall_check * HZ + RCU_STALL_DELAY_DELTA; } EXPORT_SYMBOL_GPL(rcu_jiffies_till_stall_check); /** * rcu_gp_might_be_stalled - Is it likely that the grace period is stalled? * * Returns @true if the current grace period is sufficiently old that * it is reasonable to assume that it might be stalled. This can be * useful when deciding whether to allocate memory to enable RCU-mediated * freeing on the one hand or just invoking synchronize_rcu() on the other. * The latter is preferable when the grace period is stalled. * * Note that sampling of the .gp_start and .gp_seq fields must be done * carefully to avoid false positives at the beginnings and ends of * grace periods. */ bool rcu_gp_might_be_stalled(void) { unsigned long d = rcu_jiffies_till_stall_check() / RCU_STALL_MIGHT_DIV; unsigned long j = jiffies; if (d < RCU_STALL_MIGHT_MIN) d = RCU_STALL_MIGHT_MIN; smp_mb(); // jiffies before .gp_seq to avoid false positives. if (!rcu_gp_in_progress()) return false; // Long delays at this point avoids false positive, but a delay // of ULONG_MAX/4 jiffies voids your no-false-positive warranty. smp_mb(); // .gp_seq before second .gp_start // And ditto here. return !time_before(j, READ_ONCE(rcu_state.gp_start) + d); } /* Don't do RCU CPU stall warnings during long sysrq printouts. */ void rcu_sysrq_start(void) { if (!rcu_cpu_stall_suppress) rcu_cpu_stall_suppress = 2; } void rcu_sysrq_end(void) { if (rcu_cpu_stall_suppress == 2) rcu_cpu_stall_suppress = 0; } /* Don't print RCU CPU stall warnings during a kernel panic. */ static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr) { rcu_cpu_stall_suppress = 1; return NOTIFY_DONE; } static struct notifier_block rcu_panic_block = { .notifier_call = rcu_panic, }; static int __init check_cpu_stall_init(void) { atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block); return 0; } early_initcall(check_cpu_stall_init); /* If so specified via sysctl, panic, yielding cleaner stall-warning output. */ static void panic_on_rcu_stall(void) { static int cpu_stall; if (++cpu_stall < sysctl_max_rcu_stall_to_panic) return; if (sysctl_panic_on_rcu_stall) panic("RCU Stall\n"); } /** * rcu_cpu_stall_reset - restart stall-warning timeout for current grace period * * To perform the reset request from the caller, disable stall detection until * 3 fqs loops have passed. This is required to ensure a fresh jiffies is * loaded. It should be safe to do from the fqs loop as enough timer * interrupts and context switches should have passed. * * The caller must disable hard irqs. */ void rcu_cpu_stall_reset(void) { WRITE_ONCE(rcu_state.nr_fqs_jiffies_stall, 3); WRITE_ONCE(rcu_state.jiffies_stall, ULONG_MAX); } ////////////////////////////////////////////////////////////////////////////// // // Interaction with RCU grace periods /* Start of new grace period, so record stall time (and forcing times). */ static void record_gp_stall_check_time(void) { unsigned long j = jiffies; unsigned long j1; WRITE_ONCE(rcu_state.gp_start, j); j1 = rcu_jiffies_till_stall_check(); smp_mb(); // ->gp_start before ->jiffies_stall and caller's ->gp_seq. WRITE_ONCE(rcu_state.nr_fqs_jiffies_stall, 0); WRITE_ONCE(rcu_state.jiffies_stall, j + j1); rcu_state.jiffies_resched = j + j1 / 2; rcu_state.n_force_qs_gpstart = READ_ONCE(rcu_state.n_force_qs); } /* Zero ->ticks_this_gp and snapshot the number of RCU softirq handlers. */ static void zero_cpu_stall_ticks(struct rcu_data *rdp) { rdp->ticks_this_gp = 0; rdp->softirq_snap = kstat_softirqs_cpu(RCU_SOFTIRQ, smp_processor_id()); WRITE_ONCE(rdp->last_fqs_resched, jiffies); } /* * If too much time has passed in the current grace period, and if * so configured, go kick the relevant kthreads. */ static void rcu_stall_kick_kthreads(void) { unsigned long j; if (!READ_ONCE(rcu_kick_kthreads)) return; j = READ_ONCE(rcu_state.jiffies_kick_kthreads); if (time_after(jiffies, j) && rcu_state.gp_kthread && (rcu_gp_in_progress() || READ_ONCE(rcu_state.gp_flags))) { WARN_ONCE(1, "Kicking %s grace-period kthread\n", rcu_state.name); rcu_ftrace_dump(DUMP_ALL); wake_up_process(rcu_state.gp_kthread); WRITE_ONCE(rcu_state.jiffies_kick_kthreads, j + HZ); } } /* * Handler for the irq_work request posted about halfway into the RCU CPU * stall timeout, and used to detect excessive irq disabling. Set state * appropriately, but just complain if there is unexpected state on entry. */ static void rcu_iw_handler(struct irq_work *iwp) { struct rcu_data *rdp; struct rcu_node *rnp; rdp = container_of(iwp, struct rcu_data, rcu_iw); rnp = rdp->mynode; raw_spin_lock_rcu_node(rnp); if (!WARN_ON_ONCE(!rdp->rcu_iw_pending)) { rdp->rcu_iw_gp_seq = rnp->gp_seq; rdp->rcu_iw_pending = false; } raw_spin_unlock_rcu_node(rnp); } ////////////////////////////////////////////////////////////////////////////// // // Printing RCU CPU stall warnings #ifdef CONFIG_PREEMPT_RCU /* * Dump detailed information for all tasks blocking the current RCU * grace period on the specified rcu_node structure. */ static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp) { unsigned long flags; struct task_struct *t; raw_spin_lock_irqsave_rcu_node(rnp, flags); if (!rcu_preempt_blocked_readers_cgp(rnp)) { raw_spin_unlock_irqrestore_rcu_node(rnp, flags); return; } t = list_entry(rnp->gp_tasks->prev, struct task_struct, rcu_node_entry); list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) { /* * We could be printing a lot while holding a spinlock. * Avoid triggering hard lockup. */ touch_nmi_watchdog(); sched_show_task(t); } raw_spin_unlock_irqrestore_rcu_node(rnp, flags); } // Communicate task state back to the RCU CPU stall warning request. struct rcu_stall_chk_rdr { int nesting; union rcu_special rs; bool on_blkd_list; }; /* * Report out the state of a not-running task that is stalling the * current RCU grace period. */ static bool check_slow_task(struct task_struct *t, void *arg) { struct rcu_stall_chk_rdr *rscrp = arg; if (task_curr(t)) return false; // It is running, so decline to inspect it. rscrp->nesting = t->rcu_read_lock_nesting; rscrp->rs = t->rcu_read_unlock_special; rscrp->on_blkd_list = !list_empty(&t->rcu_node_entry); return true; } /* * Scan the current list of tasks blocked within RCU read-side critical * sections, printing out the tid of each of the first few of them. */ static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags) __releases(rnp->lock) { int i = 0; int ndetected = 0; struct rcu_stall_chk_rdr rscr; struct task_struct *t; struct task_struct *ts[8]; lockdep_assert_irqs_disabled(); if (!rcu_preempt_blocked_readers_cgp(rnp)) { raw_spin_unlock_irqrestore_rcu_node(rnp, flags); return 0; } pr_err("\tTasks blocked on level-%d rcu_node (CPUs %d-%d):", rnp->level, rnp->grplo, rnp->grphi); t = list_entry(rnp->gp_tasks->prev, struct task_struct, rcu_node_entry); list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) { get_task_struct(t); ts[i++] = t; if (i >= ARRAY_SIZE(ts)) break; } raw_spin_unlock_irqrestore_rcu_node(rnp, flags); while (i) { t = ts[--i]; if (!try_invoke_on_locked_down_task(t, check_slow_task, &rscr)) pr_cont(" P%d", t->pid); else pr_cont(" P%d/%d:%c%c%c%c", t->pid, rscr.nesting, ".b"[rscr.rs.b.blocked], ".q"[rscr.rs.b.need_qs], ".e"[rscr.rs.b.exp_hint], ".l"[rscr.on_blkd_list]); lockdep_assert_irqs_disabled(); put_task_struct(t); ndetected++; } pr_cont("\n"); return ndetected; } #else /* #ifdef CONFIG_PREEMPT_RCU */ /* * Because preemptible RCU does not exist, we never have to check for * tasks blocked within RCU read-side critical sections. */ static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp) { } /* * Because preemptible RCU does not exist, we never have to check for * tasks blocked within RCU read-side critical sections. */ static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags) __releases(rnp->lock) { raw_spin_unlock_irqrestore_rcu_node(rnp, flags); return 0; } #endif /* #else #ifdef CONFIG_PREEMPT_RCU */ /* * Dump stacks of all tasks running on stalled CPUs. First try using * NMIs, but fall back to manual remote stack tracing on architectures * that don't support NMI-based stack dumps. The NMI-triggered stack * traces are more accurate because they are printed by the target CPU. */ static void rcu_dump_cpu_stacks(void) { int cpu; unsigned long flags; struct rcu_node *rnp; rcu_for_each_leaf_node(rnp) { raw_spin_lock_irqsave_rcu_node(rnp, flags); for_each_leaf_node_possible_cpu(rnp, cpu) if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) { if (cpu_is_offline(cpu)) pr_err("Offline CPU %d blocking current GP.\n", cpu); else if (!trigger_single_cpu_backtrace(cpu)) dump_cpu_task(cpu); } raw_spin_unlock_irqrestore_rcu_node(rnp, flags); } } #ifdef CONFIG_RCU_FAST_NO_HZ static void print_cpu_stall_fast_no_hz(char *cp, int cpu) { struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu); sprintf(cp, "last_accelerate: %04lx/%04lx dyntick_enabled: %d", rdp->last_accelerate & 0xffff, jiffies & 0xffff, !!rdp->tick_nohz_enabled_snap); } #else /* #ifdef CONFIG_RCU_FAST_NO_HZ */ static void print_cpu_stall_fast_no_hz(char *cp, int cpu) { *cp = '\0'; } #endif /* #else #ifdef CONFIG_RCU_FAST_NO_HZ */ static const char * const gp_state_names[] = { [RCU_GP_IDLE] = "RCU_GP_IDLE", [RCU_GP_WAIT_GPS] = "RCU_GP_WAIT_GPS", [RCU_GP_DONE_GPS] = "RCU_GP_DONE_GPS", [RCU_GP_ONOFF] = "RCU_GP_ONOFF", [RCU_GP_INIT] = "RCU_GP_INIT", [RCU_GP_WAIT_FQS] = "RCU_GP_WAIT_FQS", [RCU_GP_DOING_FQS] = "RCU_GP_DOING_FQS", [RCU_GP_CLEANUP] = "RCU_GP_CLEANUP", [RCU_GP_CLEANED] = "RCU_GP_CLEANED", }; /* * Convert a ->gp_state value to a character string. */ static const char *gp_state_getname(short gs) { if (gs < 0 || gs >= ARRAY_SIZE(gp_state_names)) return "???"; return gp_state_names[gs]; } /* Is the RCU grace-period kthread being starved of CPU time? */ static bool rcu_is_gp_kthread_starving(unsigned long *jp) { unsigned long j = jiffies - READ_ONCE(rcu_state.gp_activity); if (jp) *jp = j; return j > 2 * HZ; } /* * Print out diagnostic information for the specified stalled CPU. * * If the specified CPU is aware of the current RCU grace period, then * print the number of scheduling clock interrupts the CPU has taken * during the time that it has been aware. Otherwise, print the number * of RCU grace periods that this CPU is ignorant of, for example, "1" * if the CPU was aware of the previous grace period. * * Also print out idle and (if CONFIG_RCU_FAST_NO_HZ) idle-entry info. */ static void print_cpu_stall_info(int cpu) { unsigned long delta; bool falsepositive; char fast_no_hz[72]; struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu); char *ticks_title; unsigned long ticks_value; /* * We could be printing a lot while holding a spinlock. Avoid * triggering hard lockup. */ touch_nmi_watchdog(); ticks_value = rcu_seq_ctr(rcu_state.gp_seq - rdp->gp_seq); if (ticks_value) { ticks_title = "GPs behind"; } else { ticks_title = "ticks this GP"; ticks_value = rdp->ticks_this_gp; } print_cpu_stall_fast_no_hz(fast_no_hz, cpu); delta = rcu_seq_ctr(rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq); falsepositive = rcu_is_gp_kthread_starving(NULL) && rcu_dynticks_in_eqs(rcu_dynticks_snap(rdp)); pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%03x/%ld/%#lx softirq=%u/%u fqs=%ld %s%s\n", cpu, "O."[!!cpu_online(cpu)], "o."[!!(rdp->grpmask & rdp->mynode->qsmaskinit)], "N."[!!(rdp->grpmask & rdp->mynode->qsmaskinitnext)], !IS_ENABLED(CONFIG_IRQ_WORK) ? '?' : rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '0' : "!."[!delta], ticks_value, ticks_title, rcu_dynticks_snap(rdp) & 0xfff, rdp->dynticks_nesting, rdp->dynticks_nmi_nesting, rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu), data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart, fast_no_hz, falsepositive ? " (false positive?)" : ""); } /* Complain about starvation of grace-period kthread. */ static void rcu_check_gp_kthread_starvation(void) { int cpu; struct task_struct *gpk = rcu_state.gp_kthread; unsigned long j; if (rcu_is_gp_kthread_starving(&j)) { cpu = gpk ? task_cpu(gpk) : -1; pr_err("%s kthread starved for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x ->cpu=%d\n", rcu_state.name, j, (long)rcu_seq_current(&rcu_state.gp_seq), data_race(READ_ONCE(rcu_state.gp_flags)), gp_state_getname(rcu_state.gp_state), data_race(READ_ONCE(rcu_state.gp_state)), gpk ? data_race(READ_ONCE(gpk->__state)) : ~0, cpu); if (gpk) { pr_err("\tUnless %s kthread gets sufficient CPU time, OOM is now expected behavior.\n", rcu_state.name); pr_err("RCU grace-period kthread stack dump:\n"); sched_show_task(gpk); if (cpu >= 0) { if (cpu_is_offline(cpu)) { pr_err("RCU GP kthread last ran on offline CPU %d.\n", cpu); } else { pr_err("Stack dump where RCU GP kthread last ran:\n"); if (!trigger_single_cpu_backtrace(cpu)) dump_cpu_task(cpu); } } wake_up_process(gpk); } } } /* Complain about missing wakeups from expired fqs wait timer */ static void rcu_check_gp_kthread_expired_fqs_timer(void) { struct task_struct *gpk = rcu_state.gp_kthread; short gp_state; unsigned long jiffies_fqs; int cpu; /* * Order reads of .gp_state and .jiffies_force_qs. * Matching smp_wmb() is present in rcu_gp_fqs_loop(). */ gp_state = smp_load_acquire(&rcu_state.gp_state); jiffies_fqs = READ_ONCE(rcu_state.jiffies_force_qs); if (gp_state == RCU_GP_WAIT_FQS && time_after(jiffies, jiffies_fqs + RCU_STALL_MIGHT_MIN) && gpk && !READ_ONCE(gpk->on_rq)) { cpu = task_cpu(gpk); pr_err("%s kthread timer wakeup didn't happen for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x\n", rcu_state.name, (jiffies - jiffies_fqs), (long)rcu_seq_current(&rcu_state.gp_seq), data_race(rcu_state.gp_flags), gp_state_getname(RCU_GP_WAIT_FQS), RCU_GP_WAIT_FQS, data_race(READ_ONCE(gpk->__state))); pr_err("\tPossible timer handling issue on cpu=%d timer-softirq=%u\n", cpu, kstat_softirqs_cpu(TIMER_SOFTIRQ, cpu)); } } static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps) { int cpu; unsigned long flags; unsigned long gpa; unsigned long j; int ndetected = 0; struct rcu_node *rnp; long totqlen = 0; lockdep_assert_irqs_disabled(); /* Kick and suppress, if so configured. */ rcu_stall_kick_kthreads(); if (rcu_stall_is_suppressed()) return; /* * OK, time to rat on our buddy... * See Documentation/RCU/stallwarn.rst for info on how to debug * RCU CPU stall warnings. */ trace_rcu_stall_warning(rcu_state.name, TPS("StallDetected")); pr_err("INFO: %s detected stalls on CPUs/tasks:\n", rcu_state.name); rcu_for_each_leaf_node(rnp) { raw_spin_lock_irqsave_rcu_node(rnp, flags); if (rnp->qsmask != 0) { for_each_leaf_node_possible_cpu(rnp, cpu) if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) { print_cpu_stall_info(cpu); ndetected++; } } ndetected += rcu_print_task_stall(rnp, flags); // Releases rnp->lock. lockdep_assert_irqs_disabled(); } for_each_possible_cpu(cpu) totqlen += rcu_get_n_cbs_cpu(cpu); pr_cont("\t(detected by %d, t=%ld jiffies, g=%ld, q=%lu)\n", smp_processor_id(), (long)(jiffies - gps), (long)rcu_seq_current(&rcu_state.gp_seq), totqlen); if (ndetected) { rcu_dump_cpu_stacks(); /* Complain about tasks blocking the grace period. */ rcu_for_each_leaf_node(rnp) rcu_print_detail_task_stall_rnp(rnp); } else { if (rcu_seq_current(&rcu_state.gp_seq) != gp_seq) { pr_err("INFO: Stall ended before state dump start\n"); } else { j = jiffies; gpa = data_race(READ_ONCE(rcu_state.gp_activity)); pr_err("All QSes seen, last %s kthread activity %ld (%ld-%ld), jiffies_till_next_fqs=%ld, root ->qsmask %#lx\n", rcu_state.name, j - gpa, j, gpa, data_race(READ_ONCE(jiffies_till_next_fqs)), data_race(READ_ONCE(rcu_get_root()->qsmask))); } } /* Rewrite if needed in case of slow consoles. */ if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall))) WRITE_ONCE(rcu_state.jiffies_stall, jiffies + 3 * rcu_jiffies_till_stall_check() + 3); rcu_check_gp_kthread_expired_fqs_timer(); rcu_check_gp_kthread_starvation(); panic_on_rcu_stall(); rcu_force_quiescent_state(); /* Kick them all. */ } static void print_cpu_stall(unsigned long gps) { int cpu; unsigned long flags; struct rcu_data *rdp = this_cpu_ptr(&rcu_data); struct rcu_node *rnp = rcu_get_root(); long totqlen = 0; lockdep_assert_irqs_disabled(); /* Kick and suppress, if so configured. */ rcu_stall_kick_kthreads(); if (rcu_stall_is_suppressed()) return; /* * OK, time to rat on ourselves... * See Documentation/RCU/stallwarn.rst for info on how to debug * RCU CPU stall warnings. */ trace_rcu_stall_warning(rcu_state.name, TPS("SelfDetected")); pr_err("INFO: %s self-detected stall on CPU\n", rcu_state.name); raw_spin_lock_irqsave_rcu_node(rdp->mynode, flags); print_cpu_stall_info(smp_processor_id()); raw_spin_unlock_irqrestore_rcu_node(rdp->mynode, flags); for_each_possible_cpu(cpu) totqlen += rcu_get_n_cbs_cpu(cpu); pr_cont("\t(t=%lu jiffies g=%ld q=%lu)\n", jiffies - gps, (long)rcu_seq_current(&rcu_state.gp_seq), totqlen); rcu_check_gp_kthread_expired_fqs_timer(); rcu_check_gp_kthread_starvation(); rcu_dump_cpu_stacks(); raw_spin_lock_irqsave_rcu_node(rnp, flags); /* Rewrite if needed in case of slow consoles. */ if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall))) WRITE_ONCE(rcu_state.jiffies_stall, jiffies + 3 * rcu_jiffies_till_stall_check() + 3); raw_spin_unlock_irqrestore_rcu_node(rnp, flags); panic_on_rcu_stall(); /* * Attempt to revive the RCU machinery by forcing a context switch. * * A context switch would normally allow the RCU state machine to make * progress and it could be we're stuck in kernel space without context * switches for an entirely unreasonable amount of time. */ set_tsk_need_resched(current); set_preempt_need_resched(); } static void check_cpu_stall(struct rcu_data *rdp) { bool didstall = false; unsigned long gs1; unsigned long gs2; unsigned long gps; unsigned long j; unsigned long jn; unsigned long js; struct rcu_node *rnp; lockdep_assert_irqs_disabled(); if ((rcu_stall_is_suppressed() && !READ_ONCE(rcu_kick_kthreads)) || !rcu_gp_in_progress()) return; rcu_stall_kick_kthreads(); /* * Check if it was requested (via rcu_cpu_stall_reset()) that the FQS * loop has to set jiffies to ensure a non-stale jiffies value. This * is required to have good jiffies value after coming out of long * breaks of jiffies updates. Not doing so can cause false positives. */ if (READ_ONCE(rcu_state.nr_fqs_jiffies_stall) > 0) return; j = jiffies; /* * Lots of memory barriers to reject false positives. * * The idea is to pick up rcu_state.gp_seq, then * rcu_state.jiffies_stall, then rcu_state.gp_start, and finally * another copy of rcu_state.gp_seq. These values are updated in * the opposite order with memory barriers (or equivalent) during * grace-period initialization and cleanup. Now, a false positive * can occur if we get an new value of rcu_state.gp_start and a old * value of rcu_state.jiffies_stall. But given the memory barriers, * the only way that this can happen is if one grace period ends * and another starts between these two fetches. This is detected * by comparing the second fetch of rcu_state.gp_seq with the * previous fetch from rcu_state.gp_seq. * * Given this check, comparisons of jiffies, rcu_state.jiffies_stall, * and rcu_state.gp_start suffice to forestall false positives. */ gs1 = READ_ONCE(rcu_state.gp_seq); smp_rmb(); /* Pick up ->gp_seq first... */ js = READ_ONCE(rcu_state.jiffies_stall); smp_rmb(); /* ...then ->jiffies_stall before the rest... */ gps = READ_ONCE(rcu_state.gp_start); smp_rmb(); /* ...and finally ->gp_start before ->gp_seq again. */ gs2 = READ_ONCE(rcu_state.gp_seq); if (gs1 != gs2 || ULONG_CMP_LT(j, js) || ULONG_CMP_GE(gps, js)) return; /* No stall or GP completed since entering function. */ rnp = rdp->mynode; jn = jiffies + ULONG_MAX / 2; if (rcu_gp_in_progress() && (READ_ONCE(rnp->qsmask) & rdp->grpmask) && cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) { /* * If a virtual machine is stopped by the host it can look to * the watchdog like an RCU stall. Check to see if the host * stopped the vm. */ if (kvm_check_and_clear_guest_paused()) return; /* We haven't checked in, so go dump stack. */ print_cpu_stall(gps); if (READ_ONCE(rcu_cpu_stall_ftrace_dump)) rcu_ftrace_dump(DUMP_ALL); didstall = true; } else if (rcu_gp_in_progress() && ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) && cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) { /* * If a virtual machine is stopped by the host it can look to * the watchdog like an RCU stall. Check to see if the host * stopped the vm. */ if (kvm_check_and_clear_guest_paused()) return; /* They had a few time units to dump stack, so complain. */ print_other_cpu_stall(gs2, gps); if (READ_ONCE(rcu_cpu_stall_ftrace_dump)) rcu_ftrace_dump(DUMP_ALL); didstall = true; } if (didstall && READ_ONCE(rcu_state.jiffies_stall) == jn) { jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3; WRITE_ONCE(rcu_state.jiffies_stall, jn); } } ////////////////////////////////////////////////////////////////////////////// // // RCU forward-progress mechanisms, including of callback invocation. /* * Check to see if a failure to end RCU priority inversion was due to * a CPU not passing through a quiescent state. When this happens, there * is nothing that RCU priority boosting can do to help, so we shouldn't * count this as an RCU priority boosting failure. A return of true says * RCU priority boosting is to blame, and false says otherwise. If false * is returned, the first of the CPUs to blame is stored through cpup. * If there was no CPU blocking the current grace period, but also nothing * in need of being boosted, *cpup is set to -1. This can happen in case * of vCPU preemption while the last CPU is reporting its quiscent state, * for example. * * If cpup is NULL, then a lockless quick check is carried out, suitable * for high-rate usage. On the other hand, if cpup is non-NULL, each * rcu_node structure's ->lock is acquired, ruling out high-rate usage. */ bool rcu_check_boost_fail(unsigned long gp_state, int *cpup) { bool atb = false; int cpu; unsigned long flags; struct rcu_node *rnp; rcu_for_each_leaf_node(rnp) { if (!cpup) { if (data_race(READ_ONCE(rnp->qsmask))) { return false; } else { if (READ_ONCE(rnp->gp_tasks)) atb = true; continue; } } *cpup = -1; raw_spin_lock_irqsave_rcu_node(rnp, flags); if (rnp->gp_tasks) atb = true; if (!rnp->qsmask) { // No CPUs without quiescent states for this rnp. raw_spin_unlock_irqrestore_rcu_node(rnp, flags); continue; } // Find the first holdout CPU. for_each_leaf_node_possible_cpu(rnp, cpu) { if (rnp->qsmask & (1UL << (cpu - rnp->grplo))) { raw_spin_unlock_irqrestore_rcu_node(rnp, flags); *cpup = cpu; return false; } } raw_spin_unlock_irqrestore_rcu_node(rnp, flags); } // Can't blame CPUs, so must blame RCU priority boosting. return atb; } EXPORT_SYMBOL_GPL(rcu_check_boost_fail); /* * Show the state of the grace-period kthreads. */ void show_rcu_gp_kthreads(void) { unsigned long cbs = 0; int cpu; unsigned long j; unsigned long ja; unsigned long jr; unsigned long js; unsigned long jw; struct rcu_data *rdp; struct rcu_node *rnp; struct task_struct *t = READ_ONCE(rcu_state.gp_kthread); j = jiffies; ja = j - data_race(READ_ONCE(rcu_state.gp_activity)); jr = j - data_race(READ_ONCE(rcu_state.gp_req_activity)); js = j - data_race(READ_ONCE(rcu_state.gp_start)); jw = j - data_race(READ_ONCE(rcu_state.gp_wake_time)); pr_info("%s: wait state: %s(%d) ->state: %#x ->rt_priority %u delta ->gp_start %lu ->gp_activity %lu ->gp_req_activity %lu ->gp_wake_time %lu ->gp_wake_seq %ld ->gp_seq %ld ->gp_seq_needed %ld ->gp_max %lu ->gp_flags %#x\n", rcu_state.name, gp_state_getname(rcu_state.gp_state), data_race(READ_ONCE(rcu_state.gp_state)), t ? data_race(READ_ONCE(t->__state)) : 0x1ffff, t ? t->rt_priority : 0xffU, js, ja, jr, jw, (long)data_race(READ_ONCE(rcu_state.gp_wake_seq)), (long)data_race(READ_ONCE(rcu_state.gp_seq)), (long)data_race(READ_ONCE(rcu_get_root()->gp_seq_needed)), data_race(READ_ONCE(rcu_state.gp_max)), data_race(READ_ONCE(rcu_state.gp_flags))); rcu_for_each_node_breadth_first(rnp) { if (ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq), READ_ONCE(rnp->gp_seq_needed)) && !data_race(READ_ONCE(rnp->qsmask)) && !data_race(READ_ONCE(rnp->boost_tasks)) && !data_race(READ_ONCE(rnp->exp_tasks)) && !data_race(READ_ONCE(rnp->gp_tasks))) continue; pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld ->qsmask %#lx %c%c%c%c ->n_boosts %ld\n", rnp->grplo, rnp->grphi, (long)data_race(READ_ONCE(rnp->gp_seq)), (long)data_race(READ_ONCE(rnp->gp_seq_needed)), data_race(READ_ONCE(rnp->qsmask)), ".b"[!!data_race(READ_ONCE(rnp->boost_kthread_task))], ".B"[!!data_race(READ_ONCE(rnp->boost_tasks))], ".E"[!!data_race(READ_ONCE(rnp->exp_tasks))], ".G"[!!data_race(READ_ONCE(rnp->gp_tasks))], data_race(READ_ONCE(rnp->n_boosts))); if (!rcu_is_leaf_node(rnp)) continue; for_each_leaf_node_possible_cpu(rnp, cpu) { rdp = per_cpu_ptr(&rcu_data, cpu); if (READ_ONCE(rdp->gpwrap) || ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq), READ_ONCE(rdp->gp_seq_needed))) continue; pr_info("\tcpu %d ->gp_seq_needed %ld\n", cpu, (long)data_race(READ_ONCE(rdp->gp_seq_needed))); } } for_each_possible_cpu(cpu) { rdp = per_cpu_ptr(&rcu_data, cpu); cbs += data_race(READ_ONCE(rdp->n_cbs_invoked)); if (rcu_segcblist_is_offloaded(&rdp->cblist)) show_rcu_nocb_state(rdp); } pr_info("RCU callbacks invoked since boot: %lu\n", cbs); show_rcu_tasks_gp_kthreads(); } EXPORT_SYMBOL_GPL(show_rcu_gp_kthreads); /* * This function checks for grace-period requests that fail to motivate * RCU to come out of its idle mode. */ static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp, const unsigned long gpssdelay) { unsigned long flags; unsigned long j; struct rcu_node *rnp_root = rcu_get_root(); static atomic_t warned = ATOMIC_INIT(0); if (!IS_ENABLED(CONFIG_PROVE_RCU) || rcu_gp_in_progress() || ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq), READ_ONCE(rnp_root->gp_seq_needed)) || !smp_load_acquire(&rcu_state.gp_kthread)) // Get stable kthread. return; j = jiffies; /* Expensive access, and in common case don't get here. */ if (time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) || time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) || atomic_read(&warned)) return; raw_spin_lock_irqsave_rcu_node(rnp, flags); j = jiffies; if (rcu_gp_in_progress() || ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq), READ_ONCE(rnp_root->gp_seq_needed)) || time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) || time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) || atomic_read(&warned)) { raw_spin_unlock_irqrestore_rcu_node(rnp, flags); return; } /* Hold onto the leaf lock to make others see warned==1. */ if (rnp_root != rnp) raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */ j = jiffies; if (rcu_gp_in_progress() || ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq), READ_ONCE(rnp_root->gp_seq_needed)) || time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) || time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) || atomic_xchg(&warned, 1)) { if (rnp_root != rnp) /* irqs remain disabled. */ raw_spin_unlock_rcu_node(rnp_root); raw_spin_unlock_irqrestore_rcu_node(rnp, flags); return; } WARN_ON(1); if (rnp_root != rnp) raw_spin_unlock_rcu_node(rnp_root); raw_spin_unlock_irqrestore_rcu_node(rnp, flags); show_rcu_gp_kthreads(); } /* * Do a forward-progress check for rcutorture. This is normally invoked * due to an OOM event. The argument "j" gives the time period during * which rcutorture would like progress to have been made. */ void rcu_fwd_progress_check(unsigned long j) { unsigned long cbs; int cpu; unsigned long max_cbs = 0; int max_cpu = -1; struct rcu_data *rdp; if (rcu_gp_in_progress()) { pr_info("%s: GP age %lu jiffies\n", __func__, jiffies - data_race(READ_ONCE(rcu_state.gp_start))); show_rcu_gp_kthreads(); } else { pr_info("%s: Last GP end %lu jiffies ago\n", __func__, jiffies - data_race(READ_ONCE(rcu_state.gp_end))); preempt_disable(); rdp = this_cpu_ptr(&rcu_data); rcu_check_gp_start_stall(rdp->mynode, rdp, j); preempt_enable(); } for_each_possible_cpu(cpu) { cbs = rcu_get_n_cbs_cpu(cpu); if (!cbs) continue; if (max_cpu < 0) pr_info("%s: callbacks", __func__); pr_cont(" %d: %lu", cpu, cbs); if (cbs <= max_cbs) continue; max_cbs = cbs; max_cpu = cpu; } if (max_cpu >= 0) pr_cont("\n"); } EXPORT_SYMBOL_GPL(rcu_fwd_progress_check); /* Commandeer a sysrq key to dump RCU's tree. */ static bool sysrq_rcu; module_param(sysrq_rcu, bool, 0444); /* Dump grace-period-request information due to commandeered sysrq. */ static void sysrq_show_rcu(int key) { show_rcu_gp_kthreads(); } static const struct sysrq_key_op sysrq_rcudump_op = { .handler = sysrq_show_rcu, .help_msg = "show-rcu(y)", .action_msg = "Show RCU tree", .enable_mask = SYSRQ_ENABLE_DUMP, }; static int __init rcu_sysrq_init(void) { if (sysrq_rcu) return register_sysrq_key('y', &sysrq_rcudump_op); return 0; } early_initcall(rcu_sysrq_init);