--- zzzz-none-000/linux-2.6.39.4/kernel/Kconfig.preempt 2011-08-03 19:43:28.000000000 +0000 +++ puma6-arm-6490-729/linux-2.6.39.4/kernel/Kconfig.preempt 2021-11-10 13:23:10.000000000 +0000 @@ -1,7 +1,6 @@ - choice - prompt "Preemption Model" - default PREEMPT_NONE + prompt "Preemption Mode" + default PREEMPT_RT config PREEMPT_NONE bool "No Forced Preemption (Server)" @@ -33,7 +32,7 @@ Select this if you are building a kernel for a desktop system. -config PREEMPT +config PREEMPT_DESKTOP bool "Preemptible Kernel (Low-Latency Desktop)" help This option reduces the latency of the kernel by making @@ -41,14 +40,138 @@ preemptible. This allows reaction to interactive events by permitting a low priority process to be preempted involuntarily even if it is in kernel mode executing a system call and would - otherwise not be about to reach a natural preemption point. - This allows applications to run more 'smoothly' even when the - system is under load, at the cost of slightly lower throughput - and a slight runtime overhead to kernel code. + otherwise not about to reach a preemption point. This allows + applications to run more 'smoothly' even when the system is + under load, at the cost of slighly lower throughput and a + slight runtime overhead to kernel code. + + (According to profiles, when this mode is selected then even + during kernel-intense workloads the system is in an immediately + preemptible state more than 50% of the time.) Select this if you are building a kernel for a desktop or embedded system with latency requirements in the milliseconds range. +config PREEMPT_RT + bool "Complete Preemption (Real-Time)" + select PREEMPT_SOFTIRQS + select PREEMPT_HARDIRQS + select PREEMPT_RCU + help + This option further reduces the scheduling latency of the + kernel by replacing almost every spinlock used by the kernel + with preemptible mutexes and thus making all but the most + critical kernel code involuntarily preemptible. The remaining + handful of lowlevel non-preemptible codepaths are short and + have a deterministic latency of a couple of tens of + microseconds (depending on the hardware). This also allows + applications to run more 'smoothly' even when the system is + under load, at the cost of lower throughput and runtime + overhead to kernel code. + + (According to profiles, when this mode is selected then even + during kernel-intense workloads the system is in an immediately + preemptible state more than 95% of the time.) + + Select this if you are building a kernel for a desktop, + embedded or real-time system with guaranteed latency + requirements of 100 usecs or lower. + endchoice +config PREEMPT + bool + default y + depends on PREEMPT_DESKTOP || PREEMPT_RT + +config PREEMPT_SOFTIRQS + bool "Thread Softirqs" + default n +# depends on PREEMPT + help + This option reduces the latency of the kernel by 'threading' + soft interrupts. This means that all softirqs will execute + in softirqd's context. While this helps latency, it can also + reduce performance. + + The threading of softirqs can also be controlled via + /proc/sys/kernel/softirq_preemption runtime flag and the + sofirq-preempt=0/1 boot-time option. + + Say N if you are unsure. + +config PREEMPT_HARDIRQS + bool "Thread Hardirqs" + default n +# depends on PREEMPT + help + This option reduces the latency of the kernel by 'threading' + hardirqs. This means that all (or selected) hardirqs will run + in their own kernel thread context. While this helps latency, + this feature can also reduce performance. + + The threading of hardirqs can also be controlled via the + /proc/sys/kernel/hardirq_preemption runtime flag and the + hardirq-preempt=0/1 boot-time option. Per-irq threading can + be enabled/disable via the /proc/irq///threaded + runtime flags. + + Say N if you are unsure. + +config SPINLOCK_BKL + bool "Old-Style Big Kernel Lock" + depends on (PREEMPT || SMP) && !PREEMPT_RT + default n + help + This option increases the latency of the kernel by making the + big kernel lock spinlock-based (which is bad for latency). + However, enable this option if you see any problems to revert + back to the traditional spinlock BKL design. + + Say Y here if you are building a kernel for a desktop system. + Say N if you are unsure. + +config PREEMPT_BKL + bool + depends on PREEMPT_RT || !SPINLOCK_BKL + default n if !PREEMPT + default n + +choice + prompt "RCU implementation type:" + default CLASSIC_RCU + +config CLASSIC_RCU + bool "Classic RCU" + depends on !PREEMPT_RT + help + This option selects the classic RCU implementation that is + designed for best read-side performance on non-realtime + systems. + + Say Y if you are unsure. + +config PREEMPT_RCU + bool "Preemptible RCU" + help + This option reduces the latency of the kernel by making certain + RCU sections preemptible. Normally RCU code is non-preemptible, if + this option is selected then read-only RCU sections become + preemptible. This helps latency, but may expose bugs due to + now-naive assumptions about each RCU read-side critical section + remaining on a given CPU through its execution. + + Say N if you are unsure. + +endchoice + +config RCU_TRACE + bool "Enable tracing for RCU - currently stats in /proc" + default y + help + This option provides tracing in RCU which presents /proc + stats for debugging RCU implementation. + + Say Y here if you want to enable RCU tracing + Say N if you are unsure.