Messages in this thread Patch in this message | | | Date | Fri, 9 Oct 2020 13:17:03 +0200 | From | Michal Hocko <> | Subject | Re: [RFC PATCH] kernel: allow to configure PREEMPT_NONE, PREEMPT_VOLUNTARY on kernel command line |
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On Fri 09-10-20 12:48:09, Michal Hocko wrote: [...] > I will add the CONFIG_PREEMPT_DYNAMIC in the next version. I just have > to think whether flipping the direction is really safe and easier in the > end. For our particular usecase we are more interested in > NONE<->VOLUNTARY at this moment and having full preemption in the mix > later is just fine. If you insist on the other direction then we can > work on that.
This is a quick implementation of PREEMPT_DYNAMIC (still with the original approach to start from non preemptive kernels). It is a bit hairy but I suspect that starting with CONFIG_PREEMPTION wouldn't make it very much easier but I haven't tried that yet. Advantage of this approach is that it is clear that it is safe as NONE/VOLUNTARY are trivial to examine.
diff --git a/include/linux/kernel.h b/include/linux/kernel.h index d2d37bd5ecd5..b61ab02dba84 100644 --- a/include/linux/kernel.h +++ b/include/linux/kernel.h @@ -193,20 +193,36 @@ struct completion; struct pt_regs; struct user; +/* + * cond_resched() and cond_resched_lock(): latency reduction via + * explicit rescheduling in places that are safe. The return + * value indicates whether a reschedule was done in fact. + * cond_resched_lock() will drop the spinlock before scheduling, + */ #ifndef CONFIG_PREEMPTION +extern int _cond_resched(void); +#else +static inline int _cond_resched(void) { return 0; } +#endif + +#ifdef CONFIG_PREEMPT_DYNAMIC #ifdef CONFIG_PREEMPT_VOLUNTARY DECLARE_STATIC_KEY_TRUE(preempt_voluntary_key); #else DECLARE_STATIC_KEY_FALSE(preempt_voluntary_key); #endif -extern int _cond_resched(void); # define might_resched() \ do { if (static_branch_likely(&preempt_voluntary_key)) _cond_resched(); } while (0) #else + +#ifdef CONFIG_PREEMPT_VOLUNTARY # define might_resched() \ - do { } while (0) + do { _cond_resched(); } while (0) +#else +# define might_resched() do { } while (0) #endif +#endif /* CONFIG_PREEMPT_DYNAMIC */ #ifdef CONFIG_DEBUG_ATOMIC_SLEEP extern void ___might_sleep(const char *file, int line, int preempt_offset); diff --git a/include/linux/sched.h b/include/linux/sched.h index afe01e232935..184b5e162184 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1812,18 +1812,6 @@ static inline int test_tsk_need_resched(struct task_struct *tsk) return unlikely(test_tsk_thread_flag(tsk,TIF_NEED_RESCHED)); } -/* - * cond_resched() and cond_resched_lock(): latency reduction via - * explicit rescheduling in places that are safe. The return - * value indicates whether a reschedule was done in fact. - * cond_resched_lock() will drop the spinlock before scheduling, - */ -#ifndef CONFIG_PREEMPTION -extern int _cond_resched(void); -#else -static inline int _cond_resched(void) { return 0; } -#endif - #define cond_resched() ({ \ ___might_sleep(__FILE__, __LINE__, 0); \ _cond_resched(); \ diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt index bf82259cff96..a73b5564cc51 100644 --- a/kernel/Kconfig.preempt +++ b/kernel/Kconfig.preempt @@ -74,6 +74,25 @@ config PREEMPT_RT endchoice +config PREEMPT_DYNAMIC + bool "Allow boot time preemption model selection" + depends on PREEMPT_NONE || PREEMPT_VOLUNTARY + help + This option allows to define the preemption model on the kernel + command line parameter and thus override the default preemption + model defined during compile time. + + The feature is primarily interesting for Linux distributions which + provide a pre-built kernel binary to reduce the number of kernel + flavors they offer while still offering different usecases. + + The runtime overhead is negligible with JUMP_LABELS enabled but if + runtime patching is not available for the specific architecture then + the potential overhead should be considered. + + Select if you the same pre-built kernel should be used for both Server + and Desktop workloads. + config PREEMPT_COUNT bool diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 07d37d862637..fe22b2fca864 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -43,6 +43,8 @@ EXPORT_TRACEPOINT_SYMBOL_GPL(sched_update_nr_running_tp); DEFINE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues); +#ifdef CONFIG_PREEMPT_DYNAMIC + #ifdef CONFIG_PREEMPT_VOLUNTARY DEFINE_STATIC_KEY_TRUE(preempt_voluntary_key); #else @@ -51,6 +53,8 @@ DEFINE_STATIC_KEY_FALSE(preempt_voluntary_key); #endif EXPORT_SYMBOL(preempt_voluntary_key); +#endif + #if defined(CONFIG_SCHED_DEBUG) && defined(CONFIG_JUMP_LABEL) /* * Debugging: various feature bits @@ -8491,7 +8495,7 @@ void call_trace_sched_update_nr_running(struct rq *rq, int count) trace_sched_update_nr_running_tp(rq, count); } -#ifndef CONFIG_PREEMPTION +#ifdef CONFIG_PREEMPT_DYNAMIC static int __init setup_non_preempt_mode(char *str) { if (!strcmp(str, "none")) { -- Michal Hocko SUSE Labs
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