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sUdq2e7mDktJOhxWJAHGxmTNQoKwvduM X-Proofpoint-GUID: sUdq2e7mDktJOhxWJAHGxmTNQoKwvduM Tianchen Ding writes: > On 2024/5/28 08:35, Ankur Arora wrote: >> The default preemption policy for voluntary preemption under >> PREEMPT_AUTO is to schedule eagerly for tasks of higher scheduling >> class, and lazily for well-behaved, non-idle tasks. >> This is the same policy as preempt=none, with an eager handling of >> higher priority scheduling classes. >> Comparing a cyclictest workload with a background kernel load of >> 'stress-ng --mmap', shows that both the average and the maximum >> latencies improve: >> # stress-ng --mmap 0 & >> # cyclictest --mlockall --smp --priority=80 --interval=200 --distance=0 -q -D 300 >> Min ( %stdev ) Act ( %stdev >> ) Avg ( %stdev ) Max ( %stdev ) >> PREEMPT_AUTO, preempt=voluntary 1.73 ( +- 25.43% ) 62.16 ( +- >> 303.39% ) 14.92 ( +- 17.96% ) 2778.22 ( +- 15.04% ) >> PREEMPT_DYNAMIC, preempt=voluntary 1.83 ( +- 20.76% ) 253.45 ( +- 233.21% ) 18.70 ( +- 15.88% ) 2992.45 ( +- 15.95% ) >> The table above shows the aggregated latencies across all CPUs. >> Cc: Ingo Molnar >> Cc: Peter Ziljstra >> Cc: Juri Lelli >> Cc: Vincent Guittot >> Originally-by: Thomas Gleixner >> Link: https://lore.kernel.org/lkml/87jzshhexi.ffs@tglx/ >> Signed-off-by: Ankur Arora >> --- >> kernel/sched/core.c | 12 ++++++++---- >> kernel/sched/sched.h | 6 ++++++ >> 2 files changed, 14 insertions(+), 4 deletions(-) >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >> index c25cccc09b65..2bc3ae21a9d0 100644 >> --- a/kernel/sched/core.c >> +++ b/kernel/sched/core.c >> @@ -1052,6 +1052,9 @@ static resched_t resched_opt_translate(struct task_struct *curr, >> if (preempt_model_preemptible()) >> return RESCHED_NOW; >> + if (preempt_model_voluntary() && opt == RESCHED_PRIORITY) >> + return RESCHED_NOW; >> + >> if (is_idle_task(curr)) >> return RESCHED_NOW; >> @@ -2289,7 +2292,7 @@ void wakeup_preempt(struct rq *rq, struct task_struct >> *p, int flags) >> if (p->sched_class == rq->curr->sched_class) >> rq->curr->sched_class->wakeup_preempt(rq, p, flags); >> else if (sched_class_above(p->sched_class, rq->curr->sched_class)) >> - resched_curr(rq); >> + resched_curr_priority(rq); >> > Besides the conditions about higher class, can we do resched_curr_priority() in the same class? > For example, in fair class, we can do it when SCHED_NORMAL vs SCHED_IDLE. So, I agree about the specific case of SCHED_NORMAL vs SCHED_IDLE. (And, that case is already handled by resched_opt_translate() explicitly promoting idle tasks to TIF_NEED_RESCHED.) But, on the general question of doing resched_curr_priority() in the same class: I did consider it. But, it seemed to me that we want to keep run to completion semantics for lazy scheduling, and so not enforcing priority in a scheduling class was a good line. (Note that resched_curr_priority(), at least as it stands, is going away for v3. I'll be folding lazy scheduling as a single model under PREEMPT_DYNAMIC. So, no separate lazy=none, lazy=voluntary.) Thanks -- ankur