Tianchen Ding <dtcccc@xxxxxxxxxxxxxxxxx> writes:
On 2024/5/28 08:35, Ankur Arora wrote:
The default preemption policy for voluntary preemption underBesides the conditions about higher class, can we do resched_curr_priority() in the same class?
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 <mingo@xxxxxxxxxx>
Cc: Peter Ziljstra <peterz@xxxxxxxxxxxxx>
Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Originally-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/lkml/87jzshhexi.ffs@tglx/
Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
---
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);
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.