commit 97450eb90965 ("sched/pelt: Remove shift of thermal clock")
removed the decay_shift for hw_pressure. This commit uses the
sched_clock_task() in sched_tick() while it replaces the
sched_clock_task() with rq_clock_pelt() in __update_blocked_others().
This could bring inconsistence. One possible scenario I can think of
is in ___update_load_sum():

u64 delta = now - sa->last_update_time

'now' could be calculated by rq_clock_pelt() from
__update_blocked_others(), and last_update_time was calculated by
rq_clock_task() previously from sched_tick(). Usually the former
chases after the latter, it cause a very large 'delta' and brings
unexpected behavior.

Fixes: 97450eb90965 ("sched/pelt: Remove shift of thermal clock")
Reviewed-by: Hongyan Xia <hongyan.xia2@arm.com>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
---
v3->v4:
  Revert to v2 to keep the clock outside of pelt functions.
  (Vincent Guittot)
  Added comments for hw_pressure within __update_blocked_others().
  (Qais Yousef)
v2>v3:
  call rq_clock_task() inside update_hw_load_avg() to avoid any
  inconsistence in the future. (Qais Yousef)
  Added comments around update_hw_load_avg(). (Qais Yousef)
v1->v2:
  Added Hongyan's Reviewed-by tag.
  Removed the Reported-by/Closes tags because they are not related
  to this fix.(Hongyan Xia)
---
 kernel/sched/fair.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9057584ec06d..5176db6ae13d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9360,9 +9360,10 @@ static bool __update_blocked_others(struct rq *rq, bool *done)
 
 	hw_pressure = arch_scale_hw_pressure(cpu_of(rq));
 
+	/* hw_pressure doesn't care about invariance */
 	decayed = update_rt_rq_load_avg(now, rq, curr_class == &rt_sched_class) |
 		  update_dl_rq_load_avg(now, rq, curr_class == &dl_sched_class) |
-		  update_hw_load_avg(now, rq, hw_pressure) |
+		  update_hw_load_avg(rq_clock_task(rq), rq, hw_pressure) |
 		  update_irq_load_avg(rq, 0);
 
 	if (others_have_blocked(rq))
-- 
2.25.1

On Tue, 6 Aug 2024 at 03:54, Chen Yu <yu.c.chen@intel.com> wrote:
>
> commit 97450eb90965 ("sched/pelt: Remove shift of thermal clock")
> removed the decay_shift for hw_pressure. This commit uses the
> sched_clock_task() in sched_tick() while it replaces the
> sched_clock_task() with rq_clock_pelt() in __update_blocked_others().
> This could bring inconsistence. One possible scenario I can think of
> is in ___update_load_sum():
>
> u64 delta = now - sa->last_update_time
>
> 'now' could be calculated by rq_clock_pelt() from
> __update_blocked_others(), and last_update_time was calculated by
> rq_clock_task() previously from sched_tick(). Usually the former
> chases after the latter, it cause a very large 'delta' and brings
> unexpected behavior.
>
> Fixes: 97450eb90965 ("sched/pelt: Remove shift of thermal clock")
> Reviewed-by: Hongyan Xia <hongyan.xia2@arm.com>
> Signed-off-by: Chen Yu <yu.c.chen@intel.com>

Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>


> ---
> v3->v4:
>   Revert to v2 to keep the clock outside of pelt functions.
>   (Vincent Guittot)
>   Added comments for hw_pressure within __update_blocked_others().
>   (Qais Yousef)
> v2>v3:
>   call rq_clock_task() inside update_hw_load_avg() to avoid any
>   inconsistence in the future. (Qais Yousef)
>   Added comments around update_hw_load_avg(). (Qais Yousef)
> v1->v2:
>   Added Hongyan's Reviewed-by tag.
>   Removed the Reported-by/Closes tags because they are not related
>   to this fix.(Hongyan Xia)
> ---
>  kernel/sched/fair.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 9057584ec06d..5176db6ae13d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9360,9 +9360,10 @@ static bool __update_blocked_others(struct rq *rq, bool *done)
>
>         hw_pressure = arch_scale_hw_pressure(cpu_of(rq));
>
> +       /* hw_pressure doesn't care about invariance */
>         decayed = update_rt_rq_load_avg(now, rq, curr_class == &rt_sched_class) |
>                   update_dl_rq_load_avg(now, rq, curr_class == &dl_sched_class) |
> -                 update_hw_load_avg(now, rq, hw_pressure) |
> +                 update_hw_load_avg(rq_clock_task(rq), rq, hw_pressure) |
>                   update_irq_load_avg(rq, 0);
>
>         if (others_have_blocked(rq))
> --
> 2.25.1
>