Re: [PATCH v2] sched/pelt: Use rq_clock_task() for hw_pressure

[Chen Yu <yu.c.chen@intel.com>]

Hi Qais,

thanks for taking a look,

On 2024-07-28 at 21:10:28 +0100, Qais Yousef wrote:
> On 07/25/24 23:08, Chen Yu 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>
> > ---
> > 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 | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 9057584ec06d..cfd4755954fd 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -9362,7 +9362,7 @@ static bool __update_blocked_others(struct rq *rq, bool *done)
> >  
> >  	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) |
> 
> NIT:
> 
> Wouldn't it be better to remove 'now' and call rq_clock_task() inside
> update_hw_load_avg()? Adding a comment on why we should use this not clock_pelt
> would be helpful too. hw_pressure doesn't care about invariance.
>

OK, will do in the next version.
 
> ie:
> 
> update_hw_load_avg(rq, hw_pressure)
> {
> }
> 
> LGTM anyway. I think this is called most of the time from idle when clock_pelt
> is synced with clock_task. So the impact is low, I believe.
>

Yes, when the current task is found to be idle, clock_pelt is synced with
clock_task by update_rq_clock(). While a case is that, in the softirq load balance,
sched_balance_softirq()->sched_balance_update_blocked_averages()->__update_blocked_others()
->update_hw_load_avg() is not always called by the idle task, and clock_pelt has not been
synced with clock_task yet.

thanks,
Chenyu