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

[Qais Yousef]

On 07/29/24 10:28, Chen Yu wrote:
> 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@xxxxxxx>
> > > Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> > > ---
> > > 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.

Yes. I think it is less often happens from there though, that's why I said the
impact is low. But yes there's a problem.