Re: [PATCH] sched/fair: don't assign runtime for throttled cfs_rq

[Liangyan]

On 19/8/27 äå1:38, bsegall@xxxxxxxxxx wrote:
> Valentin Schneider <valentin.schneider@xxxxxxx> writes:
>
> > On 23/08/2019 21:00, bsegall@xxxxxxxxxx wrote:
> > [...]
> > > Could you mention in the message that this a throttled cfs_rq can have
> > > account_cfs_rq_runtime called on it because it is throttled before
> > > idle_balance, and the idle_balance calls update_rq_clock to add time
> > > that is accounted to the task.
> >
> > Mayhaps even a comment for the extra condition.
> >
> > > I think this solution is less risky than unthrottling
> > > in this area, so other than that:
> > >
> > > Reviewed-by: Ben Segall <bsegall@xxxxxxxxxx>
> >
> > If you don't mind squashing this in:
> >
> > -----8<-----
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index b1d9cec9b1ed..b47b0bcf56bc 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -4630,6 +4630,10 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b, u64 remaining)
> >   		if (!cfs_rq_throttled(cfs_rq))
> >   			goto next;
> >   
> > +		/* By the above check, this should never be true */
> > +		WARN_ON(cfs_rq->runtime_remaining > 0);
> > +
> > +		/* Pick the minimum amount to return to a positive quota state */
> >   		runtime = -cfs_rq->runtime_remaining + 1;
> >   		if (runtime > remaining)
> >   			runtime = remaining;
> > ----->8-----
> >
> > I'm not adamant about the extra comment, but the WARN_ON would be nice IMO.
> >
> >
> > @Ben, do you reckon we want to strap
> >
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Fixes: ec12cb7f31e2 ("sched: Accumulate per-cfs_rq cpu usage and charge against bandwidth")
> >
> > to the thing? AFAICT the pick_next_task_fair() + idle_balance() dance you
> > described should still be possible on that commit.
>
> I'm not sure about stable policy in general, but it seems reasonable.
> The WARN_ON might want to be WARN_ON_ONCE, and it seems fine to have it
> or not.

Thanks Ben and Valentin for all of the comments. Per Xunlei's 
suggestion, I used SCHED_WARN_ON instead in v3. Regarding whether cc 
stable, I'm also not sure.

> > Other than that,
> >
> > Reviewed-by: Valentin Schneider <valentin.schneider@xxxxxxx>
> >
> > [...]