Re: [PATCH v4 2/2] sched: Skip priority checks with SCHED_FLAG_KEEP_PARAMS

From: Quentin Perret <qperret@google.com>
To: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: mingo@redhat.com, peterz@infradead.org,
	vincent.guittot@linaro.org, qais.yousef@arm.com,
	rickyiu@google.com, wvw@google.com, patrick.bellasi@matbug.net,
	xuewen.yan94@gmail.com, linux-kernel@vger.kernel.org,
	kernel-team@android.com
Subject: Re: [PATCH v4 2/2] sched: Skip priority checks with SCHED_FLAG_KEEP_PARAMS
Date: Tue, 27 Jul 2021 11:16:48 +0100	[thread overview]
Message-ID: <YP/dEMeULqozIqZd@google.com> (raw)
In-Reply-To: <YP6++lClPCQvTLcK@google.com>

On Monday 26 Jul 2021 at 14:56:10 (+0100), Quentin Perret wrote:
> On Thursday 22 Jul 2021 at 10:47:33 (+0200), Dietmar Eggemann wrote:
> > (*) This changes the behaviour when setting uclamp values on a DL task.
> > 
> > Before uclamp values could be set but now, because of
> > 
> >   void __getparam_dl(struct task_struct *p, struct sched_attr *attr)
> >     ..
> >     attr->sched_flags = dl_se->flags
> > 
> > SCHED_FLAG_UTIL_CLAMP gets overwritten and  __sched_setscheduler() bails in:
> > 
> >     if (unlikely(policy == p->policy)) {
> >       ...
> >       retval = 0;
> >       goto unlock;
> >     }
> >   change:
> > 
> > I.e. the:
> > 
> >       if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)
> >         goto change;
> > 
> > can't trigger anymore.
> 
> Bah, as you said it doesn't seem to be a big deal, but clearly that was
> unintentional. Let me try and fix this.

While looking at this I found existing bugs in the area. Fixes are here:

https://lore.kernel.org/lkml/20210727101103.2729607-1-qperret@google.com/

And with the above series applied this patch should behave correctly
now.

Thanks,
Quentin