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Date:   Fri, 1 Oct 2021 17:48:32 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc:     Intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
        linux-kernel@vger.kernel.org,
        Tvrtko Ursulin <tvrtko.ursulin@intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>
Subject: Re: [RFC 1/6] sched: Add nice value change notifier
Message-ID: <YVct0D9kB1KtrwZ3@hirez.programming.kicks-ass.net>
References: <20210930171552.501553-1-tvrtko.ursulin@linux.intel.com>
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 <20210930183316.GC4323@worktop.programming.kicks-ass.net>
 <4aca656d-678f-4d61-38a4-d2e7a8fd89ab@linux.intel.com>
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On Fri, Oct 01, 2021 at 11:32:16AM +0100, Tvrtko Ursulin wrote:
> 
> On 01/10/2021 10:04, Tvrtko Ursulin wrote:
> > 
> > Hi Peter,
> > 
> > On 30/09/2021 19:33, Peter Zijlstra wrote:
> > > On Thu, Sep 30, 2021 at 06:15:47PM +0100, Tvrtko Ursulin wrote:
> > > > ? void set_user_nice(struct task_struct *p, long nice)
> > > > ? {
> > > > ????? bool queued, running;
> > > > -??? int old_prio;
> > > > +??? int old_prio, ret;
> > > > ????? struct rq_flags rf;
> > > > ????? struct rq *rq;
> > > > @@ -6913,6 +6945,9 @@ void set_user_nice(struct task_struct *p,
> > > > long nice)
> > > > ?????? */
> > > > ????? p->sched_class->prio_changed(rq, p, old_prio);
> > > > +??? ret = atomic_notifier_call_chain(&user_nice_notifier_list,
> > > > nice, p);
> > > > +??? WARN_ON_ONCE(ret != NOTIFY_DONE);
> > > > +
> > > > ? out_unlock:
> > > > ????? task_rq_unlock(rq, p, &rf);
> > > > ? }
> > > 
> > > No, we're not going to call out to exported, and potentially unbounded,
> > > functions under scheduler locks.
> > 
> > Agreed, that's another good point why it is even more hairy, as I have
> > generally alluded in the cover letter.
> > 
> > Do you have any immediate thoughts on possible alternatives?
> > 
> > Like for instance if I did a queue_work from set_user_nice and then ran
> > a notifier chain async from a worker? I haven't looked at yet what
> > repercussion would that have in terms of having to cancel the pending
> > workers when tasks exit. I can try and prototype that and see how it
> > would look.
> 
> Hm or I simply move calling the notifier chain to after task_rq_unlock? That
> would leave it run under the tasklist lock so probably still quite bad.

Hmm? That's for normalize_rt_tasks() only, right? Just don't have it
call the notifier in that special case (that's a magic sysrq thing
anyway).