linux-kernel - Re: [Intel-gfx] [PATCH v3 6/6] freezer, sched: Rewrite core freezer logic

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Message-ID: <Y21elgaCwy3FNk70@intel.com>
Date:   Thu, 10 Nov 2022 22:27:02 +0200
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        bigeasy@...utronix.de, rjw@...ysocki.net, oleg@...hat.com,
        rostedt@...dmis.org, mingo@...nel.org, mgorman@...e.de,
        intel-gfx@...ts.freedesktop.org, tj@...nel.org,
        Will Deacon <will@...nel.org>, dietmar.eggemann@....com,
        ebiederm@...ssion.com
Subject: Re: [Intel-gfx] [PATCH v3 6/6] freezer, sched: Rewrite core freezer
 logic

On Mon, Nov 07, 2022 at 01:47:23PM +0200, Ville Syrjälä wrote:
> On Wed, Nov 02, 2022 at 11:16:48PM +0100, Peter Zijlstra wrote:
> > On Wed, Nov 02, 2022 at 06:57:51PM +0200, Ville Syrjälä wrote:
> > > On Thu, Oct 27, 2022 at 06:53:23PM +0200, Peter Zijlstra wrote:
> > > > On Thu, Oct 27, 2022 at 04:09:01PM +0300, Ville Syrjälä wrote:
> > > > > On Wed, Oct 26, 2022 at 01:43:00PM +0200, Peter Zijlstra wrote:
> > > > 
> > > > > > Could you please give the below a spin?
> > > > > 
> > > > > Thanks. I've added this to our CI branch. I'll try to keep and eye
> > > > > on it in the coming days and let you know if anything still trips.
> > > > > And I'll report back maybe ~middle of next week if we haven't caught
> > > > > anything by then.
> > > > 
> > > > Thanks!
> > > 
> > > Looks like we haven't caught anything since I put the patch in.
> > > So the fix seems good.
> > 
> > While writing up the Changelog, it occured to me it might be possible to
> > fix another way, could I bother you to also run the below patch for a
> > bit?
> 
> I swapped in the new patch to the CI branch. I'll check back
> after a few days.

CI hasn't had anything new to report AFAICS, so looks like this
version is good as well.

> 
> > 
> > ---
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index cb2aa2b54c7a..daff72f00385 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -4200,6 +4200,40 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> >  	return success;
> >  }
> >  
> > +static bool __task_needs_rq_lock(struct task_struct *p)
> > +{
> > +	unsigned int state = READ_ONCE(p->__state);
> > +
> > +	/*
> > +	 * Since pi->lock blocks try_to_wake_up(), we don't need rq->lock when
> > +	 * the task is blocked. Make sure to check @state since ttwu() can drop
> > +	 * locks at the end, see ttwu_queue_wakelist().
> > +	 */
> > +	if (state == TASK_RUNNING || state == TASK_WAKING)
> > +		return true;
> > +
> > +	/*
> > +	 * Ensure we load p->on_rq after p->__state, otherwise it would be
> > +	 * possible to, falsely, observe p->on_rq == 0.
> > +	 *
> > +	 * See try_to_wake_up() for a longer comment.
> > +	 */
> > +	smp_rmb();
> > +	if (p->on_rq)
> > +		return true;
> > +
> > +#ifdef CONFIG_SMP
> > +	/*
> > +	 * Ensure the task has finished __schedule() and will not be referenced
> > +	 * anymore. Again, see try_to_wake_up() for a longer comment.
> > +	 */
> > +	smp_rmb();
> > +	smp_cond_load_acquire(&p->on_cpu, !VAL);
> > +#endif
> > +
> > +	return false;
> > +}
> > +
> >  /**
> >   * task_call_func - Invoke a function on task in fixed state
> >   * @p: Process for which the function is to be invoked, can be @current.
> > @@ -4217,28 +4251,12 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> >  int task_call_func(struct task_struct *p, task_call_f func, void *arg)
> >  {
> >  	struct rq *rq = NULL;
> > -	unsigned int state;
> >  	struct rq_flags rf;
> >  	int ret;
> >  
> >  	raw_spin_lock_irqsave(&p->pi_lock, rf.flags);
> >  
> > -	state = READ_ONCE(p->__state);
> > -
> > -	/*
> > -	 * Ensure we load p->on_rq after p->__state, otherwise it would be
> > -	 * possible to, falsely, observe p->on_rq == 0.
> > -	 *
> > -	 * See try_to_wake_up() for a longer comment.
> > -	 */
> > -	smp_rmb();
> > -
> > -	/*
> > -	 * Since pi->lock blocks try_to_wake_up(), we don't need rq->lock when
> > -	 * the task is blocked. Make sure to check @state since ttwu() can drop
> > -	 * locks at the end, see ttwu_queue_wakelist().
> > -	 */
> > -	if (state == TASK_RUNNING || state == TASK_WAKING || p->on_rq)
> > +	if (__task_needs_rq_lock(p))
> >  		rq = __task_rq_lock(p, &rf);
> >  
> >  	/*
> 
> -- 
> Ville Syrjälä
> Intel

-- 
Ville Syrjälä
Intel