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

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Date:   Mon, 7 Nov 2022 13:47:23 +0200
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rjw@...ysocki.net, oleg@...hat.com, mingo@...nel.org,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, mgorman@...e.de, ebiederm@...ssion.com,
        bigeasy@...utronix.de, Will Deacon <will@...nel.org>,
        linux-kernel@...r.kernel.org, tj@...nel.org,
        linux-pm@...r.kernel.org, intel-gfx@...ts.freedesktop.org
Subject: Re: [PATCH v3 6/6] freezer,sched: Rewrite core freezer logic

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.

> 
> ---
> 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