linux-kernel - Re: [PATCH v7 05/23] locking/mutex: Rework task_struct::blocked

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Date: Thu, 21 Dec 2023 09:52:52 -0800
From: John Stultz <jstultz@...gle.com>
To: Metin Kaya <metin.kaya@....com>
Cc: LKML <linux-kernel@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Joel Fernandes <joelaf@...gle.com>, Qais Yousef <qyousef@...gle.com>, Ingo Molnar <mingo@...hat.com>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Valentin Schneider <vschneid@...hat.com>, 
	Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, 
	Zimuzo Ezeozue <zezeozue@...gle.com>, Youssef Esmat <youssefesmat@...gle.com>, 
	Mel Gorman <mgorman@...e.de>, Daniel Bristot de Oliveira <bristot@...hat.com>, Will Deacon <will@...nel.org>, 
	Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>, 
	"Paul E. McKenney" <paulmck@...nel.org>, Xuewen Yan <xuewen.yan94@...il.com>, 
	K Prateek Nayak <kprateek.nayak@....com>, Thomas Gleixner <tglx@...utronix.de>, kernel-team@...roid.com, 
	"Connor O'Brien" <connoro@...gle.com>
Subject: Re: [PATCH v7 05/23] locking/mutex: Rework task_struct::blocked_on

On Thu, Dec 21, 2023 at 2:13 AM Metin Kaya <metin.kaya@....com> wrote:
> On 20/12/2023 12:18 am, John Stultz wrote:
> > +static inline struct mutex *get_task_blocked_on(struct task_struct *p)
> > +{
> > +     lockdep_assert_held(&p->blocked_lock);
> > +
> > +     return p->blocked_on;
> > +}
> > +
> > +static inline struct mutex *get_task_blocked_on_once(struct task_struct *p)
> > +{
> > +     return READ_ONCE(p->blocked_on);
> > +}
>
> These functions make me think we should use [get, set]_task_blocked_on()
> for accessing blocked_on & blocked_on_state fields, but there are some
> references in this patch which we directly access aforementioned fields.
> Is this OK?

Yeah. In the reworks I've probably added some subtle uses that should
be switched to the accessors or better commented.


 > @@ -4341,6 +4342,11 @@ int try_to_wake_up(struct task_struct *p,
unsigned int state, int wake_flags)
> >
> >               ttwu_queue(p, cpu, wake_flags);
> >       }
> > +     /* XXX can we do something better here for !CONFIG_SCHED_PROXY_EXEC case */
>
> blocked_on* fields are now used even in !CONFIG_SCHED_PROXY_EXEC case.
> I'm unsure if we can get rid of lock & unlock lines or entire hunk, but
> would this be too ugly? I wish we could convert blocked_on_state to an
> atomic variable.

Well, atomics have their own costs, but it's something I'll think
about. In the comment above, the idea I'm pondering is that in the
!PROXY_EXEC case the blocked_on_state doesn't provide much utility, so
maybe there's a way to opt out of that portion (while keeping the
blocked_on for debug checking).  Even in the PROXY_EXEC case, we might
be able to move this check to proxy_needs_return(), but I need to
think the logic out to make sure we'd always hit that path when we
need to make the transition.

I've also wondered if the blocked_on_state might be able to be merged
into the task->__state, but the rules there are more subtle so for my
sanity I've kept it separate here for now.

thanks
-john