linux-kernel - Re: [PATCH v4 07/14] sched: Introduce restrict_cpus_allowed

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Message-ID: <20201202130640.zg2iijbnxzq2zhu3@e107158-lin.cambridge.arm.com>
Date:   Wed, 2 Dec 2020 13:06:40 +0000
From:   Qais Yousef <qais.yousef@....com>
To:     Will Deacon <will@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Quentin Perret <qperret@...gle.com>, Tejun Heo <tj@...nel.org>,
        Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        kernel-team@...roid.com
Subject: Re: [PATCH v4 07/14] sched: Introduce restrict_cpus_allowed_ptr() to
 limit task CPU affinity

On 12/01/20 16:56, Will Deacon wrote:
> On Fri, Nov 27, 2020 at 01:19:16PM +0000, Qais Yousef wrote:
> > On 11/24/20 15:50, Will Deacon wrote:
> > 
> > [...]
> > 
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index d2003a7d5ab5..818c8f7bdf2a 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -1860,24 +1860,18 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
> > >  }
> > >  
> > >  /*
> > > - * Change a given task's CPU affinity. Migrate the thread to a
> > > - * proper CPU and schedule it away if the CPU it's executing on
> > > - * is removed from the allowed bitmask.
> > > - *
> > > - * NOTE: the caller must have a valid reference to the task, the
> > > - * task must not exit() & deallocate itself prematurely. The
> > > - * call is not atomic; no spinlocks may be held.
> > > + * Called with both p->pi_lock and rq->lock held; drops both before returning.
> > 
> > nit: wouldn't it be better for the caller to acquire and release the locks?
> > Not a big deal but it's always confusing when half of the work done outside the
> > function and the other half done inside.
> 
> That came up in the last version of the patches iirc, but the problem is
> that __set_cpus_allowed_ptr_locked() can trigger migration, which can
> drop the lock and take another one for the new runqueue.
> 
> Given that this function is internal to the scheduler, I think we can
> probably live with it.

I guess task_rq_lock() always entails be prepared for surprises!

Works for me.

Thanks

--
Qais Yousef