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From: Yosry Ahmed <yosryahmed@google.com>
Date: Thu, 18 Apr 2024 13:39:41 -0700
Message-ID: <CAJD7tkYJZgWOeFuTMYNoyH=9+uX2qaRdwc4cNuFN9wdhneuHfA@mail.gmail.com>
Subject: Re: [PATCH v1 2/3] cgroup/rstat: convert cgroup_rstat_lock back to mutex
To: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Jesper Dangaard Brouer <hawk@kernel.org>, tj@kernel.org, hannes@cmpxchg.org, 
	lizefan.x@bytedance.com, cgroups@vger.kernel.org, longman@redhat.com, 
	netdev@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, 
	kernel-team@cloudflare.com, Arnaldo Carvalho de Melo <acme@kernel.org>, 
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>, mhocko@kernel.org
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On Thu, Apr 18, 2024 at 7:49=E2=80=AFAM Shakeel Butt <shakeel.butt@linux.de=
v> wrote:
>
> On Thu, Apr 18, 2024 at 11:02:06AM +0200, Jesper Dangaard Brouer wrote:
> >
> >
> > On 18/04/2024 04.19, Yosry Ahmed wrote:
> [...]
> > >
> > > I will keep the high-level conversation about using the mutex here in
> > > the cover letter thread, but I am wondering why we are keeping the
> > > lock dropping logic here with the mutex?
> > >
> >
> > I agree that yielding the mutex in the loop makes less sense.
> > Especially since the raw_spin_unlock_irqrestore(cpu_lock, flags) call
> > will be a preemption point for my softirq.   But I kept it because, we
> > are running a CONFIG_PREEMPT_VOLUNTARY kernel, so I still worried that
> > there was no sched point for other userspace processes while holding th=
e
> > mutex, but I don't fully know the sched implication when holding a mute=
x.
> >
>
> Are the softirqs you are interested in, raised from the same cpu or
> remote cpu? What about local_softirq_pending() check in addition to
> need_resched() and spin_needbreak() checks? If softirq can only be
> raised on local cpu then convert the spin_lock to non-irq one (Please
> correct me if I am wrong but on return from hard irq and not within bh
> or irq disabled spin_lock, the kernel will run the pending softirqs,
> right?). Did you get the chance to test these two changes or something
> similar in your prod environment?

I tried making the spinlock a non-irq lock before, but Tejun objected [1].

Perhaps we could experiment with always dropping the lock at CPU
boundaries instead?

[1]https://lore.kernel.org/lkml/ZBz%2FV5a7%2F6PZeM7S@slm.duckdns.org/