linux-kernel - Re: [PATCH v1 2/3] cgroup/rstat: convert cgroup_rstat

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Date: Thu, 18 Apr 2024 13:39:41 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Jesper Dangaard Brouer <hawk@...nel.org>, tj@...nel.org, hannes@...xchg.org, 
	lizefan.x@...edance.com, cgroups@...r.kernel.org, longman@...hat.com, 
	netdev@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	kernel-team@...udflare.com, Arnaldo Carvalho de Melo <acme@...nel.org>, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, mhocko@...nel.org
Subject: Re: [PATCH v1 2/3] cgroup/rstat: convert cgroup_rstat_lock back to mutex

On Thu, Apr 18, 2024 at 7:49 AM Shakeel Butt <shakeel.butt@...ux.dev> 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 the
> > mutex, but I don't fully know the sched implication when holding a mutex.
> >
>
> 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/