linux-kernel - Re: [PATCH v2 3/3] memcg: no irq disable for memcg stock lock

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Message-ID: <CAGj-7pXrKBr+LC_Mbj+xyud=tXpR3bCYwzOQTUgM8aSZ0qNnhA@mail.gmail.com>
Date: Fri, 2 May 2025 16:40:53 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Johannes Weiner <hannes@...xchg.org>, 
	Michal Hocko <mhocko@...nel.org>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Muchun Song <muchun.song@...ux.dev>, Alexei Starovoitov <ast@...nel.org>, linux-mm <linux-mm@...ck.org>, 
	"open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, Meta kernel team <kernel-team@...a.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH v2 3/3] memcg: no irq disable for memcg stock lock

On Fri, May 2, 2025 at 4:28 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
[...]
> > >
> > > I don't think it works.
> > > When there is a normal irq and something doing regular GFP_NOWAIT
> > > allocation gfpflags_allow_spinning() will be true and
> > > local_lock() will reenter and complain that lock->acquired is
> > > already set... but only with lockdep on.
> >
> > Yes indeed. I dropped the first patch and didn't fix this one
> > accordingly. I think the fix can be as simple as checking for
> > in_task() here instead of gfp_mask. That should work for both RT and
> > non-RT kernels.
>
> Like:
> if (in_task())
>   local_lock(...);
> else if (!local_trylock(...))
>
> Most of the networking runs in bh, so it will be using
> local_trylock() path which is probably ok in !PREEMPT_RT,
> but will cause random performance issues in PREEMP_RT,
> since rt_spin_trylock() will be randomly failing and taking
> slow path of charging. It's not going to cause permanent
> nginx 3x regression :), but unlucky slowdowns will be seen.
> A task can grab that per-cpu rt_spin_lock and preempted
> by network processing.

Does networking run in bh for PREEMPT_RT as well?

I think I should get networking & RT folks opinion on this one. I will
decouple this irq patch from the decoupling lock patches and start a
separate discussion thread.