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

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Message-ID: <20250505090617.Q6tHb1NH@linutronix.de>
Date: Mon, 5 May 2025 11:06:17 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>,
	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>
Subject: Re: [PATCH v2 3/3] memcg: no irq disable for memcg stock lock

On 2025-05-02 16:40:53 [-0700], Shakeel Butt wrote:
> 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?

It does but BH is preemptible.

> 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.

Sebastian