linux-kernel - Re: [PATCH] memcg: decouple memcg_hotplug_cpu_dead from stock

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Message-ID: <CAGj-7pXRmG2D+5=yj-5uuciiNccWws6erBg_hSm9S6coEhN+3Q@mail.gmail.com>
Date: Fri, 11 Apr 2025 14:12:46 -0400
From: Shakeel Butt <shakeel.butt@...il.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Shakeel Butt <shakeel.butt@...ux.dev>, 
	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>, Yosry Ahmed <yosry.ahmed@...ux.dev>, 
	Waiman Long <llong@...hat.com>, linux-mm@...ck.org, cgroups@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH] memcg: decouple memcg_hotplug_cpu_dead from stock_lock

On Fri, Apr 11, 2025 at 2:06 PM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 4/11/25 19:54, Shakeel Butt wrote:
> > (my migadu/linux.dev stopped working and I have to send through gmail,
> > sorry for any formatting issue)
> >
> > I don't see how local_irq_save() will break anything. We are working on
> > a stock of a dead remote cpu. We actually don't even need to disable irq
> > or need local cpu's local_lock. It is actually the calls to
> > __mod_memcg_lruvec_state() and __mod_memcg_state() in
> > __drain_obj_stock() which need irq-disabled on non-RT kernels and for
> > RT-kernels they already have preempt_disable_nested().
> >
> > Disabling irq even on RT seems excessive but this is not a performance
> > critical code, so I don't see an issue unless there is
> > local_lock_irqsave() alternative which does not disables irqs on RT
> > kernels.
>
> local_lock_irqsave() does not disable irqs on RT kernels :)

Sorry, I wanted to say local_irq_save() here instead of local_lock_irqsave().

> so keeping
> local_lock as is would do the irq disable on !RT and be more RT-friendly on
> RT. It's just wrong from the logical scope of the lock to perform it on a
> different cpu than the stock we modify. If one day we have some runtime
> checks for that, they would complain.

Basically I don't want to use stock_lock here. Maybe I should explore
adding a new local_lock for __mod_memcg_lruvec_state and
__mod_memcg_state.