linux-kernel - Re: [PATCH 7/9] memcg: use __mod_memcg_state in drain_obj

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Message-ID: <6n7rsw565dy4kt7yxmik5kpxdz2b5h2bdsysfvi2rwmvl4juml@npkqfiyzfqua>
Date: Mon, 17 Mar 2025 14:54:45 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Vlastimil Babka <vbabka@...e.cz>
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>, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, linux-mm@...ck.org, cgroups@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH 7/9] memcg: use __mod_memcg_state in drain_obj_stock

On Mon, Mar 17, 2025 at 09:56:39PM +0100, Vlastimil Babka wrote:
> On 3/15/25 18:49, Shakeel Butt wrote:
> > For non-PREEMPT_RT kernels, drain_obj_stock() is always called with irq
> > disabled, so we can use __mod_memcg_state() instead of
> > mod_memcg_state(). For PREEMPT_RT, we need to add memcg_stats_[un]lock
> > in __mod_memcg_state().
> > 
> > Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>
> > Reviewed-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> 
> I've asked in the RFC and from Sebastian's answer I think my question was
> misunderstod, so let me try again.
> 
> After this patch we'll have from mod_memcg_state():
> 
> mod_memcg_state()
>   local_irq_save(flags);
>   __mod_memcg_state()
>     memcg_stats_lock(); <- new and unnecessary?
> 
> Instead of modifying __mod_memcg_state() we could be more targetted and just
> do in drain_obj_stock():
> 
> memcg_stats_lock();
> __mod_memcg_state();
> memcg_stats_unlock();
> 
> Am I missing something?

This seems unnecessary because this patch is adding the first user of
__mod_memcg_state() but I think maintainability is better with
memcg_stats_[un]lock() inside __mod_memcg_state().

Let's take the example of __mod_memcg_lruvec_state(). It is being
called from places where non-RT kernel, the irqs are disabled through
spin_lock_irq*, so on RT kernel, the irq would not be disabled and
thus explicitly need preemption disabled. What if in future
__mod_memcg_state() is being used by a caller which assumes preemption
is disabled through irq disable. The RT kernel would be buggy there.

I am not sure if it is easy to force the future users to explicitly add
memcg_stats_[un]lock() across the call to __mod_memcg_state().