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

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Message-ID: <24a8fa02-6504-4597-a445-a8124919410f@suse.cz>
Date: Tue, 18 Mar 2025 09:02:16 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Shakeel Butt <shakeel.butt@...ux.dev>
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 3/17/25 22:54, Shakeel Butt wrote:
> 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()

You mean first other user than mod_memcg_state() itself.

> 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().

I see the point. Well least memcg_stats_lock() isn't expensive, and it's a
no-op on non-debug !RT anyway.

Acked-by: Vlastimil Babka <vbabka@...e.cz>