linux-kernel - Re: [PATCH v2] memcg: simple cleanup of stats update functions

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Date: Tue, 28 May 2024 14:17:44 +0200
From: "Vlastimil Babka (SUSE)" <vbabka@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: 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>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] memcg: simple cleanup of stats update functions

On 5/28/24 12:19 PM, Sebastian Andrzej Siewior wrote:
> On 2024-05-28 10:10:50 [+0200], Vlastimil Babka (SUSE) wrote:
>> > I don't mind sending a patch. I'm just not sure if the lock is the right
>> > thing to do. However it should ensure that interrupts are disabled on
>> > !RT for the sake of the counter update (if observed in IRQ context).
>> 
>> Looks like some places there use VM_WARN_ON_IRQS_ENABLED() that's turned off
>> for PREEMPT_RT, so maybe that's what should replace the current
>> lockdep_assert, perhaps together with
>> lockdep_assert_held(this_cpu_ptr(&memcg_stock.stock_lock));
>> 
>> But also __mod_memcg_lruvec_state() already has that VM_WARN_ON.
> 
> This "VM_WARN_ON_IRQS_ENABLED" is the initial assert for "interrupts
> must be disabled while change the counter".
> You want to replace it with lockdep? Part of its requirement was that it
> yells with lockdep disabled.

No I meant the other way around, that we'de use VM_WARN_ON_IRQS_ENABLED
instead of the current lockdep_assert() that doesn't work on RT.

> Currently I am leaning towards removing the
> lockdep_assert_irqs_disabled() from __mod_objcg_mlstate(). Nothing but
> the counter need it and they have their own check. So?

Yeah that might be sufficient.

> Sebastian