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Date: Tue, 28 May 2024 14:17:44 +0200
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Subject: Re: [PATCH v2] memcg: simple cleanup of stats update functions
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To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Shakeel Butt <shakeel.butt@linux.dev>,
 Andrew Morton <akpm@linux-foundation.org>,
 Johannes Weiner <hannes@cmpxchg.org>, Michal Hocko <mhocko@kernel.org>,
 Roman Gushchin <roman.gushchin@linux.dev>,
 Muchun Song <muchun.song@linux.dev>, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>
References: <20240420232505.2768428-1-shakeel.butt@linux.dev>
 <20240527152200.P1rU7FaG@linutronix.de>
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 <20240528075623.oFcU1JLj@linutronix.de>
 <69c0e008-ddde-4a38-b856-b765dc2b9745@kernel.org>
 <20240528101906.ts4NjHvU@linutronix.de>
From: "Vlastimil Babka (SUSE)" <vbabka@kernel.org>
In-Reply-To: <20240528101906.ts4NjHvU@linutronix.de>
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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