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

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Message-ID: <20250411140646.05s0O5SY@linutronix.de>
Date: Fri, 11 Apr 2025 16:06:46 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Vlastimil Babka <vbabka@...e.cz>
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>,
	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 2025-04-11 10:55:31 [+0200], Vlastimil Babka wrote:
> > @@ -1964,10 +1964,10 @@ static int memcg_hotplug_cpu_dead(unsigned int cpu)
> >  
> >  	stock = &per_cpu(memcg_stock, cpu);
> >  
> > -	/* drain_obj_stock requires stock_lock */
> > -	local_lock_irqsave(&memcg_stock.stock_lock, flags);
> > -	drain_obj_stock(stock);
> > -	local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
> > +	local_irq_save(flag);
> 
> I think for RT this is not great? At least in theory, probably it's not
> actually used together with cpu hotplug? As it relies on memcg_stats_lock()
> I think no irq save/enable is necessary there. local_lock_irqsave wasn't
> actually a irq disable on RT. I don't know if there's a handy wrapper for this.

No seeing the whole context but memcg_hotplug_cpu_dead() should be
invoked the control CPU while "cpu" is already gone. So the local_lock
should be acquired and the target CPU needs no locks since it is
offline. local_irq_save() will break things.

Sebastian