linux-kernel - Re: [PATCH] memcg: multi-memcg percpu charge cache

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Message-ID: <20250423153046.54d135f2@kernel.org>
Date: Wed, 23 Apr 2025 15:30:46 -0700
From: Jakub Kicinski <kuba@...nel.org>
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>, Vlastimil
 Babka <vbabka@...e.cz>, Eric Dumazet <edumazet@...gle.com>, Soheil Hassas
 Yeganeh <soheil@...gle.com>, linux-mm@...ck.org, cgroups@...r.kernel.org,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org, Meta kernel team
 <kernel-team@...a.com>
Subject: Re: [PATCH] memcg: multi-memcg percpu charge cache

On Wed, 23 Apr 2025 15:16:56 -0700 Shakeel Butt wrote:
> > > -	if (!local_trylock_irqsave(&memcg_stock.stock_lock, flags)) {
> > > +	if (nr_pages > MEMCG_CHARGE_BATCH ||
> > > +	    !local_trylock_irqsave(&memcg_stock.stock_lock, flags)) {
> > >  		/*
> > > -		 * In case of unlikely failure to lock percpu stock_lock
> > > -		 * uncharge memcg directly.
> > > +		 * In case of larger than batch refill or unlikely failure to
> > > +		 * lock the percpu stock_lock, uncharge memcg directly.
> > >  		 */  
> > 
> > We're bypassing the cache for > CHARGE_BATCH because the u8 math 
> > may overflow? Could be useful to refocus the comment on the 'why'
> 
> We actually never put more than MEMCG_CHARGE_BATCH in the cache and thus
> we can use u8 as type here. Though we may increase the batch size in
> future, so I should put a BUILD_BUG_ON somewhere here.

No idea if this matters enough to deserve its own commit but basically
I was wondering if that behavior change is a separate optimization.

Previously we'd check if the cache was for the releasing cgroup and sum
was over BATCH - drain its stock completely. Now we bypass looking at
the cache if nr_pages > BATCH so the cgroup may retain some stock.