linux-kernel - Re: [PATCH] memcg: no refill for offlined objcg

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Message-ID: <ttlcvw2jiwloctvqblvoo4mdn7cg7av6uzauzjlwnpyevghnpt@jthjpbztrvii>
Date: Mon, 14 Apr 2025 11:44:49 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	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: no refill for offlined objcg

On Thu, Apr 10, 2025 at 11:59:47PM +0000, Roman Gushchin wrote:
> Shakeel Butt <shakeel.butt@...ux.dev> writes:
> 
> > In our fleet, we are observing refill_obj_stock() spending a lot of cpu
> > in obj_cgroup_get() and on further inspection it seems like the given
> > objcg is offlined and the kernel has to take the slow path i.e. atomic
> > operations for objcg reference counting.
> >
> > Other than expensive atomic operations, refilling stock of an offlined
> > objcg is a waster as there will not be new allocations for the offlined
> > objcg. In addition, refilling triggers flush of the previous objcg which
> > might be used in future. So, let's just avoid refilling the stock with
> > the offlined objcg.
> 
> Hm, but on the other side if there are multiple uncharges in a row,
> refilling obj stocks might be still cheaper?
> 

Thanks for the review. I looked at the fleet data again and what you are
suspecting i.e. multiple objects of same objcg getting freed closeby is
possible. I think I should be optimizing/batching at the upper layer. I
will be looking at rcu freeing side which is the most obvious batching
opportunity.

> In general I think that switching to atomic css refcnt on memcg
> offlining is a mistake - it makes memory reclaim generally more
> expensive. We can simple delay it until the approximate refcnt
> number reaches some low value, e.g. 100 objects.

This is a good idea. For the memcg, I think after Muchun's LRU
reparenting, we should not have zombie refcnt slowdown but for objcg
this idea might help. Anyways this is for later.