linux-kernel - Re: [PATCH RFC 3/3] slub: reparent memcg caches' slabs on memcg offline

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Date:	Thu, 15 May 2014 11:16:52 +0400
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Christoph Lameter <cl@...ux.com>
CC:	<hannes@...xchg.org>, <mhocko@...e.cz>,
	<akpm@...ux-foundation.org>, <linux-kernel@...r.kernel.org>,
	<linux-mm@...ck.org>
Subject: Re: [PATCH RFC 3/3] slub: reparent memcg caches' slabs on memcg
 offline

On Wed, May 14, 2014 at 11:20:51AM -0500, Christoph Lameter wrote:
> On Tue, 13 May 2014, Vladimir Davydov wrote:
> 
> > Since the "slow" and the "normal" free's can't coexist at the same time,
> > we must assure all conventional free's have finished before switching
> > all further free's to the "slow" mode and starting reparenting. To
> > achieve that, a percpu refcounter is used. It is taken and held during
> > each "normal" free. The refcounter is killed on memcg offline, and the
> > cache's pages migration is initiated from the refcounter's release
> > function. If we fail to take a ref on kfree, it means all "normal"
> > free's have been completed and the cache is being reparented right now,
> > so we should free the object using the "slow" mode.
> 
> Argh adding more code to the free path touching more cachelines in the
> process.

Actually, there is not that much active code added, IMO. In fact, it's
only percpu ref get/put for per memcg caches plus a couple of
conditionals. The "slow" mode code is meant to be executed very rarely,
so we can move it to a separate function under unlikely optimization.

I admit that's far not perfect, because kfree is really a hot path,
where every byte of code matters, but unfortunately I don't see how we
can avoid this in case we want slab re-parenting.

Again, I'd like to hear from you if there is any point in moving in this
direction, or I should give up and concentrate on some other approach,
because you'll never accept it.

Thanks.
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