From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754441AbaEOHRA (ORCPT ); Thu, 15 May 2014 03:17:00 -0400 Received: from mx2.parallels.com ([199.115.105.18]:60944 "EHLO mx2.parallels.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751345AbaEOHQ7 (ORCPT ); Thu, 15 May 2014 03:16:59 -0400 Date: Thu, 15 May 2014 11:16:52 +0400 From: Vladimir Davydov To: Christoph Lameter CC: , , , , Subject: Re: [PATCH RFC 3/3] slub: reparent memcg caches' slabs on memcg offline Message-ID: <20140515071650.GB32113@esperanza> References: <6eafe1e95d9a934228e9af785f5b5de38955aa6a.1399982635.git.vdavydov@parallels.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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.