From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752863Ab1AQWAc (ORCPT ); Mon, 17 Jan 2011 17:00:32 -0500 Received: from mx1.redhat.com ([209.132.183.28]:18493 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752780Ab1AQWAa (ORCPT ); Mon, 17 Jan 2011 17:00:30 -0500 From: Jeff Moyer To: Vivek Goyal Cc: linux kernel mailing list , Jens Axboe Subject: Re: [PATCH] blkio-throttle: Avoid calling blkiocg_lookup_group() for root group References: <20110117215606.GI5624@redhat.com> X-PGP-KeyID: 1F78E1B4 X-PGP-CertKey: F6FE 280D 8293 F72C 65FD 5A58 1FF8 A7CA 1F78 E1B4 X-PCLoadLetter: What the f**k does that mean? Date: Mon, 17 Jan 2011 17:00:27 -0500 In-Reply-To: <20110117215606.GI5624@redhat.com> (Vivek Goyal's message of "Mon, 17 Jan 2011 16:56:06 -0500") Message-ID: User-Agent: Gnus/5.110011 (No Gnus v0.11) Emacs/23.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Vivek Goyal writes: > o Jeff Moyer was doing some testing on a RAM backed disk and > blkiocg_lookup_group() showed up high overhead after memcpy(). Similarly > somebody else reported that blkiocg_lookup_group() is eating 6% extra > cpu. Though looking at the code I can't think why the overhead of > this function is so high. One thing is that it is called with very high > frequency (once for every IO). > > o For lot of folks blkio controller will be compiled in but they might > not have actually created cgroups. Hence optimize the case of root > cgroup where we can avoid calling blkiocg_lookup_group() if IO is happening > in root group (common case). > > Reported-by: Jeff Moyer > Signed-off-by: Vivek Goyal Acked-by: Jeff Moyer In case it wasn't obvious, this patch was tested and confirmed to fix the performance degradation. Cheers, Jeff > --- > block/blk-throttle.c | 10 +++++++++- > 1 file changed, 9 insertions(+), 1 deletion(-) > > Index: linux-2.6/block/blk-throttle.c > =================================================================== > --- linux-2.6.orig/block/blk-throttle.c 2011-01-17 16:23:37.041280712 -0500 > +++ linux-2.6/block/blk-throttle.c 2011-01-17 16:36:09.362940975 -0500 > @@ -168,7 +168,15 @@ static struct throtl_grp * throtl_find_a > * tree of blkg (instead of traversing through hash list all > * the time. > */ > - tg = tg_of_blkg(blkiocg_lookup_group(blkcg, key)); > + > + /* > + * This is the common case when there are no blkio cgroups. > + * Avoid lookup in this case > + */ > + if (blkcg == &blkio_root_cgroup) > + tg = &td->root_tg; > + else > + tg = tg_of_blkg(blkiocg_lookup_group(blkcg, key)); > > /* Fill in device details for root group */ > if (tg && !tg->blkg.dev && bdi->dev && dev_name(bdi->dev)) {