From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752849Ab1AQV4M (ORCPT ); Mon, 17 Jan 2011 16:56:12 -0500 Received: from mx1.redhat.com ([209.132.183.28]:3565 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752724Ab1AQV4J (ORCPT ); Mon, 17 Jan 2011 16:56:09 -0500 Date: Mon, 17 Jan 2011 16:56:06 -0500 From: Vivek Goyal To: linux kernel mailing list , Jens Axboe Cc: Moyer Jeff Moyer Subject: [PATCH] blkio-throttle: Avoid calling blkiocg_lookup_group() for root group Message-ID: <20110117215606.GI5624@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- 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)) {