From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-15.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E311CC433DB for ; Tue, 2 Feb 2021 12:29:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AD31764F05 for ; Tue, 2 Feb 2021 12:29:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231402AbhBBM3W (ORCPT ); Tue, 2 Feb 2021 07:29:22 -0500 Received: from mx2.suse.de ([195.135.220.15]:58126 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231366AbhBBM3T (ORCPT ); Tue, 2 Feb 2021 07:29:19 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id C65A4AC45; Tue, 2 Feb 2021 12:28:37 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 2E3A41E14C3; Tue, 2 Feb 2021 13:28:36 +0100 (CET) Date: Tue, 2 Feb 2021 13:28:36 +0100 From: Jan Kara To: Lin Feng Cc: axboe@kernel.dk, paolo.valente@linaro.org, jack@suse.cz, linux-kernel@vger.kernel.org, linux-block@vger.kernel.org Subject: Re: [PATCH] Revert "bfq: Fix computation of shallow depth" Message-ID: <20210202122836.GC17147@quack2.suse.cz> References: <20210129111808.45796-1-linf@wangsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210129111808.45796-1-linf@wangsu.com> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org Hello! On Fri 29-01-21 19:18:08, Lin Feng wrote: > This reverts commit 6d4d273588378c65915acaf7b2ee74e9dd9c130a. > > bfq.limit_depth passes word_depths[] as shallow_depth down to sbitmap core > sbitmap_get_shallow, which uses just the number to limit the scan depth of > each bitmap word, formula: > scan_percentage_for_each_word = shallow_depth / (1 << sbimap->shift) * 100% Looking at sbitmap_get_shallow() again more carefully, I agree that I misunderstood how shallow_depth argument gets used and the original code was correct and I broke it. Thanks for spotting this! What I didn't notice is that shallow_depth indeed gets used for each bitmap word separately and not for bitmap as a whole. I'd say this could use some more documentation but that's unrelated to your revert. So feel free to add: Reviewed-by: Jan Kara to your patch. Thanks. Honza > > That means the comments's percentiles 50%, 75%, 18%, 37% of bfq are correct. > But after commit patch 'bfq: Fix computation of shallow depth', we use > sbitmap.depth instead, as a example in following case: > > sbitmap.depth = 256, map_nr = 4, shift = 6; sbitmap_word.depth = 64. > The resulsts of computed bfqd->word_depths[] are {128, 192, 48, 96}, and > three of the numbers exceed core dirver's 'sbitmap_word.depth=64' limit > nothing. Do we really don't want limit depth for such workloads, or we > just want to bump up the percentiles to 100%? > > Please correct me if I miss something, thanks. > > Signed-off-by: Lin Feng > --- > block/bfq-iosched.c | 8 ++++---- > 1 file changed, 4 insertions(+), 4 deletions(-) > > diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c > index 9e4eb0fc1c16..9e81d1052091 100644 > --- a/block/bfq-iosched.c > +++ b/block/bfq-iosched.c > @@ -6332,13 +6332,13 @@ static unsigned int bfq_update_depths(struct bfq_data *bfqd, > * limit 'something'. > */ > /* no more than 50% of tags for async I/O */ > - bfqd->word_depths[0][0] = max(bt->sb.depth >> 1, 1U); > + bfqd->word_depths[0][0] = max((1U << bt->sb.shift) >> 1, 1U); > /* > * no more than 75% of tags for sync writes (25% extra tags > * w.r.t. async I/O, to prevent async I/O from starving sync > * writes) > */ > - bfqd->word_depths[0][1] = max((bt->sb.depth * 3) >> 2, 1U); > + bfqd->word_depths[0][1] = max(((1U << bt->sb.shift) * 3) >> 2, 1U); > > /* > * In-word depths in case some bfq_queue is being weight- > @@ -6348,9 +6348,9 @@ static unsigned int bfq_update_depths(struct bfq_data *bfqd, > * shortage. > */ > /* no more than ~18% of tags for async I/O */ > - bfqd->word_depths[1][0] = max((bt->sb.depth * 3) >> 4, 1U); > + bfqd->word_depths[1][0] = max(((1U << bt->sb.shift) * 3) >> 4, 1U); > /* no more than ~37% of tags for sync writes (~20% extra tags) */ > - bfqd->word_depths[1][1] = max((bt->sb.depth * 6) >> 4, 1U); > + bfqd->word_depths[1][1] = max(((1U << bt->sb.shift) * 6) >> 4, 1U); > > for (i = 0; i < 2; i++) > for (j = 0; j < 2; j++) > -- > 2.25.4 > -- Jan Kara SUSE Labs, CR