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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham 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 36D6AC43441 for ; Sat, 17 Nov 2018 02:34:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DC6422075B for ; Sat, 17 Nov 2018 02:34:29 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org DC6422075B Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-block-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729793AbeKQMt0 (ORCPT ); Sat, 17 Nov 2018 07:49:26 -0500 Received: from mx1.redhat.com ([209.132.183.28]:43246 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729714AbeKQMt0 (ORCPT ); Sat, 17 Nov 2018 07:49:26 -0500 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 33F123082AFB; Sat, 17 Nov 2018 02:34:29 +0000 (UTC) Received: from ming.t460p (ovpn-8-19.pek2.redhat.com [10.72.8.19]) by smtp.corp.redhat.com (Postfix) with ESMTPS id D10176051E; Sat, 17 Nov 2018 02:34:23 +0000 (UTC) Date: Sat, 17 Nov 2018 10:34:18 +0800 From: Ming Lei To: Greg Kroah-Hartman Cc: Jens Axboe , linux-block@vger.kernel.org, "jianchao.wang" , Guenter Roeck Subject: Re: [PATCH V2 for-4.21 2/2] blk-mq: alloc q->queue_ctx as normal array Message-ID: <20181117023417.GC8872@ming.t460p> References: <20181116112311.4117-1-ming.lei@redhat.com> <20181116112311.4117-3-ming.lei@redhat.com> <20181116140623.GC4595@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181116140623.GC4595@kroah.com> User-Agent: Mutt/1.9.1 (2017-09-22) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.45]); Sat, 17 Nov 2018 02:34:29 +0000 (UTC) Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Fri, Nov 16, 2018 at 06:06:23AM -0800, Greg Kroah-Hartman wrote: > On Fri, Nov 16, 2018 at 07:23:11PM +0800, Ming Lei wrote: > > Now q->queue_ctx is just one read-mostly table for query the > > 'blk_mq_ctx' instance from one cpu index, it isn't necessary > > to allocate it as percpu variable. One simple array may be > > more efficient. > > "may be", have you run benchmarks to be sure? If so, can you add the > results of them to this changelog? If there is no measurable > difference, then why make this change at all? __blk_mq_get_ctx() is used in fast path, what do you think about which one is more efficient? - *per_cpu_ptr(q->queue_ctx, cpu); - q->queue_ctx[cpu] At least the latter isn't worse than the former. Especially q->queue_ctx is just a read-only look-up table, it doesn't make sense to make it percpu any more. Not mention q->queue_ctx[cpu] is more clean/readable. Thanks, Ming