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=-3.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,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 A5391C43441 for ; Sat, 17 Nov 2018 10:03:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4A3A720817 for ; Sat, 17 Nov 2018 10:03:47 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="GpMWYXHQ" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4A3A720817 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=linuxfoundation.org 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 S1725927AbeKQUTz (ORCPT ); Sat, 17 Nov 2018 15:19:55 -0500 Received: from mail.kernel.org ([198.145.29.99]:48356 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725854AbeKQUTz (ORCPT ); Sat, 17 Nov 2018 15:19:55 -0500 Received: from localhost (5356596B.cm-6-7b.dynamic.ziggo.nl [83.86.89.107]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 2880A206BB; Sat, 17 Nov 2018 10:03:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1542449025; bh=H80ELgAmb0oFAbVUpYbGzFXpxU0+0qqQfrigZJCPJnY=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=GpMWYXHQgJu/0sKC9d6CuLz/3umTStUgmqrA6crdjvMwpeWe3qYBMFBPtnt9g6mNb gSWW2aWdva79DIGhycVwWGZVlbr0iKD7CyespPGmdlB9fHoeyr7GrYmnBoHsjtz7+f 19uJFyrwK7B0Zk6VinY7ADlcBvK5nyAostuf/dDY= Date: Sat, 17 Nov 2018 11:03:42 +0100 From: Greg Kroah-Hartman To: Ming Lei 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: <20181117100342.GB1482@kroah.com> References: <20181116112311.4117-1-ming.lei@redhat.com> <20181116112311.4117-3-ming.lei@redhat.com> <20181116140623.GC4595@kroah.com> <20181117023417.GC8872@ming.t460p> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181117023417.GC8872@ming.t460p> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Sat, Nov 17, 2018 at 10:34:18AM +0800, Ming Lei wrote: > 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] You need to actually test to see which one is faster, you might be surprised :) In other words, do not just guess. > At least the latter isn't worse than the former. How do you know? > 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. Again, please test to verify this. thanks, greg k-h