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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 CC505C433E2 for ; Mon, 18 May 2020 10:43:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B625E207ED for ; Mon, 18 May 2020 10:43:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726180AbgERKnI (ORCPT ); Mon, 18 May 2020 06:43:08 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50492 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726127AbgERKnH (ORCPT ); Mon, 18 May 2020 06:43:07 -0400 Received: from Galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B0271C061A0C for ; Mon, 18 May 2020 03:43:07 -0700 (PDT) Received: from p5de0bf0b.dip0.t-ipconnect.de ([93.224.191.11] helo=nanos.tec.linutronix.de) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1jadEV-0007Op-JQ; Mon, 18 May 2020 12:42:55 +0200 Received: by nanos.tec.linutronix.de (Postfix, from userid 1000) id F1236100606; Mon, 18 May 2020 12:42:54 +0200 (CEST) From: Thomas Gleixner To: Ming Lei Cc: Christoph Hellwig , linux-block@vger.kernel.org, John Garry , Bart Van Assche , Hannes Reinecke Subject: Re: [PATCH 5/9] blk-mq: don't set data->ctx and data->hctx in blk_mq_alloc_request_hctx In-Reply-To: <20200518093155.GB35380@T590> Date: Mon, 18 May 2020 12:42:54 +0200 Message-ID: <87imgty15d.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org Ming Lei writes: > On Mon, May 18, 2020 at 10:32:22AM +0200, Thomas Gleixner wrote: >> Christoph Hellwig writes: >> Is this absolutely necessary to be a smp function call? That's going to > > I think it is. > > Request is bound to the allocation CPU and the hw queue(hctx) which is > mapped from the allocation CPU. > > If request is allocated from one cpu which is going to offline, we can't > handle that easily. That's a pretty handwavy explanation and does not give any reason why this needs to be a smp function call and cannot be solved otherwise, e.g. by delegating this to a work queue. >> be problematic vs. RT. Same applies to the explicit preempt_disable() in >> patch 7. > > I think it is true and the reason is same too, but the period is quite short, > and it is just taken for iterating several bitmaps for finding one free bit. And takes spinlocks along the way.... See: https://www.kernel.org/doc/html/latest/locking/locktypes.html for a full explanation why this can't work on RT. And that's the same reason why the smp function call will fall apart on a RT enabled kernel. Thanks, tglx