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=-0.9 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 908BCC433E0 for ; Mon, 18 May 2020 11:55:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 69A432065F for ; Mon, 18 May 2020 11:55:10 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Bab6Uq15" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726557AbgERLzJ (ORCPT ); Mon, 18 May 2020 07:55:09 -0400 Received: from us-smtp-2.mimecast.com ([207.211.31.81]:58286 "EHLO us-smtp-delivery-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726413AbgERLzJ (ORCPT ); Mon, 18 May 2020 07:55:09 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1589802908; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=/bYPv19dlypm02w8pxLkkDOLuFfUFPx0vbP628J02dc=; b=Bab6Uq15cLAa/mGpCvz6oGs55jrqNpD82JjGkyK67hV7Cz924PrNJyxJnBnWOtaFm3CYBs zCHrqgxPmrx5HmkGp2e1SKXDb/A8PjuzbJWI6Tt7c48bzWmy6go40v1FLX0jZ0oAEEgmKc ZUPN2maIKcaFKtBc+HgLlG9Jrl2/MH8= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-119-CMlDpOC-N6eeozz-YPh-nw-1; Mon, 18 May 2020 07:55:06 -0400 X-MC-Unique: CMlDpOC-N6eeozz-YPh-nw-1 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 mimecast-mx01.redhat.com (Postfix) with ESMTPS id 1FD49872FE0; Mon, 18 May 2020 11:55:05 +0000 (UTC) Received: from T590 (ovpn-13-68.pek2.redhat.com [10.72.13.68]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 42B5178B5E; Mon, 18 May 2020 11:54:58 +0000 (UTC) Date: Mon, 18 May 2020 19:54:54 +0800 From: Ming Lei To: Thomas Gleixner 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 Message-ID: <20200518115454.GA46364@T590> References: <20200518093155.GB35380@T590> <87imgty15d.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87imgty15d.fsf@nanos.tec.linutronix.de> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Mon, May 18, 2020 at 12:42:54PM +0200, Thomas Gleixner wrote: > 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. I guess I misunderstood your point, sorry for that. The requirement is just that the request needs to be allocated on one online CPU after INACTIVE is set, and we can use a workqueue to do that. > > >> 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. We do want to avoid the cost of any lock, because it is in the fast IO path. Looks preempt_disable in patch 7 can't be avoided. Thanks, Ming