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Mon, 26 Dec 2022 23:33:48 -0800 (PST) X-Google-Smtp-Source: AMrXdXuclLqFnR9qkTo2hfrgyxoL/HUwscW2Ss3HBVsduIc3VD7zOi8RbL5ZWgzxX6AOL6LqRjw/Mg== X-Received: by 2002:a5d:6849:0:b0:280:70e1:c24f with SMTP id o9-20020a5d6849000000b0028070e1c24fmr2305128wrw.68.1672126427814; Mon, 26 Dec 2022 23:33:47 -0800 (PST) Received: from redhat.com ([2.52.151.85]) by smtp.gmail.com with ESMTPSA id c17-20020adffb51000000b0027cb20605e3sm5064232wrs.105.2022.12.26.23.33.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 26 Dec 2022 23:33:47 -0800 (PST) Date: Tue, 27 Dec 2022 02:33:43 -0500 From: "Michael S. Tsirkin" To: Jason Wang Subject: Re: [PATCH 3/4] virtio_ring: introduce a per virtqueue waitqueue Message-ID: <20221227022255-mutt-send-email-mst@kernel.org> References: <20221226074908.8154-1-jasowang@redhat.com> <20221226074908.8154-4-jasowang@redhat.com> <20221226183705-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Disposition: inline Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, eperezma@redhat.com, edumazet@google.com, maxime.coquelin@redhat.com, kuba@kernel.org, pabeni@redhat.com, davem@davemloft.net X-BeenThere: virtualization@lists.linux-foundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Linux virtualization List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: virtualization-bounces@lists.linux-foundation.org Sender: "Virtualization" On Tue, Dec 27, 2022 at 12:30:35PM +0800, Jason Wang wrote: > > But device is still going and will later use the buffers. > > > > Same for timeout really. > > Avoiding infinite wait/poll is one of the goals, another is to sleep. > If we think the timeout is hard, we can start from the wait. > > Thanks If the goal is to avoid disrupting traffic while CVQ is in use, that sounds more reasonable. E.g. someone is turning on promisc, a spike in CPU usage might be unwelcome. things we should be careful to address then: 1- debugging. Currently it's easy to see a warning if CPU is stuck in a loop for a while, and we also get a backtrace. E.g. with this - how do we know who has the RTNL? We need to integrate with kernel/watchdog.c for good results and to make sure policy is consistent. 2- overhead. In a very common scenario when device is in hypervisor, programming timers etc has a very high overhead, at bootup lots of CVQ commands are run and slowing boot down is not nice. let's poll for a bit before waiting? 3- suprise removal. need to wake up thread in some way. what about other cases of device breakage - is there a chance this introduces new bugs around that? at least enumerate them please. -- MST _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization 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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7C2BFC10F1B for ; Tue, 27 Dec 2022 07:34:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229961AbiL0Hen (ORCPT ); Tue, 27 Dec 2022 02:34:43 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46388 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229496AbiL0Heh (ORCPT ); Tue, 27 Dec 2022 02:34:37 -0500 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6B4CE10AF for ; Mon, 26 Dec 2022 23:33:51 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1672126430; 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=h2F2GH9r8G5/9JPGcKQm0Kg39TfbvH77Y/WP493sark=; b=JWY/+/+CRUIyTNRTfzoCCrxhJHYa/KCLdrUGXBXo8KpzUlGUL9AiC+VMe/HDudvpuu2QQS l9M/6hGF4qZaGDhrlJJIxH89WwIM8b/ZzeB0SLAbdD5YOaGE+Z3W7KehF4gn24BqNBM13q 6r5OnX63mXc/V3hkhBtyVBAqVDeUai8= Received: from mail-wm1-f71.google.com (mail-wm1-f71.google.com [209.85.128.71]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_128_GCM_SHA256) id us-mta-219-vu8JubGjMrOLtDsHMkpRXg-1; Tue, 27 Dec 2022 02:33:49 -0500 X-MC-Unique: vu8JubGjMrOLtDsHMkpRXg-1 Received: by mail-wm1-f71.google.com with SMTP id c66-20020a1c3545000000b003d355c13229so9137166wma.0 for ; Mon, 26 Dec 2022 23:33:48 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=h2F2GH9r8G5/9JPGcKQm0Kg39TfbvH77Y/WP493sark=; b=tXnVqquyjZYsmmdR22yPmk7F/FUY0DS99/5WBUrepTKTx/eeWG5vSifNhKVpeOdCNz ssQAa9JLSHiuEdJqsSWZp1NE60LiU1/RCWLpbc6utYeZ/zEGXDDmCcXNJoa0JSVOR6xC gxKRR2fSfO17bDv02jx4C5q+oDWKFCCGv5Z7qlZvwtg+ZaDp8ZGL07g4fB2uePJm8I2Y 8qF89qYrDVpYLbVJ+F89jacT11JCpyIkVgPYzsCdsTsLHOi8GbGXu+lqE4HjpaEENhnl zmNRlVf2Aww7hy+dyGQuwSW1B2OTeflVq4YhXLu6epTYqZbYOIFq30ywxqfUfcGv1/9p 2CDw== X-Gm-Message-State: AFqh2kqHOw6Zi05AM68T0wOHO+dDdmsgT07j/3hZS1LX+RqnfGUIWgjO rO+XLrGGcttEvCKhPSoBNtSj7Ji57HZHGCxtgTyDPL/TM1BJU8JySVHB3pDT7v93E8hhKhzQfNv WRYGUFOz5u1LNcijDmxjUHVBL X-Received: by 2002:a5d:6849:0:b0:280:70e1:c24f with SMTP id o9-20020a5d6849000000b0028070e1c24fmr2305147wrw.68.1672126428060; Mon, 26 Dec 2022 23:33:48 -0800 (PST) X-Google-Smtp-Source: AMrXdXuclLqFnR9qkTo2hfrgyxoL/HUwscW2Ss3HBVsduIc3VD7zOi8RbL5ZWgzxX6AOL6LqRjw/Mg== X-Received: by 2002:a5d:6849:0:b0:280:70e1:c24f with SMTP id o9-20020a5d6849000000b0028070e1c24fmr2305128wrw.68.1672126427814; Mon, 26 Dec 2022 23:33:47 -0800 (PST) Received: from redhat.com ([2.52.151.85]) by smtp.gmail.com with ESMTPSA id c17-20020adffb51000000b0027cb20605e3sm5064232wrs.105.2022.12.26.23.33.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 26 Dec 2022 23:33:47 -0800 (PST) Date: Tue, 27 Dec 2022 02:33:43 -0500 From: "Michael S. Tsirkin" To: Jason Wang Cc: davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, maxime.coquelin@redhat.com, alvaro.karsz@solid-run.com, eperezma@redhat.com Subject: Re: [PATCH 3/4] virtio_ring: introduce a per virtqueue waitqueue Message-ID: <20221227022255-mutt-send-email-mst@kernel.org> References: <20221226074908.8154-1-jasowang@redhat.com> <20221226074908.8154-4-jasowang@redhat.com> <20221226183705-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Dec 27, 2022 at 12:30:35PM +0800, Jason Wang wrote: > > But device is still going and will later use the buffers. > > > > Same for timeout really. > > Avoiding infinite wait/poll is one of the goals, another is to sleep. > If we think the timeout is hard, we can start from the wait. > > Thanks If the goal is to avoid disrupting traffic while CVQ is in use, that sounds more reasonable. E.g. someone is turning on promisc, a spike in CPU usage might be unwelcome. things we should be careful to address then: 1- debugging. Currently it's easy to see a warning if CPU is stuck in a loop for a while, and we also get a backtrace. E.g. with this - how do we know who has the RTNL? We need to integrate with kernel/watchdog.c for good results and to make sure policy is consistent. 2- overhead. In a very common scenario when device is in hypervisor, programming timers etc has a very high overhead, at bootup lots of CVQ commands are run and slowing boot down is not nice. let's poll for a bit before waiting? 3- suprise removal. need to wake up thread in some way. what about other cases of device breakage - is there a chance this introduces new bugs around that? at least enumerate them please. -- MST