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 62373C4332F for ; Thu, 29 Dec 2022 08:11:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232981AbiL2ILl (ORCPT ); Thu, 29 Dec 2022 03:11:41 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37026 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229747AbiL2ILh (ORCPT ); Thu, 29 Dec 2022 03:11:37 -0500 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DD74FB48F for ; Thu, 29 Dec 2022 00:10:46 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1672301446; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=gI6AlEc7KfZ1avo1jwJKrSQequUz5Q+UUl+tM82CWOM=; b=AV9zpFHRCl1/eZolQsiw8myZfyda+u6h+arE3PQfj+6tYKyuR45RnctNO4tHZfMNsKtyoD 4NB++hTajBIfGANUkw/ie9x/tM9dpu+A0LIvZTuGL5HlGOodubr6pI2uJSp/jkNiiZa+Vt oBg5sFrfG4A7+yNZSaQzJXC/0oaGuH8= Received: from mail-wm1-f70.google.com (mail-wm1-f70.google.com [209.85.128.70]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_128_GCM_SHA256) id us-mta-450-QGuZlwxoPcSfIULsVsuHRw-1; Thu, 29 Dec 2022 03:10:44 -0500 X-MC-Unique: QGuZlwxoPcSfIULsVsuHRw-1 Received: by mail-wm1-f70.google.com with SMTP id k20-20020a05600c1c9400b003d9717c8b11so7424135wms.7 for ; Thu, 29 Dec 2022 00:10:43 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=in-reply-to:content-transfer-encoding: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=gI6AlEc7KfZ1avo1jwJKrSQequUz5Q+UUl+tM82CWOM=; b=3I9724fSAZZNZcUOJaSBoe6d4vra9PQ6jA3da24NkZ3IBDYqR8F/ELAOfkzy/Z5ccp Wey+L2xWqB27fG2lqrG2+y2+netNhoVeZawvZ7dw4ExL/W/qniLbZGiaR8Fn83f1VTMb flxCtAMC18yfure67kZmcGk5yPivViLzmD/4hG5u+OdPoFzX7/YvwDVNR7vevV5nSTwx uEh+dhsdB8zP4rV4vmV0GpvqMuWBKimkbXbZEIUUKlwhnL3jFvDVX6aNTL92IjmUwytH xVRb9YBzvdOA6je5FiWpMKaoEIVruRrB1FwnV6x9fjmwwtpog8RrRKxWlliox45QGyW5 fbYA== X-Gm-Message-State: AFqh2ko1XEbdiYjReYyGqtt1PrkV+NI1SVjfrUUrbC0ShAQBdEbZE9dU ll/KHHd+3KIcFr+xANtHWf9NL6VnygvrlMF7DsnLjj00VYf3jNCL986QrBpgGzrTAs6j2BzUV4N aqYWwMSpcN/EYhxST+x/45yMt X-Received: by 2002:adf:ce0a:0:b0:246:e6df:86e7 with SMTP id p10-20020adfce0a000000b00246e6df86e7mr17498204wrn.5.1672301442825; Thu, 29 Dec 2022 00:10:42 -0800 (PST) X-Google-Smtp-Source: AMrXdXtCwrO0CDFtRs3sThb9MNmLR5zMGV1PR7/QCvy+zp7ENhYRIgm6RfCU8IAhgG7fUSbme3pHsQ== X-Received: by 2002:adf:ce0a:0:b0:246:e6df:86e7 with SMTP id p10-20020adfce0a000000b00246e6df86e7mr17498181wrn.5.1672301442504; Thu, 29 Dec 2022 00:10:42 -0800 (PST) Received: from redhat.com ([2.52.151.85]) by smtp.gmail.com with ESMTPSA id a6-20020adff7c6000000b002421db5f279sm17347928wrq.78.2022.12.29.00.10.40 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 29 Dec 2022 00:10:41 -0800 (PST) Date: Thu, 29 Dec 2022 03:10:38 -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: <20221229030633-mutt-send-email-mst@kernel.org> References: <20221226074908.8154-4-jasowang@redhat.com> <20221226183705-mutt-send-email-mst@kernel.org> <20221227022255-mutt-send-email-mst@kernel.org> <20221227043148-mutt-send-email-mst@kernel.org> <0d9f1b89-9374-747b-3fb0-b4b28ad0ace1@redhat.com> <20221229020553-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 29, 2022 at 04:04:13PM +0800, Jason Wang wrote: > On Thu, Dec 29, 2022 at 3:07 PM Michael S. Tsirkin wrote: > > > > On Wed, Dec 28, 2022 at 07:53:08PM +0800, Jason Wang wrote: > > > On Wed, Dec 28, 2022 at 2:34 PM Jason Wang wrote: > > > > > > > > > > > > 在 2022/12/27 17:38, Michael S. Tsirkin 写道: > > > > > On Tue, Dec 27, 2022 at 05:12:58PM +0800, Jason Wang wrote: > > > > >> 在 2022/12/27 15:33, Michael S. Tsirkin 写道: > > > > >>> 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. > > > > >> > > > > >> Yes, this would be more obvious is UP is used. > > > > >> > > > > >> > > > > >>> 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. > > > > >> > > > > >> That's fine, will consider this. > > > > > > So after some investigation, it seems the watchdog.c doesn't help. The > > > only export helper is touch_softlockup_watchdog() which tries to avoid > > > triggering the lockups warning for the known slow path. > > > > I never said you can just use existing exporting APIs. You'll have to > > write new ones :) > > Ok, I thought you wanted to trigger similar warnings as a watchdog. > > Btw, I wonder what kind of logic you want here. If we switch to using > sleep, there won't be soft lockup anymore. A simple wait + timeout + > warning seems sufficient? > > Thanks I'd like to avoid need to teach users new APIs. So watchdog setup to apply to this driver. The warning can be different. > > > > > And before the patch, we end up with a real infinite loop which could > > > be caught by RCU stall detector which is not the case of the sleep. > > > What we can do is probably do a periodic netdev_err(). > > > > > > Thanks > > > > Only with a bad device. > > > > > > >> > > > > >> > > > > >>> 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? > > > > >> > > > > >> Then we go back to the question of choosing a good timeout for poll. And > > > > >> poll seems problematic in the case of UP, scheduler might not have the > > > > >> chance to run. > > > > > Poll just a bit :) Seriously I don't know, but at least check once > > > > > after kick. > > > > > > > > > > > > I think it is what the current code did where the condition will be > > > > check before trying to sleep in the wait_event(). > > > > > > > > > > > > > > > > > >>> 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. > > > > >> > > > > >> The current code did: > > > > >> > > > > >> 1) check for vq->broken > > > > >> 2) wakeup during BAD_RING() > > > > >> > > > > >> So we won't end up with a never woke up process which should be fine. > > > > >> > > > > >> Thanks > > > > > > > > > > BTW BAD_RING on removal will trigger dev_err. Not sure that is a good > > > > > idea - can cause crashes if kernel panics on error. > > > > > > > > > > > > Yes, it's better to use __virtqueue_break() instead. > > > > > > > > But consider we will start from a wait first, I will limit the changes > > > > in virtio-net without bothering virtio core. > > > > > > > > Thanks > > > > > > > > > > > > > > > > > >>> > >