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Tsirkin" X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com 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="utf-8" Content-Transfer-Encoding: base64 Errors-To: virtualization-bounces@lists.linux-foundation.org Sender: "Virtualization" T24gTW9uLCBKYW4gMzAsIDIwMjMgYXQgMTo0MyBQTSBNaWNoYWVsIFMuIFRzaXJraW4gPG1zdEBy ZWRoYXQuY29tPiB3cm90ZToKPgo+IE9uIE1vbiwgSmFuIDMwLCAyMDIzIGF0IDEwOjUzOjU0QU0g KzA4MDAsIEphc29uIFdhbmcgd3JvdGU6Cj4gPiBPbiBTdW4sIEphbiAyOSwgMjAyMyBhdCAzOjMw IFBNIE1pY2hhZWwgUy4gVHNpcmtpbiA8bXN0QHJlZGhhdC5jb20+IHdyb3RlOgo+ID4gPgo+ID4g 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SMTP id k30-20020a056870959e00b001639ceaeea7mr296168oao.35.1675064675626; Sun, 29 Jan 2023 23:44:35 -0800 (PST) X-Google-Smtp-Source: AK7set+aQPAKyAPfHNJ0Xk76LMscbAi71N7VBp8BSAhSQ5QVi89w2CuqgyJztGEUVEEMiWNskmo6eTc++9HGfQU0UPw= X-Received: by 2002:a05:6870:959e:b0:163:9cea:eea7 with SMTP id k30-20020a056870959e00b001639ceaeea7mr296160oao.35.1675064675326; Sun, 29 Jan 2023 23:44:35 -0800 (PST) MIME-Version: 1.0 References: <0d9f1b89-9374-747b-3fb0-b4b28ad0ace1@redhat.com> <20221229020553-mutt-send-email-mst@kernel.org> <20221229030633-mutt-send-email-mst@kernel.org> <20230127053112-mutt-send-email-mst@kernel.org> <20230129022809-mutt-send-email-mst@kernel.org> <20230130003334-mutt-send-email-mst@kernel.org> In-Reply-To: <20230130003334-mutt-send-email-mst@kernel.org> From: Jason Wang Date: Mon, 30 Jan 2023 15:44:24 +0800 Message-ID: Subject: Re: [PATCH 3/4] virtio_ring: introduce a per virtqueue waitqueue To: "Michael S. Tsirkin" 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 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 30, 2023 at 1:43 PM Michael S. Tsirkin wrote: > > On Mon, Jan 30, 2023 at 10:53:54AM +0800, Jason Wang wrote: > > On Sun, Jan 29, 2023 at 3:30 PM Michael S. Tsirkin wro= te: > > > > > > On Sun, Jan 29, 2023 at 01:48:49PM +0800, Jason Wang wrote: > > > > On Fri, Jan 27, 2023 at 6:35 PM Michael S. Tsirkin = wrote: > > > > > > > > > > On Fri, Dec 30, 2022 at 11:43:08AM +0800, Jason Wang wrote: > > > > > > On Thu, Dec 29, 2022 at 4:10 PM Michael S. Tsirkin wrote: > > > > > > > > > > > > > > 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 wrot= e: > > > > > > > > > > On Wed, Dec 28, 2022 at 2:34 PM Jason Wang wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > =E5=9C=A8 2022/12/27 17:38, Michael S. Tsirkin =E5=86= =99=E9=81=93: > > > > > > > > > > > > On Tue, Dec 27, 2022 at 05:12:58PM +0800, Jason Wan= g wrote: > > > > > > > > > > > >> =E5=9C=A8 2022/12/27 15:33, Michael S. Tsirkin =E5= =86=99=E9=81=93: > > > > > > > > > > > >>> On Tue, Dec 27, 2022 at 12:30:35PM +0800, Jason W= ang wrote: > > > > > > > > > > > >>>>> But device is still going and will later use th= e 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 fr= om the wait. > > > > > > > > > > > >>>> > > > > > > > > > > > >>>> Thanks > > > > > > > > > > > >>> If the goal is to avoid disrupting traffic while = CVQ is in use, > > > > > > > > > > > >>> that sounds more reasonable. E.g. someone is turn= ing 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 warnin= g if CPU is stuck > > > > > > > > > > > >>> in a loop for a while, and we also get a bac= ktrace. > > > > > > > > > > > >>> 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 do= esn'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. Yo= u'll have to > > > > > > > > > write new ones :) > > > > > > > > > > > > > > > > Ok, I thought you wanted to trigger similar warnings as a w= atchdog. > > > > > > > > > > > > > > > > Btw, I wonder what kind of logic you want here. If we switc= h 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 s= etup to apply > > > > > > > to this driver. The warning can be different. > > > > > > > > > > > > Right, so it looks to me the only possible setup is the > > > > > > watchdog_thres. I plan to trigger the warning every watchdog_th= res * 2 > > > > > > second (as softlockup did). > > > > > > > > > > > > And I think it would still make sense to fail, we can start wit= h a > > > > > > very long timeout like 1 minutes and break the device. Does thi= s make > > > > > > sense? > > > > > > > > > > > > Thanks > > > > > > > > > > I'd say we need to make this manageable then. > > > > > > > > Did you mean something like sysfs or module parameters? > > > > > > No I'd say pass it with an ioctl. > > > > > > > > Can't we do it normally > > > > > e.g. react to an interrupt to return to userspace? > > > > > > > > I didn't get the meaning of this. Sorry. > > > > > > > > Thanks > > > > > > Standard way to handle things that can timeout and where userspace > > > did not supply the time is to block until an interrupt > > > then return EINTR. > > > > Well this seems to be a huge change, ioctl(2) doesn't say it can > > return EINTR now. > > the one on fedora 37 does not but it says: > No single standard. Arguments, returns, and semantics of ioctl() = vary according to the device driver in question (the call is > used as a catch-all for operations that don't cleanly fit the UNIX= stream I/O model). > > so it depends on the device e.g. for a streams device it does: > https://pubs.opengroup.org/onlinepubs/9699919799/functions/ioctl.html > has EINTR. Ok, I saw signal(7) also mention about EINTR for ioctl(2): """ If a blocked call to one of the following interfaces is interrupted by a signal handler, then the call is automatically restarted after the signal handler re=E2=80=90 turns if the SA_RESTART flag was used; otherwise the call fails with the error EINTR: * read(2), readv(2), write(2), writev(2), and ioctl(2) calls on "slow" devices. A "slow" device is one where the I/O call may block for an indefinite time, for example, a terminal, pipe, or socket. If an I/O call on a slow device has already transferred some data by the time it is interrupted by a signal handler, then the call will return a success status (normally, the number of bytes transferred). Note that a (local) disk is not a slow device according to this defi=E2=80=90 nition; I/O operations on disk devices are not interrupted by sign= als. """ > > > > > Actually, a driver timeout is used by other drivers when using > > controlq/adminq (e.g i40e). Starting from a sane value (e.g 1 minutes > > to avoid false negatives) seems to be a good first step. > > Well because it's specific hardware so timeout matches what it can > promise. virtio spec does not give guarantees. One issue is with > software implementations. At the moment I can set a breakpoint in qemu > or vhost user backend and nothing bad happens in just continues. Yes but it should be no difference from using a kgdb to debug i40e drivers. > > > > > Userspace controls the timeout by > > > using e.g. alarm(2). > > > > Not used in iproute2 after a git grep. > > > > Thanks > > No need for iproute2 to do it user can just do it from shell. Or user can= just press CTRL-C. Yes, but iproute2 needs to deal with EINTR, that is the challenge part, if we simply return an error, the next cvq command might get confused. Thanks > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > And before the patch, we end up with a real infinite lo= op which could > > > > > > > > > > be caught by RCU stall detector which is not the case o= f 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 devic= e is in hypervisor, > > > > > > > > > > > >>> programming timers etc has a very high overh= ead, at bootup > > > > > > > > > > > >>> lots of CVQ commands are run and slowing boo= t 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, schedule= r might not have the > > > > > > > > > > > >> chance to run. > > > > > > > > > > > > Poll just a bit :) Seriously I don't know, but at l= east check once > > > > > > > > > > > > after kick. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > I think it is what the current code did where the con= dition will be > > > > > > > > > > > check before trying to sleep in the wait_event(). > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > >>> 3- suprise removal. need to wake up thread in som= e way. what about > > > > > > > > > > > >>> other cases of device breakage - is there a = chance this > > > > > > > > > > > >>> introduces new bugs around that? at least en= umerate 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 wh= ich should be fine. > > > > > > > > > > > >> > > > > > > > > > > > >> Thanks > > > > > > > > > > > > > > > > > > > > > > > > BTW BAD_RING on removal will trigger dev_err. Not s= ure 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 > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > >>> > > > > > > > > > > > > > > > > > > > > > > > > >