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Date: Wed, 28 Dec 2022 14:34:20 +0800 From: Jason Wang <jasowang@...hat.com> To: "Michael S. Tsirkin" <mst@...hat.com> Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com, virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, maxime.coquelin@...hat.com, alvaro.karsz@...id-run.com, eperezma@...hat.com Subject: Re: [PATCH 3/4] virtio_ring: introduce a per virtqueue waitqueue 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. >> >> >>> 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 > >>>
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