On Wed, Dec 28, 2022 at 2:34 PM Jason Wang <jasowang@xxxxxxxxxx> 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. 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 > >> > >> > >>> 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 > > > > > >>> _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
