Cornelia Huck <cornelia.huck@xxxxxxxxxx> writes: > On Wed, 4 Mar 2015 11:25:56 +0100 > "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > >> On Wed, Mar 04, 2015 at 04:44:54PM +1030, Rusty Russell wrote: >> > "Michael S. Tsirkin" <mst@xxxxxxxxxx> writes: >> > > On Mon, Mar 02, 2015 at 10:37:26AM +1030, Rusty Russell wrote: >> > >> Thomas Huth <thuth@xxxxxxxxxxxxxxxxxx> writes: >> > >> > On Thu, 26 Feb 2015 11:50:42 +1030 >> > >> > Rusty Russell <rusty@xxxxxxxxxxxxxxx> wrote: >> > >> > >> > >> >> Thomas Huth <thuth@xxxxxxxxxxxxxxxxxx> writes: >> > >> >> > Hi all, >> > >> >> > >> > >> >> > with the recent kernel 3.19, I get a kernel warning when I start my >> > >> >> > KVM guest on s390 with virtio balloon enabled: >> > >> >> >> > >> >> The deeper problem is that virtio_ccw_get_config just silently fails on >> > >> >> OOM. >> > >> >> >> > >> >> Neither get_config nor set_config are expected to fail. >> > >> > >> > >> > AFAIK this is currently not a problem. According to >> > >> > http://lwn.net/Articles/627419/ these kmalloc calls never >> > >> > fail because they allocate less than a page. >> > >> >> > >> I strongly suggest you unlearn that fact. >> > >> The fix for this is in two parts: >> > >> >> > >> 1) Annotate using sched_annotate_sleep() and add a comment: we may spin >> > >> a few times in low memory situations, but this isn't a high >> > >> performance path. >> > >> >> > >> 2) Handle get_config (and other) failure in some more elegant way. >> > >> >> > >> Cheers, >> > >> Rusty. >> > > >> > > I agree, but I'd like to point out that even without kmalloc, >> > > on s390 get_config is blocking - it's waiting >> > > for a hardware interrupt. >> > > >> > > And it makes sense: config is not data path, I don't think >> > > we should spin there. >> > > >> > > So I think besides these two parts, we still need my two patches: >> > > virtio-balloon: do not call blocking ops when !TASK_RUNNING >> > >> > I prefer to annotate, over trying to fix this. >> > >> > Because it's not important. We might spin a few times, but it's very >> > unlikely, and it's certainly not performance critical. >> > >> > Thanks, >> > Rusty. >> > >> > Subject: virtio_balloon: annotate possible sleep waiting for event. >> > >> > CCW (s390) does this. >> > >> > Reported-by: Thomas Huth <thuth@xxxxxxxxxxxxxxxxxx> >> > Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx> >> > >> > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c >> > index 0413157f3b49..3f4d5acdbde0 100644 >> > --- a/drivers/virtio/virtio_balloon.c >> > +++ b/drivers/virtio/virtio_balloon.c >> > @@ -340,6 +340,15 @@ static int balloon(void *_vballoon) >> > s64 diff; >> > >> > try_to_freeze(); >> > + >> > + /* >> > + * Reading the config on the ccw backend involves an >> > + * allocation, so we may actually sleep and have an >> > + * extra iteration. It's extremely unlikely, >> >> Hmm, this part of the comment seems wrong to me. >> Reading the config on the ccw backend always sleeps >> because it's interrupt driven. > > (...) > >> So I suspect >> http://mid.gmane.org/1424874878-17155-1-git-send-email-mst@xxxxxxxxxx >> is better. >> >> What do you think? > > I'd prefer to fix this as well. While the I/O request completes > instantly on current qemu (the ssch backend handles the start function > immediately, not asynchronously as on real hardware), this (a) is an > implementation detail that may change and (b) doesn't account for the > need to deliver the interrupt to the guest - which might take non-zero > time. Ah, I see. My mistake. I've thrown out my patch, applied that one. Thanks, Rusty. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
