Xen project Mailing List

On Tue, Feb 7, 2017 at 3:09 PM, Mihai Donțu <mihai.dontu@xxxxxxxxx> wrote:

On Tue, 7 Feb 2017 22:41:57 +0200 Razvan Cojocaru wrote:
> On 02/07/2017 10:20 PM, Tamas K Lengyel wrote:
> > On Tue, Feb 7, 2017 at 11:57 AM, Razvan Cojocaru wrote:
> > On 02/07/2017 08:39 PM, Andrew Cooper wrote:
> > > On 07/02/17 18:31, Razvan Cojocaru wrote:
> > >> On 02/07/2017 08:15 PM, Tamas K Lengyel wrote:

> > >>> On Tue, Feb 7, 2017 at 9:53 AM, Razvan Cojocaru wrote:
> > >>> Hello,
> > >>>
> > >>> Setting, e.g. 16 VCPUs for a HVM guest, ends up blocking the
> > guest
> > >>> completely when subscribing to vm_events, apparently because
> > of this
> > >>> code in xen/common/vm_event.c:
> > >>>
> > >>> 315 /* Give this vCPU a black eye if necessary, on the
> > way out.
> > >>> 316 * See the comments above wake_blocked() for more
> > information
> > >>> 317 * on how this mechanism works to avoid waiting. */
> > >>> 318 avail_req = vm_event_ring_available(ved);
> > >>> 319 if( current->domain == d && avail_req < d->max_vcpus )
> > >>> 320 vm_event_mark_and_pause(current, ved);
> > >>>
> > >>> It would appear that even if the guest only has 2 online
> > VCPUs, the
> > >>> "avail_req < d->max_vcpus" condition will pause current, and we
> > >>> eventually end up with all the VCPUs paused.
> > >>>
> > >>> An ugly hack ("avail_req < 2") has allowed booting a guest
> > with many
> > >>> VCPUs (max_vcpus, the guest only brings 2 VCPUs online),
> > however that's
> > >>> just to prove that that was the culprit - a real solution to
> > this needs
> > >>> more in-depth understading of the issue and potential
> > solution. That's
> > >>> basically very old code (pre-2012 at least) that got moved
> > around into
> > >>> the current shape of Xen today - please CC anyone relevant
> > to the
> > >>> discussion that you're aware of.
> > >>>
> > >>> Thoughts?
> > >>>
> > >>>
> > >>> I think is a side-effect of the growth of the vm_event structure
> > and the
> > >>> fact that we have a single page ring. The check effectively sets a
> > >>> threshold of having enough space for each vCPU to place at least one
> > >>> more event on the ring, and if that's not the case it gets
> > paused. OTOH
> > >>> I think this would only have an effect on asynchronous events,
> > for all
> > >>> other events the vCPU is already paused. Is that the case you have?
> > >> No, on the contrary, all my events are synchronous (the VCPU is
> > paused
> > >> waiting for the vm_event reply).
> > >>
> > >> I've debugged this a bit, and the problem seems to be that
> > >> vm_event_wake_blocked() breaks here:
> > >>
> > >> 150 /* We remember which vcpu last woke up to avoid scanning
> > always
> > >> linearly
> > >> 151 * from zero and starving higher-numbered vcpus under
> > high load */
> > >> 152 if ( d->vcpu )
> > >> 153 {
> > >> 154 int i, j, k;
> > >> 155
> > >> 156 for (i = ved->last_vcpu_wake_up + 1, j = 0; j <
> > >> d->max_vcpus; i++, j++)
> > >> 157 {
> > >> 158 k = i % d->max_vcpus;
> > >> 159 v = d->vcpu[k];
> > >> 160 if ( !v )
> > >> 161 continue;
> > >> 162
> > >> 163 if ( !(ved->blocked) || online >= avail_req )
> > >> 164 break;
> > >> 165
> > >> 166 if ( test_and_clear_bit(ved->pause_flag,
> > &v->pause_flags) )
> > >> 167 {
> > >> 168 vcpu_unpause(v);
> > >> 169 online++;
> > >> 170 ved->blocked--;
> > >> 171 ved->last_vcpu_wake_up = k;
> > >> 172 }
> > >> 173 }
> > >> 174 }
> > >>
> > >> at "if ( !(ved->blocked) || online >= avail_req )". At this point,
> > >> nothing ever gets unblocked. It's hard to believe that this is
> > desired
> > >> behaviour, as I don't know what could possibly happen for that
> > condition
> > >> to become false once all the online VCPUs are stuck (especially
> > when the
> > >> guest has just started booting).
> >
> >
> > Ah I see what happens. During boot vCPU 0 generates an event and gets
> > marked blocked because the number of vCPUs is so high. The other vCPUs
> > are still unblocked since they are idle, but this test here will still
> > be true (online >= avail_req) and thus we can never unblock vCPU0. And
> > then the boot process is hanging because vCPU0 never resumes. I would
> > argue that this test should be changed to check that there is at least 1
> > spot on the ring and only break if that is not the case anymore (ie.
> > instead of incrementing online we should be decrementing avail_req).
>
> That is exactly what happens. And it can't really be fixed just by
> increasing the ring buffer (although that definitely helps a lot and
> would be a smart move): no matter how large it is, we can always ask the
> domain to use more VCPUs than there are slots in the buffer.
>
> > >
> > > I wouldn't bet that this logic has ever been tested. If you
> > recall, the
> > > addition of register state into the vm_event ring made each entry far
> > > larger, which in turns makes it more likely to hit this condition.
> > >
> > > However, simply fixing the logic to re-online the cpus isn't a good
> > > solution either, as having $N vcpus paused at any one time because of
> > > ring contention is not conducive good system performance.
> > >
> > > Realistically, the ring size needs to be max_cpus * sizeof(largest
> > > vm_event) at an absolute minimum, and I guess this is now beyond 1
> > page?
> >
> > Yes, of course the reason this triggers earlier now is the growth of the
> > request's size. Yes, using e.g. 20 VCPUs in the guest's setup will
> > exceed a page's number of slots.
> >
> > And yes, ideally we should have multi-page ring buffers - however that
> > is a long-term project that requires design changes in other parts of
> > Xen as well (Andrew, CCd here, was recently talking about one).
> >
> > However, even with a one-page ring buffer, surely it's not good to end
> > up in this situation, especially for guests such as mine, which never
> > actually bring more than 2 VCPUs online. But even if they were to use
> > more, blocking the guest on vm_event init is completely pointless - we
> > might as well return some kind of error if max_vcpus > available slots.
> >
> > I don't follow the system performance argument. Surely completely
> > blocking the guest is worse.
> >
> >
> > I also don't see the point in marking a vCPU blocked if it is already
> > paused. I think this behavior of blocking vCPUs makes only sense for
> > asynchronous events. Razvan, could you test what happens if
> > vm_event_mark_and_pause is only called if the vCPU is unpaused?
>
> It works for me with this change (using Xen 4.7 sources here):
>
> @@ -318,7 +329,11 @@ void vm_event_put_request(struct domain *d,
> * on how this mechanism works to avoid waiting. */
> avail_req = vm_event_ring_available(ved);
> if( current->domain == d && avail_req < d->max_vcpus )
> - vm_event_mark_and_pause(current, ved);
> + {
> + if ( !atomic_read( &current->vm_event_pause_count ) )
> + vm_event_mark_and_pause(current, ved);
> + }

If I'm reading the code correctly, when max_vcpus is greater than the
number of slots available in the ring, a race appears that can lead to
a ring corruption (in debug mode ASSERT(free_req > 0) will trigger).

For example, when a single slot is available, two vCPUs can race to
vm_event_put_request() after both being given a green light in
__vm_event_claim_slot(), whose return depends only on
vm_event_ring_available() returning non-zero (which it can do, for both
vCPUs at the same time).

As it turns out, the bug being talked about prevented this from showing
up.

PS: https://xenbits.xen.org/gitweb/?p=xen.git;a=commit;h=3643a961195f76ba849a213628c1979240e6fbdd

I was also looking at this but that's not the case. The function __vm_event_claim_slot will eventually trickle down to vm_event_grab_slot, which does have a lock. While holding the lock it calls vm_event_ring_available which returns how many free spots are on the ring (number of items the ring can hold - ved->target_producers - ved->foreign_producers). So if it returns 1 it will be grabbed, and either ved->target_producers or ved->foreign_producers gets incremented, and only then we unlock. The next caller to vm_event_grab_slot will not find a free spot. If that happens the caller to __vm_event_claim_slot will either gets put on the waitqueue (if sleeping_allowed is set) or returns -EBUSY (if losing the event is permissible).

Tamas

Re: [Xen-devel] Enabling vm_event for a guest with more VCPUs than available ring buffer slots freezes the virtual machine