[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] Enabling vm_event for a guest with more VCPUs than available ring buffer slots freezes the virtual machine
On 02/07/2017 10:20 PM, Tamas K Lengyel wrote: > > > On Tue, Feb 7, 2017 at 11:57 AM, Razvan Cojocaru > <rcojocaru@xxxxxxxxxxxxxxx <mailto:rcojocaru@xxxxxxxxxxxxxxx>> 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 > >>> <rcojocaru@xxxxxxxxxxxxxxx <mailto:rcojocaru@xxxxxxxxxxxxxxx> > <mailto:rcojocaru@xxxxxxxxxxxxxxx > <mailto:rcojocaru@xxxxxxxxxxxxxxx>>> 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( ¤t->vm_event_pause_count ) ) + vm_event_mark_and_pause(current, ved); + } Thanks, Razvan _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx https://lists.xen.org/xen-devel
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