On 04/10/2018 11:33 AM, Dario Faggioli wrote: > On Tue, 2018-04-10 at 09:34 +0000, George Dunlap wrote: >> Assuming the bug is this one: >> >> BUG_ON( cpu != snext->vcpu->processor ); >> > Yes, it is that one. > > Another stack trace, this time from a debug=y built hypervisor, of what > we are thinking it is the same bug (although reproduced in a slightly > different way) is this: > > (XEN) ----[ Xen-4.7.2_02-36.1.12847.11.PTF x86_64 debug=y Not tainted ]---- > (XEN) CPU: 45 > (XEN) RIP: e008:[<ffff82d08012508f>] > sched_credit.c#csched_schedule+0x361/0xaa9 > ... > (XEN) Xen call trace: > (XEN) [<ffff82d08012508f>] sched_credit.c#csched_schedule+0x361/0xaa9 > (XEN) [<ffff82d08012c233>] schedule.c#schedule+0x109/0x5d6 > (XEN) [<ffff82d08012fb5f>] softirq.c#__do_softirq+0x7f/0x8a > (XEN) [<ffff82d08012fbb4>] do_softirq+0x13/0x15 > (XEN) [<ffff82d0801fd5c5>] vmx_asm_do_vmentry+0x25/0x2a > > (I can provide it all, if necessary.) > > I've done some analysis, although when we still were not entirely sure > that changing the affinities was the actual cause (or, at least, what > is triggering the whole thing). > > In the specific case of this stack trace, the current vcpu running on > CPU 45 is d3v11. It is not in the runqueue, because it has been > removed, and not added back to it, and the reason is it is not runnable > (it has VPF_migrating on in pause_flags). > > The runqueue of pcpu 45 looks fine (i.e., it is not corrupt or anything > like that), it has d3v10,d9v1,d32767v45 in it (in this order) > > d3v11->processor is 45, so that is also fine. > > Basically, d3v11 wants to move away from pcpu 45, and this might (but > that's not certain) be the reson because we're rescheduling. The fact > that there are vcpus wanting to migrate can very well be the cause of > affinity being changed. > > Now, the problem is that, looking into the runqueue, I found out that > d3v10->processor=32. I.e., d3v10 is queued in pcpu 45's runqueue, with > processor=32, which really shouldn't happen. > > This leads to the bug triggering, as, in csched_schedule(), we read the > head of the runqueue with: > > snext = __runq_elem(runq->next); > > and then we pass snext to csched_load_balance(), where the BUG_ON is. > > Another thing that I've found out, is that all "misplaced" vcpus (i.e., > in this and also in other manifestations of this bug) have their > csched_vcpu.flags=4, which is CSCHED_FLAGS_VCPU_MIGRATING. > > This, basically, is again a sign of vcpu_migrate() having been called, > on d3v10 as well, which in turn has called csched_vcpu_pick(). > >> a nasty race condition… a vcpu has just been taken off the runqueue >> of the current pcpu, but it’s apparently been assigned to a different >> cpu. >> > Nasty indeed. I've been looking into this on and off, but so far I > haven't found the root cause. > > Now that we know for sure that it is changing affinity that trigger it, > the field of the investigation can be narrowed a little bit... But I > still am finding hard to spot where the race happens. > > I'll look more into this later in the afternoon. I'll let know if > something comes to mind.
Actually, it looks quite simple: schedule.c:vcpu_move_locked() is supposed to actually do the moving; if vcpu_scheduler()->migrate is defined, it calls that; otherwise, it just sets v->processor. Credit1 doesn't define migrate. So when changing the vcpu affinity on credit1, v->processor is simply modified without it changing runqueues. The real question is why it's so hard to actually trigger any problems! All in all it looks like the migration / cpu_pick could be made a bit more rational... we do this weird thing where we call cpu_pick, and if it's different we call migrate; but of course if the vcpu is running, we just set the VPF_migrating bit and raise a schedule_softirq, which will cause cpu_pick() to be called yet another time. But as a quick fix, implementing csched_vcpu_migrate() is probably the best solution. Do you want to pick that up, or should I? -George _______________________________________________ Xen-devel mailing list Xenemail@example.com https://lists.xenproject.org/mailman/listinfo/xen-devel