On Fri, Jan 23, 2015 at 11:45:06AM +0000, David Vrabel wrote: > On 23/01/15 00:29, Luis R. Rodriguez wrote: > > From: "Luis R. Rodriguez" <mcgrof@xxxxxxxx> > > > > Xen has support for splitting heavy work work into a series > > of hypercalls, called multicalls, and preempting them through > > what Xen calls continuation [0]. Despite this though without > > CONFIG_PREEMPT preemption won't happen, without preemption > > a system can become pretty useless on heavy handed hypercalls. > > Such is the case for example when creating a > 50 GiB HVM guest, > > we can get softlockups [1] with:. > > > > kernel: [ 802.084335] BUG: soft lockup - CPU#1 stuck for 22s! [xend:31351] > > > > The softlock up triggers on the TASK_UNINTERRUPTIBLE hanger check > > (default 120 seconds), on the Xen side in this particular case > > this happens when the following Xen hypervisor code is used: > > > > xc_domain_set_pod_target() --> > > do_memory_op() --> > > arch_memory_op() --> > > p2m_pod_set_mem_target() > > -- long delay (real or emulated) -- > > > > This happens on arch_memory_op() on the XENMEM_set_pod_target memory > > op even though arch_memory_op() can handle continuation via > > hypercall_create_continuation() for example. > > > > Machines over 50 GiB of memory are on high demand and hard to come > > by so to help replicate this sort of issue long delays on select > > hypercalls have been emulated in order to be able to test this on > > smaller machines [2]. > > > > On one hand this issue can be considered as expected given that > > CONFIG_PREEMPT=n is used however we have forced voluntary preemption > > precedent practices in the kernel even for CONFIG_PREEMPT=n through > > the usage of cond_resched() sprinkled in many places. To address > > this issue with Xen hypercalls though we need to find a way to aid > > to the schedular in the middle of hypercalls. We are motivated to > > address this issue on CONFIG_PREEMPT=n as otherwise the system becomes > > rather unresponsive for long periods of time; in the worst case, at least > > only currently by emulating long delays on select io disk bound > > hypercalls, this can lead to filesystem corruption if the delay happens > > for example on SCHEDOP_remote_shutdown (when we call 'xl <domain> shutdown'). > > > > We can address this problem by trying to check if we should schedule > > on the xen timer in the middle of a hypercall on the return from the > > timer interrupt. We want to be careful to not always force voluntary > > preemption though so to do this we only selectively enable preemption > > on very specific xen hypercalls. > [...] > > @@ -1243,6 +1247,25 @@ void xen_evtchn_do_upcall(struct pt_regs *regs) > > set_irq_regs(old_regs); > > } > > > > +/* > > + * CONFIG_PREEMPT=n kernels can end up triggering the softlock > > + * TASK_UNINTERRUPTIBLE hanger check (default 120 seconds) > > + * when certain multicalls are used [0] on large systems, in > > + * that case we need a way to voluntarily preempt. This is > > + * only an issue on CONFIG_PREEMPT=n kernels. > > Rewrite this comment as; > > * Some hypercalls issued by the toolstack can take many 10s of Its not just hypercalls though, this is all about the interactions with multicalls no? > * seconds. Allow tasks running hypercalls via the privcmd driver to be > * voluntarily preempted even if full kernel preemption is disabled. > > > + * [0] https://bugzilla.novell.com/show_bug.cgi?id=861093 > > This link isn't accessible so I don't think it should be included here. OK. Luis -- 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
