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 * 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. > + */ > +void xen_end_upcall(struct pt_regs *regs) > +{ > + if (xen_is_preemptible_hypercall(regs)) { > + int cpuid = smp_processor_id(); > + if (_cond_resched()) > + trace_xen_hypercall_preemption(cpuid); I don't think a tracepoint here is useful. > + } > +} > +NOKPROBE_SYMBOL(xen_end_upcall); Do we need this is this function is no longer notrace? David -- 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
