On Thu, Jan 22, 2015 at 4:29 PM, Luis R. Rodriguez <mcgrof@xxxxxxxxxxxxxxxx> 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. > > This enables hypercall preemption by selectively forcing checks for > voluntary preempting only on ioctl initiated private hypercalls > where we know some folks have run into reported issues [1]. > > This also adds a trace event to be able to review when xen hypercalls > are preemtped, right now we just tell you when it happens and which > CPU got preempted. > > ergon:~ # echo 1 > /sys/kernel/debug/tracing/events/xen/xen_hypercall_preemption/trigger > ergon:~ # cat /sys/kernel/debug/tracing/trace_pipe > ... > qemu-system-i38-2114 [000] .... 491.038440: xen_hypercall_preemption: on CPU 0 > qemu-system-i38-2114 [003] .... 518.138592: xen_hypercall_preemption: on CPU 3 > ... > > [0] http://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=42217cbc5b3e84b8c145d8cfb62dd5de0134b9e8;hp=3a0b9c57d5c9e82c55dd967c84dd06cb43c49ee9 > [1] https://bugzilla.novell.com/show_bug.cgi?id=861093 > [2] http://ftp.suse.com/pub/people/mcgrof/xen/emulate-long-xen-hypercalls.patch > > Based on original work by: David Vrabel <david.vrabel@xxxxxxxxxx> > Suggested-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx> > Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx> > Cc: Borislav Petkov <bp@xxxxxxx> > Cc: David Vrabel <david.vrabel@xxxxxxxxxx> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > Cc: Ingo Molnar <mingo@xxxxxxxxxx> > Cc: "H. Peter Anvin" <hpa@xxxxxxxxx> > Cc: x86@xxxxxxxxxx > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx> > Cc: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx> > Cc: Jan Beulich <JBeulich@xxxxxxxx> > Cc: linux-kernel@xxxxxxxxxxxxxxx > Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxx> > --- > arch/x86/kernel/entry_32.S | 2 ++ > arch/x86/kernel/entry_64.S | 2 ++ > drivers/xen/events/events_base.c | 23 +++++++++++++++++++++++ > include/trace/events/xen.h | 9 +++++++++ > include/xen/events.h | 1 + > 5 files changed, 37 insertions(+) > Reviewed-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx> > > +/* > + * 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. > + * > + * [0] https://bugzilla.novell.com/show_bug.cgi?id=861093 > + */ > +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); If you want to speed this up a bit, I think you could move the smp_processor_id() into the TP_fast_assign. But don't tracepoints report the cpu number even without any action? -- 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
