On Wed, Jan 21, 2015 at 6:17 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]. > > [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 | 13 +++++++++++++ > include/xen/events.h | 1 + > 4 files changed, 18 insertions(+) > > diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S > index 000d419..b4b1f42 100644 > --- a/arch/x86/kernel/entry_32.S > +++ b/arch/x86/kernel/entry_32.S > @@ -982,6 +982,8 @@ ENTRY(xen_hypervisor_callback) > ENTRY(xen_do_upcall) > 1: mov %esp, %eax > call xen_evtchn_do_upcall > + movl %esp,%eax > + call xen_end_upcall > jmp ret_from_intr > CFI_ENDPROC > ENDPROC(xen_hypervisor_callback) > diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S > index 9ebaf63..ee28733 100644 > --- a/arch/x86/kernel/entry_64.S > +++ b/arch/x86/kernel/entry_64.S > @@ -1198,6 +1198,8 @@ ENTRY(xen_do_hypervisor_callback) # do_hypervisor_callback(struct *pt_regs) > popq %rsp > CFI_DEF_CFA_REGISTER rsp > decl PER_CPU_VAR(irq_count) > + movq %rsp, %rdi /* pass pt_regs as first argument */ > + call xen_end_upcall > jmp error_exit > CFI_ENDPROC > END(xen_do_hypervisor_callback) > diff --git a/drivers/xen/events/events_base.c b/drivers/xen/events/events_base.c > index b4bca2d..23c526b 100644 > --- a/drivers/xen/events/events_base.c > +++ b/drivers/xen/events/events_base.c > @@ -32,6 +32,8 @@ > #include <linux/slab.h> > #include <linux/irqnr.h> > #include <linux/pci.h> > +#include <linux/sched.h> > +#include <linux/kprobes.h> > > #ifdef CONFIG_X86 > #include <asm/desc.h> > @@ -1243,6 +1245,17 @@ void xen_evtchn_do_upcall(struct pt_regs *regs) > set_irq_regs(old_regs); > } > > +notrace void xen_end_upcall(struct pt_regs *regs) > +{ > + if (!xen_is_preemptible_hypercall(regs) || > + __this_cpu_read(xed_nesting_count)) > + return; What's xed_nesting_count? > + > + if (_cond_resched()) > + printk(KERN_DEBUG "xen hypercall preempted\n"); Did you mean to leave this in? If so, should it be pr_debug? --Andy -- 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
