[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v2 2/2] x86/xen: allow privcmd hypercalls to be preempted
On Wed, Dec 10, 2014 at 4:55 PM, Luis R. Rodriguez <mcgrof@xxxxxxxx> wrote: > On Wed, Dec 10, 2014 at 03:51:48PM -0800, Andy Lutomirski wrote: >> On Wed, Dec 10, 2014 at 3:34 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 and while enabling >> > CONFIG_RT_GROUP_SCHED can at times help its not enough to >> > make a system usable. 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> >> > 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: Andy Lutomirski <luto@xxxxxxxxxxxxxx> >> > 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 | 21 +++++++++++++++++++++ >> > arch/x86/kernel/entry_64.S | 17 +++++++++++++++++ >> > drivers/xen/Makefile | 2 +- >> > drivers/xen/preempt.c | 17 +++++++++++++++++ >> > drivers/xen/privcmd.c | 2 ++ >> > include/xen/xen-ops.h | 26 ++++++++++++++++++++++++++ >> > 6 files changed, 84 insertions(+), 1 deletion(-) >> > create mode 100644 drivers/xen/preempt.c >> > >> > diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S >> > index 344b63f..40b5c0c 100644 >> > --- a/arch/x86/kernel/entry_32.S >> > +++ b/arch/x86/kernel/entry_32.S >> > @@ -982,7 +982,28 @@ ENTRY(xen_hypervisor_callback) >> > ENTRY(xen_do_upcall) >> > 1: mov %esp, %eax >> > call xen_evtchn_do_upcall >> > +#ifdef CONFIG_PREEMPT >> > jmp ret_from_intr >> > +#else >> > + GET_THREAD_INFO(%ebp) >> > +#ifdef CONFIG_VM86 >> > + movl PT_EFLAGS(%esp), %eax # mix EFLAGS and CS >> > + movb PT_CS(%esp), %al >> > + andl $(X86_EFLAGS_VM | SEGMENT_RPL_MASK), %eax >> > +#else >> > + movl PT_CS(%esp), %eax >> > + andl $SEGMENT_RPL_MASK, %eax >> > +#endif >> > + cmpl $USER_RPL, %eax >> > + jae resume_userspace # returning to v8086 or userspace >> > + DISABLE_INTERRUPTS(CLBR_ANY) >> > + cmpb $0,PER_CPU_VAR(xen_in_preemptible_hcall) >> > + jz resume_kernel >> > + movb $0,PER_CPU_VAR(xen_in_preemptible_hcall) >> > + call cond_resched_irq >> > + movb $1,PER_CPU_VAR(xen_in_preemptible_hcall) >> > + jmp resume_kernel >> > +#endif /* CONFIG_PREEMPT */ >> > CFI_ENDPROC >> > ENDPROC(xen_hypervisor_callback) >> > >> > diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S >> > index c0226ab..0ccdd06 100644 >> > --- a/arch/x86/kernel/entry_64.S >> > +++ b/arch/x86/kernel/entry_64.S >> > @@ -1170,7 +1170,23 @@ ENTRY(xen_do_hypervisor_callback) # >> > do_hypervisor_callback(struct *pt_regs) >> > popq %rsp >> > CFI_DEF_CFA_REGISTER rsp >> > decl PER_CPU_VAR(irq_count) >> > +#ifdef CONFIG_PREEMPT >> > jmp error_exit >> > +#else >> > + movl %ebx, %eax >> > + RESTORE_REST >> > + DISABLE_INTERRUPTS(CLBR_NONE) >> > + TRACE_IRQS_OFF >> > + GET_THREAD_INFO(%rcx) >> > + testl %eax, %eax >> > + je error_exit_user >> > + cmpb $0,PER_CPU_VAR(xen_in_preemptible_hcall) >> > + jz retint_kernel >> >> I think I understand this part. >> >> > + movb $0,PER_CPU_VAR(xen_in_preemptible_hcall) >> >> Why? Is the issue that, if preemptible hypercalls nest, you don't >> want to preempt again? > > > So this callback on the xen timer, without the CPU variable > we would not be able to selectively preempt specific hypercalls > and we'd a no-preempt kernel fully preemptive. > > I asked the same question, see: > > https://lkml.org/lkml/2014/12/3/756 > >> >> > + call cond_resched_irq >> >> On !CONFIG_PREEMPT, there's no preempt_disable, right? So how do you >> guarantee that you don't preempt something you shouldn't? > > Not sure I follow, but in essence this is no different then the use > of cond_resched() on !CONFIG_PREEMPT, the only thing here is we are > in interrupt context. If this is about abuse of making !CONFIG_PREEMPT > voluntarily preemptive at select points then I had similar concerns > and David pointed out to me the wide use of cond_resched() on the > kernel. > >> Is the idea >> that these events will only fire nested *directly* inside a >> preemptible hypercall? > > Yeah its the timer interrupt that would trigger the above. > >> Also, should you check that IRQs were on when >> the event fired? (Are they on in pt_regs?) > > Right before this xen_evtchn_do_upcall() is issued, which > saves pt_regs and then restores them. > >> > + movb $1,PER_CPU_VAR(xen_in_preemptible_hcall) >> > + jmp retint_kernel >> > +#endif /* CONFIG_PREEMPT */ >> > CFI_ENDPROC >> >> All that being said, this is IMO a bit gross. > > That was my first reaction hence my original attempt to try to get away from > this. I've learned David also tried to not go down this route too before, > more on this below. > >> You've added a bunch of >> asm that's kind of like a parallel error_exit, > > yeah.. I first tried to macro'tize this but it looked hairy, if we > wanted to do that... (although the name probably ain't best) > > 32-bit: > .macro test_from_kernel kernel_ret:req > GET_THREAD_INFO(%ebp) > #ifdef CONFIG_VM86 > movl PT_EFLAGS(%esp), %eax # mix EFLAGS and CS > movb PT_CS(%esp), %al > andl $(X86_EFLAGS_VM | SEGMENT_RPL_MASK), %eax > #else > /* > * We can be coming here from child spawned by kernel_thread(). > */ > movl PT_CS(%esp), %eax > andl $SEGMENT_RPL_MASK, %eax > #endif > cmpl $USER_RPL, %eax > jb \kernel_ret > .endm > > 64-bit: > .macro test_from_kernel kernel_ret:req > movl %ebx,%eax > RESTORE_REST > DISABLE_INTERRUPTS(CLBR_NONE) > TRACE_IRQS_OFF > GET_THREAD_INFO(%rcx) > testl %eax,%eax > jne \kernel_ret > .endm > >> and the error entry and >> exit code is hairy enough that this scares me. > > yeah... > >> Can you do this mostly in C instead? This would look a nicer if it could >> be: >> >> call xen_evtchn_do_upcall >> popq %rsp >> CFI_DEF_CFA_REGISTER rsp >> decl PER_CPU_VAR(irq_count) >> + call xen_end_upcall >> jmp error_exit > > It seems David tried this originally eons ago: > > http://lists.xen.org/archives/html/xen-devel/2014-02/msg01101.html > > and the strategy was shifted based on Jan's feedback that we could > not sched as we're on the IRQ stack. David evolved the strategy > to asm and to use preempt_schedule_irq(), this new iteratin just > revisits the same old but tries to generalize scheduling on IRQ > context on very special circumstances. Indeed. But look more closely at my proposed one-line asm patch: we're not on the irq stack. Also, to make this more obviously safe wrt preempting at the wrong time, would it be possible to check regs->ip and make sure it's pointing exactly where you expect before scheduling? (You'd need one more line of asm to get pt_regs in xen_end_upcall.) --Andy, the stack switching maestro extraordinaire :) > >> Where xen_end_upcall would be witten in C, nokprobes and notrace (if >> needed) and would check pt_regs and whatever else and just call >> schedule if needed? > > If there's a way to do it it'd be great. I am not sure if we can though. > > Luis -- Andy Lutomirski AMA Capital Management, LLC _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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