Re: [Xen-devel] [RFC v3 2/2] x86/xen: allow privcmd hypercalls to be preempted

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

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

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