Re: [Xen-devel] [RFC v4 2/2] x86/xen: allow privcmd hypercalls to be preempted

[Luis R. Rodriguez]

On Fri, Jan 23, 2015 at 11:45:06AM +0000, David Vrabel wrote:
> 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

Its not just hypercalls though, this is all about the interactions
with multicalls no?

> * 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.

OK.

 Luis
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