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

[Steven Rostedt <rostedt@xxxxxxxxxxx>]

On Thu, 22 Jan 2015 17:40:27 -0800
Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:

> >
> > +/*
> > + * 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?

Yes, but if you scheduled here, the tracepoint could happen on a
different CPU. Thus, cpuid will not equal smp_processor_id().

-- Steve

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