Re: [kvm-unit-tests PATCH] tscdeadline_latency: Check condition first before loop

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Thu, Jul 11, 2019 at 03:33:35PM +0800, Peter Xu wrote:
> On Thu, Jul 11, 2019 at 03:17:56PM +0800, Peter Xu wrote:
> > This patch fixes a tscdeadline_latency hang when specifying a very
> > small breakmax value.  It's easily reproduced on my host with
> > parameters like "200000 10000 10" (set breakmax to 10 TSC clocks).
> > 
> > The problem is test_tsc_deadline_timer() can be very slow because
> > we've got printf() in there.  So when reach the main loop we might
> > have already triggered the IRQ handler for multiple times and we might
> > have triggered the hitmax condition which will turn IRQ off.  Then
> > with no IRQ that first HLT instruction can last forever.
> > 
> > Fix this by simply checking the condition first in the loop.
> > 
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> >  x86/tscdeadline_latency.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/x86/tscdeadline_latency.c b/x86/tscdeadline_latency.c
> > index 0617a1b..4ee5917 100644
> > --- a/x86/tscdeadline_latency.c
> > +++ b/x86/tscdeadline_latency.c
> > @@ -118,9 +118,9 @@ int main(int argc, char **argv)
> >      test_tsc_deadline_timer();
> >      irq_enable();
> >  
> > -    do {
> > +    /* The condition might have triggered already, so check before HLT. */
> > +    while (!hitmax && table_idx < size)
> 
> Hmm... I think this is not ideal too in that variables (e.g., hitmax)
> could logically still change between the condition check and HLT below
> (though this patch already runs nicely here).  Maybe we can simply use
> "nop" or "pause" instead of "hlt".
> 
> I tested that using pause fixes the problem too.

Ensuring the first hlt lands in an interrupt shadow should prevent getting
into a halted state after the timer has been disabled, e.g.:

    irq_disable();
    test_tsc_deadline_timer();

    do {
        safe_halt();
    } while (!hitmax && table_idx < size);

> 
> >          asm volatile("hlt");
> > -    } while (!hitmax && table_idx < size);
> >  
> >      for (i = 0; i < table_idx; i++) {
> >          if (hitmax && i == table_idx-1)
> > -- 
> > 2.21.0
> > 
> 
> Regards,
> 
> -- 
> Peter Xu