Re: [PATCH 1/2] perf/x86/intel/pt: Fail event scheduling on conflict with VMX

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Feb 14, 2017 at 06:17:30PM +0200, Alexander Shishkin wrote:
> Peter Zijlstra <peterz@infradead.org> writes:
> 
> > On Tue, Feb 14, 2017 at 03:24:15PM +0200, Alexander Shishkin wrote:
> >> At the moment, if VMX operation prevents PT tracing, the PMU will
> >> silently return success to the event scheduling code, which will
> >> track its 'on' time, etc. Instead, report failure so that perf
> >> core knows this event is not actually on.
> >> 
> >> Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> >> Reported-by: Andi Kleen <ak@linux.intel.com>
> >> Fixes: 1c5ac21a0e ("perf/x86/intel/pt: Don't die on VMXON")
> >> ---
> >>  arch/x86/events/intel/pt.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >> 
> >> diff --git a/arch/x86/events/intel/pt.c b/arch/x86/events/intel/pt.c
> >> index d92a60ef08..9372fa4549 100644
> >> --- a/arch/x86/events/intel/pt.c
> >> +++ b/arch/x86/events/intel/pt.c
> >> @@ -1335,7 +1335,7 @@ static void pt_event_start(struct perf_event *event, int mode)
> >>  	struct pt_buffer *buf;
> >>  
> >>  	if (READ_ONCE(pt->vmx_on))
> >> -		return;
> >> +		goto fail_stop;
> >>  
> >>  	buf = perf_aux_output_begin(&pt->handle, event);
> >>  	if (!buf)
> >
> > I'm not getting it; how does this matter to the time tracking in
> > event_sched_in() / event_sched_out() ?
> >
> > That looks at event->state == PERF_EVENT_STATE*
> >
> > This goto affects event->hw.state == PERF_HES_
> >
> > The core assumes ->start() will _NOT_ fail.
> 
> This is called by pmu::add(), which checks hw.state afterwards and if it
> finds HES_STOPPED, it returns an error, which event_sched_in() captures
> and keeps the event in INACTIVE state. Should I add a comment about it?

Egads... so what if ->add() succeeds but we then hit this on
->stop()/->start() due to throttle or period adjust?

Now I suppose PT will never normally hit either of those, but you can do
IOC_PERIOD on it, just for giggles.

Yes, this very much needs a comment... Also, should not this then live
in ->add() in the first place?