Re: [PATCH 2/4] perf/x86: Fix period for non sampling events

[Jiri Olsa]

On Tue, Jan 03, 2017 at 04:09:46PM +0100, Peter Zijlstra wrote:
> On Wed, Dec 28, 2016 at 02:31:04PM +0100, Jiri Olsa wrote:
> > When in counting mode we setup the counter with the
> > longest possible period and read the value with read
> > syscall.
> > 
> > We also still setup the PMI to be triggered when such
> > counter overflow to reconfigure it.
> > 
> > We also get PEBS interrupt if such counter has precise_ip
> > set (which makes no sense, but it's possible).
> > 
> > Having such counter with:
> >   - counting mode
> >   - precise_ip set
> > 
> > I watched my server to get stuck serving PEBS interrupt
> > again and again because of following (AFAICS):
> > 
> >   - PEBS interrupt is triggered before PMI
> 
> Slightly confused, the PEBS interrupt _is_ the PMI. And how can we get
> an interrupt before the counter overflows?
> 
> >   - when PEBS handling path reconfigured counter it
> >     had remaining value of -256
> 
> You're talking about the actual counter value, right, not @left?

right

> 
> >   - the x86_perf_event_set_period does not consider this
> >     as an extreme value, so it's configured back as the
> >     new counter value
> 
> Right, a counter value of -256 would result in @left being 256 which is
> positive and not too large, so we 'retain' the value.
> 
> >   - this makes the PEBS interrupt to be triggered right
> >     away again
> 
> So I'm curious how this is even possible. The normal described way of
> things is:
> 
> 	- we program the counter with a negative value
> 	- each 'event' does a counter increment
> 	- if we 'overflow' (turn positive) we start to arm the PEBS
> 	  assist

heh, I guess I thought this could happen earlier ;-) otherwise
I dont get how could I saw -256 left in the counter value..

> 	- once the assist is armed, the next 'event' triggers a PEBS
> 	  record.
> 	- if the amount of PEBS records exceeds the DS threshold, we
> 	  set bit 62 in GLOBAL_STATUS and raise the PMI.
> 
> At which point the actual counter value should be at the very least 1
> (for having counted the event that triggers the PEBS assist into
> creating the record).

what I saw was the bit 62 set and pebs_drain->__intel_pmu_pebs_event
re-configuring the event back with -256 again and again..

I'll run fuzzer again without the fix with my debug stuff in and try
to recreate ;-)

> Did your kernel include commit:
> 
>   daa864b8f8e3 ("perf/x86/pebs: Fix handling of PEBS buffer overflows")

yep

jirka