RE: [PATCH] perf/x86: fix event counter update issue

From: Vince Weaver
Date: Thu Feb 23 2017 - 10:07:15 EST

On Wed, 22 Feb 2017, Liang, Kan wrote:

> > So from what I understand, the issue is if we have an architecture with full-
> > width counters and we trigger a x86_perf_event_update() when bit
> > 47 is set?
> No. It related to the counter width. The number of bits we can use should be
> 1 bit less than the total width. Otherwise, there will be problem.
> For big cores such as haswell, broadwell, skylake, the counter width is 48 bit.
> So we can only use 47 bits.
> For Silvermont and KNL, the counter width is only 32 bit I think. So we can only
> use 31 bits.

So on a machine with 48-bit counters I should just have a counting event
that counts to somewhere above 0x8000 0000 0001 and it should show
Because I am unable to trigger this.

But I guess if anywhere along the line x86_perf_event_update() is run
then you start over?

I noticed your original reproducer bound the event to a core, is that
needed to trigger this?

Can it happen on a fixed event or only a genearl purpose event?

> > So if I have a test that runs in a loop for 2^48 retired instructions (which
> > takes ~12 hours on a recent machine) and then reads the results, they
> > might be wrong?
> It only needs several minutes to reproduce the issue on SLM/KNL.

Yes, but I only have machines with 48-bit counters. So it's going to take
256 times as long as on a machine with 40-bit counters.

I have an assembly loop that can consistently generate 2 instructions/cycle
(I'd be glad to hear suggestions for events that count faster) and on
a broadwell-ep machine it still takes at least 7 hours or so to get up
to 0x800000000000.