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SubjectRe: [PATCH V3 1/5] perf/x86/intel: fix event update for auto-reload
On Tue, Feb 06, 2018 at 12:58:23PM -0500, Liang, Kan wrote:
>
>
> > With the exception of handling 'empty' buffers, I ended up with the
> > below. Please try again.
> >
>
> There are two small errors. After fixing them, the patch works well.

Well, it still doesn't do A, two read()s without PEBS record in between.
So that needs fixing. What 3/5 does, call x86_perf_event_update() after
drain_pebs() is actively wrong after this patch.

> > +
> > + /*
> > + * Careful, not all hw sign-extends above the physical width
> > + * of the counter.
> > + */
> > + delta = (new_raw_count << shift) - (prev_raw_count << shift);
> > + delta >>= shift;
>
> new_raw_count could be smaller than prev_raw_count.
> The sign bit will be set. The delta>> could be wrong.
>
> I think we can add a period here to prevent it.
> + delta = (period << shift) + (new_raw_count << shift) -
> + (prev_raw_count << shift);
> + delta >>= shift;
> ......
> + local64_add(delta + period * (count - 1), &event->count);
>

Right it does, but that wrecks case A again, because then we get here
with !@count.

Maybe something like:


s64 new, old;

new = ((s64)(new_raw_count << shift) >> shift);
old = ((s64)(old_raw_count << shift) >> shift);

local64_add(new - old + count * period, &event->count);


And then make intel_pmu_drain_pebs_*(), call this function even when !n.

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