Re: [PATCH V3 1/5] perf/x86/intel: fix event update for auto-reload

["Liang, Kan" <kan.liang@linux.intel.com>]

On 2/9/2018 9:09 AM, Peter Zijlstra wrote:
> 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.
>

As my understanding, for case A, drain_pebs() will return immediately. 
It cannot reach the patch.
Because there is no PEBS record ready. So the ds->pebs_index should be 
the same as ds->pebs_buffer_base.

3/5 is to handle case A.

Thanks,
Kan

>>> +
>>> +	/*
>>> +	 * 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.
>