Re: [RFC PATCH 6/7] perf, x86: large PEBS interrupt threshold

[Stephane Eranian]

On Wed, May 28, 2014 at 10:10 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Wed, May 28, 2014 at 02:18:09PM +0800, Yan, Zheng wrote:
>> PEBS always had the capability to log samples to its buffers without
>> an interrupt. Traditionally perf has not used this but always set the
>> PEBS threshold to one.
>>
>> For the common cases we still need to use the PMI because the PEBS
>> hardware has various limitations. The biggest one is that it can not
>> supply a callgraph. It also requires setting a fixed period, as the
>> hardware does not support adaptive period. Another issue is that it
>> cannot supply a time stamp and some other options.
>
> So the reason I've never done this is because Intel has never fully
> explained the demuxing of pebs events.
>
> In particular, the 0x90 offset (IA32_PERF_GLOBAL_STATUS). Intel once
> confirmed to me that that is a direct copy of the similarly named MSR at
> the time of the PEBS assist.
>
> This is a problem, since if multiple counters overflow multiple bits
> will be set and its (afaict) ambiguous which event is for which counter.
>
I am not sure how having only one entry in the PEBS buffer solves this.
I think PEBS will create only one entry if multiple counters overflow
simultaneously. That OVFL_STATUS bitmask will have multiple bits
set. I understand the problem in perf_events because you need to
assign a sample to an event and not all events may record the same
info in the sampling buffer.

> At one point it was said they'd fix this 0x90 offset to indicate which
> counter triggered the event, but I've never heard back if this happened.
>
> So until you can give an official Intel answer on how all this demuxing
> is supposed to work and be correct this patch set isn't moving anywhere.
>
>> To supply a TID it
>> requires flushing on context switch. It can however supply the IP
>
> On SNB+, previous to SNB it would need to have precise==1. I've seen no
> such logic in. Instead you seem to artificially limit it to SNB+, for no
> apparent reason to me.
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