Re: [PATCH 1/7] perf: introduce raw_type attribute to specify the type of a raw sample

[Stephane Eranian]

On Thu, May 20, 2010 at 12:37 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Thu, 2010-05-20 at 11:42 +0200, Stephane Eranian wrote:
>
>> > For Instruction-Fetch:
>
>> > Â32:47 latency    (r/w)
>
>> Your are mixing output and input parameters.
>>
>> The only input parameters you have are:
>> - sample-period, enable, random
>> The rest is output only.
>
> Ah, my bad, I thought it was a r/w field.
>
>> > encode these IBS things as:
>> >
>> > Â0x87 Instruction Fetch Stall -- Ins-Fetch
>> > Â0xC0 Retired Instructions  Â-- Ins-Exec
>> >
>> I think those events do not map to the behavior of IBS. We have
>> add that discussion before.
>
> Hrm,. so there are no regular events that count the same thing as the
> IBS things? That really sucks.
>
> So yeah, you might as well expose it as a whole separate PMU using Lin's
> stuff.

What's wrong with creating pseudo-events for IBS? We'd have to pick
two unused event codes. That would have the advantage of making it
explicit you're using IBS. I think we could still use the precise_ip field
if people are only interested in the IP. They would use PERF_SAMPLE_RAW
if they need more.

>
>> > The Ins-Exec will have to re-construct the actual event->count by adding
>> > sample-period on each interrupt, as it seems we lack an actual counter
>> > in hardware.
>> >
>> For what? counting mode?
>
> Yeah, events are supposed to count.
>
IBS is a sampling only feature. I suspect it would be okay to return 0 here
or do as you said, count the number of IBS interrupts and multiply by the
sampling period.

>> > Furthermore, these counters will have to deal with sample-period > 2^16
>> > by 'ignoring' interrupts until we get ->period_left down to 0.
>> >
>> Well, it's not 2^16, it's 2^20 but bottom 4 bits must be zero.
>> What about simply failing perf_event_open() is sample_period does not fit the
>> constraint?
>
> Why, its simple enough to ignore a few interrupts, we do the same for
> all other implementations.
>
>> > The extra data could possibly be exposed through attaching non-sampling
>> > group events and using SAMPLE_READ, like L1-misses, although
>> > reconstructing the count from just one bit seems 'interesting'.
>> >
>> > The IbsFetchLinAd/IbsOpRip would go straight into PERF_SAMPLE_IP by
>> > replacing pt_regs->ip I guess.
>> >
>> > IbsDcLinAd goes into PERF_SAMPLE_ADDR
>> >
>> What about the rest, the TLB, alignment, data sources?
>
> Dunno, reconstruct sensible counters? Surely the software that uses IBS
> does something useful with that data? What does libpfm do with the IBS
> data?
>
The common usage model is you gather the IBSop data (all of it), you save
samples into a file and then you have scripts that extract whatever fields they
need to compute the metric you want. For instance, if you want data cache misses
you extract [IP, data address, data source, miss latency], if you care about
instruction latencies, you extract [IP, tag2ret, comp2ret], and so on.

Libpfm does not handle IBS output. Its goal is to help applications setup
the events/counters. With perf_events, it does the mapping from symbolic event
names+attributes -> struct perf_event_attr. It does not make any perf_event
syscalls for you.
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