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    SubjectRe: [PATCH 1/7] perf: introduce raw_type attribute to specify the type of a raw sample
    On Thu, May 20, 2010 at 12:37 PM, Peter Zijlstra <> wrote:
    > On Thu, 2010-05-20 at 11:42 +0200, Stephane Eranian wrote:
    >> > For Instruction-Fetch:
    >> >  32:47 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:
    >> >
    >> >  0x87 Instruction Fetch Stall -- Ins-Fetch
    >> >  0xC0 Retired Instructions    -- 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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      Last update: 2010-05-20 14:17    [W:0.030 / U:8.004 seconds]
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