linux-kernel - Re: [PATCH] perf intel-pt: Synthesize cycle events

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Date:   Tue, 15 Mar 2022 19:00:27 +0100
From:   "Steinar H. Gunderson" <sesse@...gle.com>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf intel-pt: Synthesize cycle events

On Tue, Mar 15, 2022 at 01:32:38PM +0200, Adrian Hunter wrote:
>> I think the structure looks good, but I'm not sure about updating
>> e.g. ptq->last_cy_insn_cnt in both functions? Does that make sense?
> It should only be updated in the new intel_pt_synth_cycle_sample().
> intel_pt_synth_instruction_sample() should be unchanged.

Hm, OK. But something definitely changed between my original patch and
your change. (The first patch; I didn't try the last one yet.) With my
patch, I got (on a specific trace, synthing cycles only with perf report
--itrace=y0nse):

Samples: 4M of event 'cycles:uH', Event count (approx.): 4844309

With yours on the same file:

Samples: 2M of event 'cycles:uH', Event count (approx.): 77622449

The relative times between functions are also pretty different (although
none of them are obviously crazy), so one of them has to be wrong.
Is this to be expected, ie., would you expect your change to fix some
bad bug on cycle-only synth? For reference, “perf script --itrace=i0ns
-F +ipc | grep -c IPC:” (a quick proxy for the number of CYC packets :-) )
yields 4836782, so I'm a bit surprised why there are only 2M events
being emitted from that.

/* Steinar */