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

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Date:   Tue, 15 Mar 2022 22:11:54 +0200
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     "Steinar H. Gunderson" <sesse@...gle.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 15.3.2022 20.00, Steinar H. Gunderson wrote:
> 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.

Sorry, my first suggestion has issues, but the second is better.