linux-kernel - Re: [PATCH 2/2] perf cs-etm: Set time on synthesised samples to preserve ordering

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Date:   Thu, 15 Apr 2021 22:33:29 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     James Clark <james.clark@....com>
Cc:     coresight@...ts.linaro.org, al.grant@....com,
        branislav.rankov@....com, denik@...omium.org,
        suzuki.poulose@....com, Mike Leach <mike.leach@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        John Garry <john.garry@...wei.com>,
        Will Deacon <will@...nel.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        linux-arm-kernel@...ts.infradead.org,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] perf cs-etm: Set time on synthesised samples to
 preserve ordering

Hi James,

On Thu, Apr 15, 2021 at 03:51:46PM +0300, James Clark wrote:

[...]

> > For the orignal perf data file with "--per-thread" option, the decoder
> > runs into the condition for "etm->timeless_decoding"; and it doesn't
> > contain ETM timestamp.
> > 
> > Afterwards, the injected perf data file also misses ETM timestamp and
> > hit the condition "etm->timeless_decoding".
> > 
> > So I am confusing why the original perf data can be processed properly
> > but fails to handle the injected perf data file.
> 
> Hi Leo,
> 
> My patch only deals with per-cpu mode. With per-thread mode everything is already working
> because _none_ of the events have timestamps because they are not enabled by default:
> 
> 	/* In per-cpu case, always need the time of mmap events etc */
> 	if (!perf_cpu_map__empty(cpus))
> 		evsel__set_sample_bit(tracking_evsel, TIME);
> 
> When none of the events have timestamps, I think perf doesn't use the ordering code in
> ordered-events.c. So when the inject file is opened, the events are read in file order.

The explination makes sense to me.  One thinking: if the original file
doesn't use the ordered event, is it possible for the injected file to
not use the ordered event as well?

Could you confirm Intel-pt can work well for per-cpu mode for inject
file?

> So it's not really about --per-thread vs per-cpu mode, it's actually about whether
> PERF_SAMPLE_TIME is set, which is set as a by-product of per-cpu mode.
>
> I hope I understood your question properly.

Thanks for info, sorry if I miss any info you have elaborated.

Leo