linux-kernel - Re: [PATCH 1/8] perf cs-etm: Don't flush when packet

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Message-ID: <c124ad7c-31be-42cb-9eeb-507d99a97891@linaro.org>
Date: Thu, 12 Sep 2024 16:22:11 +0100
From: James Clark <james.clark@...aro.org>
To: Leo Yan <leo.yan@....com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>,
 "Liang, Kan" <kan.liang@...ux.intel.com>,
 Suzuki K Poulose <suzuki.poulose@....com>, Mike Leach
 <mike.leach@...aro.org>, John Garry <john.g.garry@...cle.com>,
 Will Deacon <will@...nel.org>, Leo Yan <leo.yan@...ux.dev>,
 Ben Gainey <ben.gainey@....com>, Ruidong Tian
 <tianruidong@...ux.alibaba.com>, Benjamin Gray <bgray@...ux.ibm.com>,
 Mathieu Poirier <mathieu.poirier@...aro.org>, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-perf-users@...r.kernel.org,
 gankulkarni@...amperecomputing.com, coresight@...ts.linaro.org,
 scclevenger@...amperecomputing.com
Subject: Re: [PATCH 1/8] perf cs-etm: Don't flush when packet_queue fills up



On 11/09/2024 09:14, Leo Yan wrote:
> 
> 
> On 9/10/2024 9:28 PM, Leo Yan wrote:
>> On 9/5/2024 11:50 AM, James Clark wrote:
>>
>> [...]
>>
>>> cs_etm__flush(), like cs_etm__sample() is an operation that generates a
>>> sample and then swaps the current with the previous packet. Calling
>>> flush after processing the queues results in two swaps which corrupts
>>> the next sample. Therefore it wasn't appropriate to call flush here so
>>> remove it.
>>
>> In the cs_etm__sample(), if the period is not overflow, it is not necessarily
>> to generate instruction samples and copy back stack entries. This is why we
>> want to call cs_etm__flush() to make sure the last packet can be recorded
>> properly for instruction sample with back stacks.
>>
>> We also need to take account into the case for the end of the session - in
>> this case we need to generate samples for the last packet for complete info.
>>
>> I am wandering should we remove the cs_etm__packet_swap() from cs_etm__sample()?
> 
> Sorry for typo. I meant to remove the cs_etm__packet_swap() from cs_etm__flush().
> 
> Thanks,
> Leo

Turns out there was already cs_etm__end_block() for the end of the 
session, but it was only called for the timeless modes. I added it for 
timestamped mode too in V2.

I also kept the existing flush() function for discontinuities. I changed 
my mind that the differences to cs_etm__sample() weren't relevant.

So I think we still need to keep the swap in flush() because otherwise 
the next sample won't start from the right place.