linux-kernel - Re: [PATCH v2 1/1] coresight: Don't immediately close events that are run on invalid CPU/sink combos

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Message-ID: <b36e8a19-4867-c788-7d68-774a3244fa61@arm.com>
Date:   Wed, 22 Sep 2021 16:20:56 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     James Clark <james.clark@....com>, mathieu.poirier@...aro.org,
        coresight@...ts.linaro.org
Cc:     Mike Leach <mike.leach@...aro.org>, Leo Yan <leo.yan@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] coresight: Don't immediately close events that are
 run on invalid CPU/sink combos

On 22/09/2021 13:51, James Clark wrote:
> When a traced process runs on a CPU that can't reach the selected sink,
> the event will be stopped with PERF_HES_STOPPED. This means that even if
> the process migrates to a valid CPU, tracing will not resume.
> 
> This can be reproduced (on N1SDP) by using taskset to start the process
> on CPU 0, and then switching it to CPU 2 (ETF 1 is only reachable from
> CPU 2):
> 
>    taskset --cpu-list 0 ./perf record -e cs_etm/@..._etf1/ --per-thread -- taskset --cpu-list 2 ls
> 
> This produces a single 0 length AUX record, and then no more trace:
> 
>    0x3c8 [0x30]: PERF_RECORD_AUX offset: 0 size: 0 flags: 0x1 [T]
> 
> After the fix, the same command produces normal AUX records. The perf
> self test "89: Check Arm CoreSight trace data recording and synthesized
> samples" no longer fails intermittently. This was because the taskset in
> the test is after the fork, so there is a period where the task is
> scheduled on a random CPU rather than forced to a valid one.
> 
> Specifically selecting an invalid CPU will still result in a failure to
> open the event because it will never produce trace:
> 
>    ./perf record -C 2 -e cs_etm/@..._etf0/
>    failed to mmap with 12 (Cannot allocate memory)
> 
> The only scenario that has changed is if the CPU mask has a valid CPU
> sink combo in it.
> 
> Testing
> =======
> 
> * Coresight self test passes consistently:
>    ./perf test Coresight
> 
> * CPU wide mode still produces trace:
>    ./perf record -e cs_etm// -a
> 
> * Invalid -C options still fail to open:
>    ./perf record -C 2,3 -e cs_etm/@..._etf0/
>    failed to mmap with 12 (Cannot allocate memory)
> 
> * Migrating a task to a valid sink/CPU now produces trace:
>    taskset --cpu-list 0 ./perf record -e cs_etm/@..._etf1/ --per-thread -- taskset --cpu-list 2 ls
> 
> * If the task remains on an invalid CPU, no trace is emitted:
>    taskset --cpu-list 0 ./perf record -e cs_etm/@..._etf1/ --per-thread -- ls
> 
> Reviewed-by: Mathieu Poirier <mathieu.poirier@...aro.org>
> Reviewed-by: Suzuki K Poulose <suzuki.poulose@....com>
> Signed-off-by: James Clark <james.clark@....com>

Thanks James, I have queued this.

Suzuki