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List:       linux-rt-users
Subject:    problem using trace-cmd and trace_marker when using only one core
From:       "Jürgen_Lanner" <jue.lanner () gmx ! at>
Date:       2014-12-01 13:22:46
Message-ID: trinity-fa84f03b-d353-462e-aae6-5a5e481afcec-1417440165730 () 3capp-gmx-bs11
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Ladies and gentlemen,

I found some odd behaviour when using trace-cmd together with trace_marker.

I try to use trace-cmd to trace scheduling events to find out latencies on my box.
    trace-cmd record -e 'sched_wakeup*' -e sched_switch

In oder to synchronize the trace I use echoing to /sys/kernel/debug/tracing/trace_marker
    echo Hallo > /sys/kernel/debug/tracing/trace_marker


This works fine so far.

My task in observation is set to run only on one core (on a quad core cpu) 
so I tried to narrow down the focus of tracing (and the data amount) using the -M option:
    trace-cmd record -M 2 -e 'sched_wakeup*' -e sched_switch

But from this point on I am no longer able to write to the trace_marker, instead I get this response:
    bash: echo: write error: Bad file descriptor

Only reboot solves the problem.

Is this some known or even intended behaviour? Or do I use it in some weird way as described?
Maybe I understood something wrong and you can give me a hint how to overcome the obstacle.

Thanks a lot

Juergen
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