linux-kernel - Re: perf: negative time

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Date:   Fri, 17 Mar 2017 10:37:48 -0400 (EDT)
From:   Vince Weaver <vincent.weaver@...ne.edu>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: perf: negative time_enabled issue

On Tue, 14 Mar 2017, Vince Weaver wrote:

> So another obscure rdpmc problem, turned up by the PAPI regression tests.
> 
> If you use rdpmc, and your event is attached measuring another process,
> and then you ENABLE/DISABLE/ENABLE/rdpmc the event
> 
> somehow the userpg->time_enabled field gets a weird value and that 
> completely confuses code expecting enabled!=running to mean multiplexing 
> is happening.

Still working on this issue.  It turns out it's not rdpmc specific, you 
can see it with a regular read() too.

So far I've traced it back far enough to see that shadow_ctx_time is 
ending up negative for some reason if you do an enable/disable/enable on 
an event attached to another process.  I'm trying to trace it back further 
but it's a slow process.

Vince