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Date: Thu, 13 Apr 2017 11:00:05 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Mel Gorman <mgorman@suse.de>, LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Matt Fleming <matt@codeblueprint.co.uk>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Subject: Re: [PATCH v2] sched: Enabled schedstat when schedstat tracepoints
 are enabled
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On Wed, Apr 12, 2017 at 10:56:07PM -0400, Steven Rostedt wrote:
> From: Steven Rostedt (VMware) <rostedt@goodmis.org>
> 
> During my tests, I see this in my dmesg:
> 
> "Scheduler tracepoints stat_sleep, stat_iowait, stat_blocked and
> stat_runtime require the kernel parameter schedstats=enabled or
> kernel.sched_schedstats=1"
> 
> And found the commit:
> 
> cb2517653fc ("sched/debug: Make schedstats a runtime tunable that is
> disabled by default")
> 
> Which states:
> 
> "For tracepoints, there is a simple warning as it's not safe to activate
>  schedstats in the context when it's known the tracepoint may be wanted
>  but is unavailable."
> 
> I'm assuming that Mel did not know about the TRACE_EVENT_FN() and
> DEFINE_EVENT_FN() that allow for callbacks for tracepoints as they are
> enabled and disabled. I do not see any reason for not enabling
> schedstat when one of its tracepoints are enabled.
> 
> The state of schedstat is saved when the first tracepoint is enabled,
> and that state is put back when the tracepoints are disabled.

There is one additional complication with all this.

Dynamically enabling the sched_stats like this doesn't guarantee correct
information. So you've now taken away the informational print and
silently generate bollocks numbers.