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Date:   Thu, 13 Apr 2017 10:08:10 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mel Gorman <mgorman@...e.de>, LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v2] sched: Enabled schedstat when schedstat tracepoints
 are enabled

On Thu, 13 Apr 2017 11:01:24 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, Apr 13, 2017 at 11:00:05AM +0200, Peter Zijlstra wrote:
> > On Wed, Apr 12, 2017 at 10:56:07PM -0400, Steven Rostedt wrote:  
> > > From: Steven Rostedt (VMware) <rostedt@...dmis.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.  
> 
> Josh has a patch like:
> 
>   http://lkml.kernel.org/r/00e8805cc094657d5ccb20bb88e0d2ce1cbb92e1.1466184592.git.jpoimboe@redhat.com
> 
> That tries to keep the few stats required for the tracepoint active at
> all times. But I never managed to get performance overhead to 0.

Interesting. I may take a look at that patch too. What benchmark did
you use to measure performance?

Thanks!

-- Steve

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