linux-kernel - Re: [PATCH v2] sched: Enabled schedstat when schedstat tracepoints are enabled

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Message-ID: <20170413100118.rggxbhlgluynysmp@suse.de>
Date:   Thu, 13 Apr 2017 11:01:19 +0100
From:   Mel Gorman <mgorman@...e.de>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2] sched: Enabled schedstat when schedstat tracepoints
 are enabled

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.
> 
> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

You're right, I wasn't aware. Patch looks ok. When it triggers, there
will be a short interval of garbage numbers but that's the same as what
happens currently. It's not deliberate action any more that would flag
to the user that there is a hazard but I think it'll be manageable.

Acked-by: Mel Gorman <mgorman@...e.de>

Thanks Steven.

-- 
Mel Gorman
SUSE Labs