linux-kernel - Re: RFC: futex_wait() can DoS the tick

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Message-ID: <1434022875.4051.5.camel@gmail.com>
Date:	Thu, 11 Jun 2015 13:41:15 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: RFC: futex_wait() can DoS the tick

On Thu, 2015-06-11 at 10:34 +0200, Thomas Gleixner wrote:

> Can you try the following, please?
> 
> Enable function tracer and hrtimer events manually. Then watch the irq
> count on cpu3. If it stalls or becomes slow, then stop the trace with
> 
>       echo 0 >/sys/kernel/debug/tracing/tracing_on
> 
> If the overhead of the function tracer hides the problem, then try just
> with hrtimer, sched_switch and irq events.

Yeah, function tracer hides it.  After some fiddling with buffer size to
be able to ever see a tick, I finally have a trace trimmed with a tick
on each end, 175ms gap between them.  It's still 1.5 after xz, sending
off list.

	-Mike


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