linux-kernel - Re: [PATCH 2/3 v2] futex: avoid double wake up in futex

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Date:	Thu, 16 Apr 2015 11:19:41 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Davidlohr Bueso <dave@...olabs.net>
cc:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Darren Hart <darren@...art.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	fredrik.markstrom@...driver.com,
	Manfred Spraul <manfred@...orfullife.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [PATCH 2/3 v2] futex: avoid double wake up in futex_wake() on
 -RT

On Wed, 15 Apr 2015, Davidlohr Bueso wrote:
> On Sun, 2015-04-12 at 20:02 -0700, Davidlohr Bueso wrote:
> > Doing the wakeups while holding the lock is also a general performance
> > issue for futex_wake. The problem being dealing with spurious wakeups
> > (wacky drivers), which makes no difference wrt nr_wake.
> 
> So I did some measurements with the patch below (Cc'ing Arnaldo for
> perf-bench consideration, albeit probably still pretty crude) and by
> doing the lockless wakeups, on avg we reduce contending waking threads
> latency in about 2x for each thread, which indicates that overall
> speedup is based on the number of futex_wake'ers.
> 
> I guess now we have the code, the numbers. I go back to auditing drivers
> *sigh*. In any case any important core-code already deals with spurious
> wakeups (the last silly offender being sysv sems), so I'm really not
> _that_ concerned -- in fact, Peter, your patch to trigger them seems to
> not trigger any issues anymore. But perhaps its late and I'm in lala
> land.

OTOH, we have quite some other code in the kernel which can generate
spurious wakeups. Just look at signals.

CPU0				CPU1

T1 random_syscall()		
   schedule_interruptible()

				Send process wide signal, wake T1
				because its the first target
T2 do_stuff()
   handle_signal()
   schedule()

T1 Deal with the spurious wakeup
  
So any code which does not handle a spurious wakeup is broken
independent of the futex changes. So really nothing to worry about.

Thanks,

	tglx
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