linux-kernel - Re: [PATCH 2/4] nohz: Synchronize sleep time stats with seqlock

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Message-ID: <20130821183157.GE31370@twins.programming.kicks-ass.net>
Date:	Wed, 21 Aug 2013 20:31:57 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	Fernando Luis Vázquez Cao 
	<fernando_b1@....ntt.co.jp>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/4] nohz: Synchronize sleep time stats with seqlock

On Wed, Aug 21, 2013 at 07:09:27PM +0200, Oleg Nesterov wrote:
> So, unlike me, you like -02 more than -Os ;)

I haven't checked the actual flags they enable in a while, but I think I
prefer something in the middle.

Esp. -freorder-blocks and the various -falign flags are something you
really want with -Os.

> > +static inline long schedule_timeout(long timeout)
> > +{
> > +	if (timeout == MAX_SCHEDULE_TIMEOUT) {
> > +		schedule();
> > +		return timeout;
> > +	}
> > +	return __schedule_timeout(timeout);
> > +}
> 
> Well this means that every caller will do the MAX_SCHEDULE_TIMEOUT
> check inline, and this case is unlikely. 

OK, so do not remove the MAX_SCHEDULE_TIMEOUT check from
__schedule_timeout() and change the above to:

static __always_inline long schedule_timeout(long timeout)
{
	if (__builtin_constant_p(timeout) && timeout == MAX_SCHEDULE_TIMEOUT) {
		schedule();
		return timeout;
	}
	return __schedule_timeout(timeout);
}

That should avoid extra code generation for the runtime sites while
still allowing what we set out to do.
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