linux-kernel - Re: [RFC] change non-atomic bitops method

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Date:	Mon, 2 Feb 2015 23:31:36 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"Wang, Yalin" <Yalin.Wang@...ymobile.com>,
	"'arnd@...db.de'" <arnd@...db.de>,
	"'linux-arch@...r.kernel.org'" <linux-arch@...r.kernel.org>,
	"'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
	"'linux-arm-kernel@...ts.infradead.org'" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC] change non-atomic bitops method

On Mon, Feb 02, 2015 at 03:29:09PM -0800, Andrew Morton wrote:
> On Mon, 2 Feb 2015 11:55:03 +0800 "Wang, Yalin" <Yalin.Wang@...ymobile.com> wrote:
> 
> > This patch change non-atomic bitops,
> > add a if() condition to test it, before set/clear the bit.
> > so that we don't need dirty the cache line, if this bit
> > have been set or clear. On SMP system, dirty cache line will
> > need invalidate other processors cache line, this will have
> > some impact on SMP systems.
> > 
> > --- a/include/asm-generic/bitops/non-atomic.h
> > +++ b/include/asm-generic/bitops/non-atomic.h
> > @@ -17,7 +17,9 @@ static inline void __set_bit(int nr, volatile unsigned long *addr)
> >  	unsigned long mask = BIT_MASK(nr);
> >  	unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
> >  
> > -	*p  |= mask;
> > +	if ((*p & mask) == 0)
> > +		*p  |= mask;
> > +
> >  }
> 
> hm, maybe.
> 
> It will speed up set_bit on an already-set bit.  But it will slow down
> set_bit on a not-set bit.  And the latter case is presumably much, much
> more common.
> 
> How do we know the patch is a net performance gain?

Yes, we do need to know the performance impact of changes like this -
as Laura said in her reply already...

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