Re: [RFC] change non-atomic bitops method

Russell King - ARM Linux
2015-02-02T23:31:47Z

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@sonymobile.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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