Messages in this thread | | | Date | Mon, 2 Feb 2015 15:29:09 -0800 | From | Andrew Morton <> | Subject | Re: [RFC] change non-atomic bitops method |
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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?
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