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@xxxxxxxxxxxxxx> 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... -- FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up according to speedtest.net. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html