Re: [RFC] change non-atomic bitops method
From: Russell King - ARM Linux <hidden>
Date: 2015-02-02 23:31:53
Also in:
linux-arm-kernel, lkml
On Mon, Feb 02, 2015 at 03:29:09PM -0800, Andrew Morton wrote:
On Mon, 2 Feb 2015 11:55:03 +0800 "Wang, Yalin" [off-list ref] wrote:quoted
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.