Messages in this thread | | | Date | Mon, 2 Nov 2015 12:15:35 -0800 | From | Davidlohr Bueso <> | Subject | Re: [PATCH 3/4] x86,asm: Re-work smp_store_mb() |
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On Tue, 27 Oct 2015, Peter Zijlstra wrote:
>On Wed, Oct 28, 2015 at 06:33:56AM +0900, Linus Torvalds wrote: >> On Wed, Oct 28, 2015 at 4:53 AM, Davidlohr Bueso <dave@stgolabs.net> wrote: >> > >> > Note that this might affect callers that could/would rely on the >> > atomicity semantics, but there are no guarantees of that for >> > smp_store_mb() mentioned anywhere, plus most archs use this anyway. >> > Thus we continue to be consistent with the memory-barriers.txt file, >> > and more importantly, maintain the semantics of the smp_ nature. >> > >> So with this patch, the whole thing becomes pointless, I feel. (Ok, so >> it may have been pointless before too, but at least before this patch >> it generated special code, now it doesn't). So why carry it along at >> all? > >So I suppose this boils down to if: XCHG ends up being cheaper than >MOV+FENCE.
So I ran some experiments on an IvyBridge (2.8GHz) and the cost of XCHG is constantly cheaper (by at least half the latency) than MFENCE. While there was a decent amount of variation, this difference remained rather constant.
Then again, I'm not sure this matters. Thoughts?
Thanks, Davidlohr
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