Messages in this thread | | | Date | Thu, 11 Oct 2018 11:21:10 -0700 | From | Andres Freund <> | Subject | Re: [PATCH v2 0/3] x86: faster mb()+other barrier.h tweaks |
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Hi,
On 2018-10-11 14:11:42 -0400, Michael S. Tsirkin wrote: > On Thu, Oct 11, 2018 at 10:37:07AM -0700, Andres Freund wrote: > > Hi, > > > > On 2016-01-26 10:20:14 +0200, Michael S. Tsirkin wrote: > > > On Tue, Jan 12, 2016 at 02:25:24PM -0800, H. Peter Anvin wrote: > > > > On 01/12/16 14:10, Michael S. Tsirkin wrote: > > > > > mb() typically uses mfence on modern x86, but a micro-benchmark shows that it's > > > > > 2 to 3 times slower than lock; addl $0,(%%e/rsp) that we use on older CPUs. > > > > > > > > > > So let's use the locked variant everywhere - helps keep the code simple as > > > > > well. > > > > > > > > > > While I was at it, I found some inconsistencies in comments in > > > > > arch/x86/include/asm/barrier.h > > > > > > > > > > I hope I'm not splitting this up too much - the reason is I wanted to isolate > > > > > the code changes (that people might want to test for performance) from comment > > > > > changes approved by Linus, from (so far unreviewed) comment change I came up > > > > > with myself. > > > > > > > > > > Lightly tested on my system. > > > > > > > > > > Michael S. Tsirkin (3): > > > > > x86: drop mfence in favor of lock+addl > > > > > x86: drop a comment left over from X86_OOSTORE > > > > > x86: tweak the comment about use of wmb for IO > > > > > > > > > > > > > I would like to get feedback from the hardware team about the > > > > implications of this change, first. > > > > > Any luck getting some feedback on this one? > > > > Ping? I just saw a bunch of kernel fences in a benchmark, making me > > wonder why linux uses mfence rather than lock addl. Leading me to this > > thread. > > > > Greetings, > > > > Andres Freund > > It doesn't do it for smp_mb any longer: > > commit 450cbdd0125cfa5d7bbf9e2a6b6961cc48d29730 > Author: Michael S. Tsirkin <mst@redhat.com> > Date: Fri Oct 27 19:14:31 2017 +0300 > > locking/x86: Use LOCK ADD for smp_mb() instead of MFENCE
Ooh, missed that one.
> I didn't bother with mb() since I didn't think it's performance > critical, and one needs to worry about drivers possibly doing > non-temporals etc which do need mfence. > > Do you see mb() in a benchmark then?
No it was a smp_mp(). It was on an older kernel (was profiling postgres on hardware I have limited control over,not the kernel. Just noticed the barrier while looking at perf output). I quickly looked into a current arch/x86/include/asm/barrier.h and still saw mfences, and then found this thread. Should have looked more carefully.
Sorry for the noise, and thanks for the quick answer.
- Andres
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