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authorMichael S. Tsirkin <mst@redhat.com>2017-10-27 19:14:31 +0300
committerIngo Molnar <mingo@kernel.org>2017-11-10 13:43:44 +0100
commit450cbdd0125cfa5d7bbf9e2a6b6961cc48d29730 (patch)
tree16db34c6e5f3bed41e4e9c14fb028d2303eabe6f /drivers/edac
parentb04db8e19fc2e9131524dec43057c1b96d5ba3ba (diff)
locking/x86: Use LOCK ADD for smp_mb() instead of MFENCE
MFENCE appears to be way slower than a locked instruction - let's use LOCK ADD unconditionally, as we always did on old 32-bit. Performance testing results: perf stat -r 10 -- ./virtio_ring_0_9 --sleep --host-affinity 0 --guest-affinity 0 Before: 0.922565990 seconds time elapsed ( +- 1.15% ) After: 0.578667024 seconds time elapsed ( +- 1.21% ) i.e. about ~60% faster. Just poking at SP would be the most natural, but if we then read the value from SP, we get a false dependency which will slow us down. This was noted in this article: http://shipilev.net/blog/2014/on-the-fence-with-dependencies/ And is easy to reproduce by sticking a barrier in a small non-inline function. So let's use a negative offset - which avoids this problem since we build with the red zone disabled. For userspace, use an address just below the redzone. The one difference between LOCK ADD and MFENCE is that LOCK ADD does not affect CLFLUSH, previous patches converted all uses of CLFLUSH to call mb(), such that changes to smp_mb() won't affect it. Update mb/rmb/wmb() on 32-bit to use the negative offset, too, for consistency. As a follow-up, it might be worth considering switching users of CLFLUSH to another API (e.g. clflush_mb()?) - we will then be able to convert mb() to smp_mb() again. Also arguably, GCC should switch to use LOCK ADD for __sync_synchronize(). This might be worth pursuing separately. Suggested-by: Andy Lutomirski <luto@amacapital.net> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Acked-by: Peter Zijlstra <peterz@infradead.org> Acked-by: Thomas Gleixner <tglx@linutronix.de> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Andy Lutomirski <luto@kernel.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Brian Gerst <brgerst@gmail.com> Cc: Denys Vlasenko <dvlasenk@redhat.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Josh Poimboeuf <jpoimboe@redhat.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: qemu-devel@nongnu.org Cc: virtualization@lists.linux-foundation.org Link: http://lkml.kernel.org/r/1509118355-4890-1-git-send-email-mst@redhat.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
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