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Michael S. Tsirkin 450cbdd012 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>
2017-11-10 13:43:44 +01:00
..
main.c ringtest: support test specific parameters 2017-05-09 16:43:23 +03:00
main.h locking/x86: Use LOCK ADD for smp_mb() instead of MFENCE 2017-11-10 13:43:44 +01:00
Makefile License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
noring.c License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
ptr_ring.c License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
README tools/virtio/ringtest: add usage example to README 2016-06-06 13:00:11 +03:00
ring.c ringtest: commonize implementation of poll_avail/poll_used 2016-10-31 00:21:43 +02:00
run-on-all.sh License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
virtio_ring_0_9.c ringtest: commonize implementation of poll_avail/poll_used 2016-10-31 00:21:43 +02:00
virtio_ring_inorder.c
virtio_ring_poll.c

README 

Partial implementation of various ring layouts, useful to tune virtio design.
Uses shared memory heavily.

Typical use:

# sh run-on-all.sh perf stat -r 10 --log-fd 1 -- ./ring