[PATCH v1 0/3] Lockless SMP function call and TLB flushing

[Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>]

Hi,

This series implements lockless SMP function call and then rewrites x86 TLB
flushing to use SMP function calls.

We have observed that the TLB flush lock can be a point of contention for
certain workloads, e.g. migrating 10 VMs off a host during a host evacuation.

Performance numbers:

I wrote a synthetic benchmark to measure the performance. The benchmark has one
or more CPUs in Xen calling on_selected_cpus() with between 1 and 64 CPUs in
the selected mask. The executed function simply delays for 500 microseconds.

The table below shows the % change in execution time of on_selected_cpus():

                  1 thread   2 threads    4 threads
1 CPU in mask     0.02       -35.23       -51.18
2 CPUs in mask    0.01       -47.20       -69.27
4 CPUs in mask    -0.02      -42.40       -66.55
8 CPUs in mask    -0.03      -47.82       -68.39
16 CPUs in mask   0.12       -41.95       -58.26
32 CPUs in mask   0.02       -25.43       -39.35
64 CPUs in mask   0.00       -24.70       -37.83

With 1 thread (i.e. no contention), there is no regression in execution time.
With multiple threads, as expected there is a significant improvement in
execution time.

As a more practical benchmark to simulate host evacuation, I measured the
memory dirtying rate across 10 VMs after enabling log dirty (on an AMD system,
so without PML). The rate increased by 16% with this patch series, even
after the recent deferred TLB flush changes.

FWIW, my first attempt at this was to port the SMP call functionality from
Linux. I found it didn't scale well as the number of CPUs in the mask
increases so I've taken a different approach here.

Thanks,
Ross

Ross Lagerwall (3):
  x86/hap: Wait for remote CPUs during TLB flush
  xen/smp: Rewrite on_selected_cpus() to be lockless
  x86/smp: Rewrite TLB flush using on_selected_cpus()

 tools/xentrace/xenalyze.c              |   2 -
 xen/arch/x86/include/asm/irq-vectors.h |   1 -
 xen/arch/x86/include/asm/irq.h         |   1 -
 xen/arch/x86/mm/hap/hap.c              |   2 +-
 xen/arch/x86/smp.c                     |  30 ++++----
 xen/arch/x86/smpboot.c                 |   1 -
 xen/common/smp.c                       | 101 ++++++++++++++++---------
 7 files changed, 80 insertions(+), 58 deletions(-)

-- 
2.53.0