[PATCH 0/11 v2] Use local_lock for pcp protection and reduce stat overhead

[Mel Gorman]

For MM people, the whole series is relevant but patch 3 needs particular
attention for memory hotremove as I had problems testing it because full
zone removal always failed for me. For RT people, the most interesting
patches are 2, 9 and 10 with 2 being the most important.

This series requires patches in Andrew's tree so for convenience, it's also available at

git://git.kernel.org/pub/scm/linux/kernel/git/mel/linux.git mm-percpu-local_lock-v2r10

The PCP (per-cpu page allocator in page_alloc.c) shares locking
requirements with vmstat and the zone lock which is inconvenient and
causes some issues. For example, the PCP list and vmstat share the same
per-cpu space meaning that it's possible that vmstat updates dirty cache
lines holding per-cpu lists across CPUs unless padding is used.  Second,
PREEMPT_RT does not want IRQs disabled in the page allocator because it's
too long for IRQs to be disabled unnecesarily.

This series splits the locking requirements and uses locks types more
suitable for PREEMPT_RT, reduces the time when special locking is required
for stats and reduces the time when IRQs need to be disabled on !PREEMPT_RT
kernels.

Why local_lock? PREEMPT_RT considers the following sequence to be unsafe
as documented in Documentation/locking/locktypes.rst

   local_irq_disable();
   raw_spin_lock(&lock);

The page allocator does not use raw_spin_lock but using local_irq_safe
is undesirable on PREEMPT_RT as it leaves IRQs disabled for an excessive
length of time. By converting to local_lock which disables migration on
PREEMPT_RT, the locking requirements can be separated and start moving
the protections for PCP, stats and the zone lock to PREEMPT_RT-safe
equivalent locking. As a bonus, local_lock also means that PROVE_LOCKING
does something useful.

After that, it was very obvious that zone_statistics in particular has
way too much overhead and leaves IRQs disabled for longer than necessary
on !PREEMPT_RT kernels. zone_statistics uses perfectly accurate counters
requiring IRQs be disabled for parallel RMW sequences when inaccurate ones
like vm_events would do. The series makes the NUMA statistics (NUMA_HIT
and friends) inaccurate counters that then require no special protection
on !PREEMPT_RT.

The bulk page allocator can then do stat updates in bulk with IRQs enabled
which should improve the efficiency.  Technically, this could have been
done without the local_lock and vmstat conversion work and the order
simply reflects the timing of when different series were implemented.

Finally, there are places where we conflate IRQs being disabled for the
PCP with the IRQ-safe zone spinlock. The remainder of the series reduces
the scope of what is protected by disabled IRQs on !PREEMPT_RT kernels.
By the end of the series, page_alloc.c does not call local_irq_save so
the locking scope is a bit clearer. The one exception is that modifying
NR_FREE_PAGES still happens in places where it's known the IRQs are
disabled as it's harmless for PREEMPT_RT and would be expensive to split
the locking there.

No performance data is included because despite the overhead of the stats,
it's within the noise for most workloads on !PREEMPT_RT. However, Jesper
Dangaard Brouer ran a page allocation microbenchmark on a E5-1650 v4 @
3.60GHz CPU on the first version of this series. Focusing on the array
variant of the bulk page allocator reveals the following.

(CPU: Intel(R) Xeon(R) CPU E5-1650 v4 @ 3.60GHz)
ARRAY variant: time_bulk_page_alloc_free_array: step=bulk size

         Baseline        Patched
 1       56.383          54.225 (+3.83%)
 2       40.047          35.492 (+11.38%)
 3       37.339          32.643 (+12.58%)
 4       35.578          30.992 (+12.89%)
 8       33.592          29.606 (+11.87%)
 16      32.362          28.532 (+11.85%)
 32      31.476          27.728 (+11.91%)
 64      30.633          27.252 (+11.04%)
 128     30.596          27.090 (+11.46%)

While this is a positive outcome, the series is more likely to be
interesting to the RT people in terms of getting parts of the PREEMPT_RT
tree into mainline.

 drivers/base/node.c    |  18 +--
 include/linux/mmzone.h |  29 ++--
 include/linux/vmstat.h |  65 +++++----
 mm/internal.h          |   2 +-
 mm/memory_hotplug.c    |  10 +-
 mm/mempolicy.c         |   2 +-
 mm/page_alloc.c        | 297 ++++++++++++++++++++++++-----------------
 mm/vmstat.c            | 250 ++++++++++++----------------------
 8 files changed, 339 insertions(+), 334 deletions(-)

-- 
2.26.2