Use local_lock for pcp protection and reduce stat overhead

Changelog since v5
o Rebase 5.13-rc1 on top of Hugh's changes to mm/vmstat.c which means the series no longer has pre-requisites from Andrew's tree
o Not mentioned in changelogs but Sebastian (cc'd) indicated it passed tests replacing the local_lock implementation in the preempt-rt tree

Changelog since v4
o Dropped local_lock embed patch due to complexity
o Fix !NUMA build
o Avoid adding pages with mt >= MIGRATE_PCPTYPES to non-existant per-cpu list

Changelog since v3
o Preserve NUMA_* counters after CPU hotplug
o Drop "mm/page_alloc: Remove duplicate checks if migratetype should be isolated"
o Add micro-optimisation tracking PFN during free_unref_page_list
o Add Acks

Changelog since v2
o Fix zonestats initialisation
o Merged memory hotplug fix separately
o Embed local_lock within per_cpu_pages

This series has no pre-requisites from Andrew's tree any more but is also available at

git://git.kernel.org/pub/scm/linux/kernel/git/mel/linux.git mm-percpu-local_lock-v6r4

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 to disable IRQs for too long in the page allocator.

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();
   spin_lock(&lock);

The pcp allocator has this sequence for rmqueue_pcplist (local_irq_save) -> __rmqueue_pcplist -> rmqueue_bulk (spin_lock). While it's possible to separate this out, it generally means there are points where we enable IRQs and reenable them again immediately. To prevent a migration and the per-cpu pointer going stale, migrate_disable is also needed. That is a custom lock that is similar, but worse, than local_lock. Furthermore, on PREEMPT_RT, it's undesirable to leave IRQs disabled for too long. 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's obvious that zone_statistics incurs 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 |  31 +++--
 include/linux/vmstat.h |  65 ++++++-----
 mm/mempolicy.c         |   2 +-
 mm/page_alloc.c        | 255 ++++++++++++++++++++++++-----------------
 mm/vmstat.c            | 246 +++++++++++++++++----------------------
 6 files changed, 324 insertions(+), 293 deletions(-)

2.26.2