[PATCH v4 00/7] Drain remote per-cpu directly

Mel Gorman
2022-06-13T17:33:51Z

This replaces the existing version on mm-unstable. The biggest difference
is replacing local_lock entirely which is the last patch. The other changes
are minor fixes reported by Hugh and Vlastimil.

Changelog since v3
o Checkpatch fixes from mm-unstable (akpm)
o Replace local_lock with spinlock (akpm)
o Remove IRQ-disabled check in free_unref_page_list as it triggers
a false positive (hughd)
o Take an unlikely check out of the rmqueue fast path (vbabka)

Some setups, notably NOHZ_FULL CPUs, may be running realtime or
latency-sensitive applications that cannot tolerate interference due to
per-cpu drain work queued by __drain_all_pages(). Introduce a new
mechanism to remotely drain the per-cpu lists. It is made possible by
remotely locking 'struct per_cpu_pages' new per-cpu spinlocks. This has
two advantages, the time to drain is more predictable and other unrelated
tasks are not interrupted.

This series has the same intent as Nicolas' series "mm/page_alloc: Remote
per-cpu lists drain support" -- avoid interference of a high priority task
due to a workqueue item draining per-cpu page lists. While many workloads
can tolerate a brief interruption, it may cause a real-time task running
on a NOHZ_FULL CPU to miss a deadline and at minimum, the draining is
non-deterministic.

Currently an IRQ-safe local_lock protects the page allocator per-cpu
lists. The local_lock on its own prevents migration and the IRQ disabling
protects from corruption due to an interrupt arriving while a page
allocation is in progress.

This series adjusts the locking. A spinlock is added to struct
per_cpu_pages to protect the list contents while local_lock_irq is
ultimately replaced by just the spinlock in the final patch. This allows
a remote CPU to safely. Follow-on work should allow the local_irq_save
to be converted to a local_irq to avoid IRQs being disabled/enabled in
most cases.

Patch 1 is a cosmetic patch to clarify when page->lru is storing buddy pages
and when it is storing per-cpu pages.

Patch 2 shrinks per_cpu_pages to make room for a spin lock. Strictly speaking
this is not necessary but it avoids per_cpu_pages consuming another
cache line.

Patch 3 is a preparation patch to avoid code duplication.

Patch 4 is a simple micro-optimisation that improves code flow necessary for
a later patch to avoid code duplication.

Patch 5 uses a spin_lock to protect the per_cpu_pages contents while still
relying on local_lock to prevent migration, stabilise the pcp
lookup and prevent IRQ reentrancy.

Patch 6 remote drains per-cpu pages directly instead of using a workqueue.

Patch 7 uses a normal spinlock instead of local_lock for remote draining

include/linux/mm_types.h | 5 +
include/linux/mmzone.h | 12 +-
mm/page_alloc.c | 404 ++++++++++++++++++++++++---------------
3 files changed, 266 insertions(+), 155 deletions(-)

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2.35.3