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Subject[RFC PATCH 0/6] Drain remote per-cpu directly
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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 be cause a real-time
task runnning on a NOHZ_FULL CPU to miss a deadline and at minimum,
the draining in 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. The locking is inherently unsafe for remote access unless
the CPU is hot-removed.

This series adjusts the locking. A spin-lock is added to struct
per_cpu_pages to protect the list contents while local_lock_irq continues
to prevent migration and IRQ reentry. This allows a remote CPU to safely
drain a remote per-cpu list.

This series is a partial series. Follow-on work would allow the
local_irq_save to be converted to a local_irq to avoid IRQs being
disabled/enabled in most cases. However, there are enough corner cases
that it deserves a series on its own separated by one kernel release and
the priority right now is to avoid interference of high priority tasks.

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.

include/linux/mm_types.h | 5 +
include/linux/mmzone.h | 12 +-
mm/page_alloc.c | 333 ++++++++++++++++++++++++---------------
3 files changed, 222 insertions(+), 128 deletions(-)

--
2.34.1

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