Drain remote per-cpu directly v2

Changelog since v1
o Fix unsafe RT locking scheme
o Use spin_trylock on UP PREEMPT_RT

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 spinlock 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 should allow the
local_irq_save to be converted to a local_irq to avoid IRQs being
disabled/enabled in most cases. Consequently, there are some TODO comments
highlighting the places that would change if local_irq was used. 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          | 342 +++++++++++++++++++++++++--------------
 3 files changed, 230 insertions(+), 129 deletions(-)

-- 
2.34.1