Re: [PATCH 0/6] Drain remote per-cpu directly v3

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 13 May 2022 15:23:30 +0100 Mel Gorman <mgorman@techsingularity.net> wrote:

> Correct.
> 
> > > the draining in non-deterministic.
> > 
> > s/n/s/;)
> > 
> 
> Think that one is ok. At least spell check did not complain.

s/in/si/

> > > 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.
> > 
> > I don't understand the final sentence here.  Which CPU and why does
> > hot-removing it make the locking safe?
> > 
> 
> The sentence can be dropped because it adds little and is potentially
> confusing. The PCP being safe to access remotely is specific to the
> context of the CPU being hot-removed and there are other special corner
> cases like zone_pcp_disable that modifies a per-cpu structure remotely
> but not in a way that causes corruption.

OK.  I pasted in your para from the other email.  Current 0/n blurb:

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 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.