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

[Minchan Kim]

On Wed, Apr 20, 2022 at 10:59:00AM +0100, Mel Gorman wrote:
> 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.

Yeah, the non-deterministic is a problem. I saw the kworker-based draining
takes 100+ms(up to 300ms observed) sometimes in alloc_contig_range if CPUs
are heavily loaded.

I am not sure Nicolas already observed. it's not only problem of
per_cpu_pages but it is also lru_pvecs (pagevec) draining.
Do we need to introduce similar(allow remote drainning with spin_lock)
solution for pagevec?

> 
> 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
> 
>