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

[Suren Baghdasaryan <surenb@google.com>]

On Mon, Apr 25, 2022 at 7:49 PM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Wed, Apr 20, 2022 at 2:59 AM Mel Gorman <mgorman@techsingularity.net> 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.
> >
> > 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.
>
> This quite possibly solves the issue I was trying to fix in
> https://lore.kernel.org/all/20220225012819.1807147-1-surenb@google.com.
> I will give it a try and see how it looks.

My test shows sizable improvement for the worst case drain_all_pages
duration. Before the change I caught cases when a drain_local_pages_wq
in the workqueue was delayed by 100+ms (not even counting
drain_local_pages_wq execution time itself). With this patchset the
worst time I was able to record for drain_all_pages duration was 17ms.

> Thanks!
>
> >
> >  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
> >
> >