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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Mon, 25 Apr 2022 19:49:46 -0700
Message-ID: <CAJuCfpH=U9E-46wZgKEYQZVKjw=2qq_KXDRF=tqDBvqpR+BBbg@mail.gmail.com>
Subject: Re: [RFC PATCH 0/6] Drain remote per-cpu directly
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Nicolas Saenz Julienne <nsaenzju@redhat.com>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Michal Hocko <mhocko@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>
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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.
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
>
>