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

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Message-ID: <9b04479e3810dbbc3cb2ebd68a3ab3fd034e125e.camel@redhat.com>
Date:   Fri, 13 May 2022 14:24:47 +0200
From:   Nicolas Saenz Julienne <nsaenzju@...hat.com>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Marcelo Tosatti <mtosatti@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 0/6] Drain remote per-cpu directly v3

Hi Mel,

On Thu, 2022-05-12 at 09:50 +0100, Mel Gorman wrote:
> Changelog since v2
> o More conversions from page->lru to page->[pcp_list|buddy_list]
> o Additional test results in changelogs
> 
> 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          | 348 +++++++++++++++++++++++++------------
> --

Thanks for the series!

I'm testing the series ATM, both with on vanilla and RT kernels. I'll have the
results by Monday.

Regards,

-- 
Nicolás Sáenz