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

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Message-Id: <20220512124325.751781bb88ceef5c37ca653e@linux-foundation.org>
Date:   Thu, 12 May 2022 12:43:25 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        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

On Thu, 12 May 2022 09:50:37 +0100 Mel Gorman <mgorman@...hsingularity.net> 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

s/may be/may/

> task runnning on a NOHZ_FULL CPU to miss a deadline and at minimum,

s/nnn/nn/

> the draining in non-deterministic.

s/n/s/;)

> 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?

> 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.