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Date:   Sun, 3 Jul 2022 16:28:21 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
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>,
        Hugh Dickins <hughd@google.com>, Yu Zhao <yuzhao@google.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>
Subject: Re: [PATCH v5 00/7] Drain remote per-cpu directly
Message-Id: <20220703162821.f097d6e4b3e2a0114820a8d9@linux-foundation.org>
In-Reply-To: <20220624125423.6126-1-mgorman@techsingularity.net>
References: <20220624125423.6126-1-mgorman@techsingularity.net>
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On Fri, 24 Jun 2022 13:54:16 +0100 Mel Gorman <mgorman@techsingularity.net> wrote:

> Some setups, notably NOHZ_FULL CPUs, may be running realtime or
> latency-sensitive applications that cannot tolerate interference due to
> per-cpu drain work queued by __drain_all_pages().  Introduce a new
> mechanism to remotely drain the per-cpu lists. It is made possible by
> remotely locking 'struct per_cpu_pages' new per-cpu spinlocks.  This has
> two advantages, the time to drain is more predictable and other unrelated
> tasks are not interrupted.
> 
> 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 cause a real-time task running
> on a NOHZ_FULL CPU to miss a deadline and at minimum, the draining is
> 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.
> 
> This series adjusts the locking.  A spinlock is added to struct
> per_cpu_pages to protect the list contents while local_lock_irq is
> ultimately replaced by just the spinlock in the final patch.  This allows
> a remote CPU to safely. Follow-on work should allow the spin_lock_irqsave
> to be converted to spin_lock to avoid IRQs being disabled/enabled in
> most cases. The follow-on patch will be one kernel release later as it
> is relatively high risk and it'll make bisections more clear if there
> are any problems.

I plan to move this and Mel's fix to [7/7] into mm-stable around July 8.