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Message-ID: <09fbcf0884ef5e03812566b9576979253e506ce9.camel@redhat.com>
Date: Wed, 18 May 2022 20:01:07 +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
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.
FWIW tested this against our RT+nohz_full workloads. I can have another go if
the locking scheme changes.
Tested-by: Nicolas Saenz Julienne <nsaenzju@...hat.com>
Thanks,
--
Nicolás Sáenz
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