Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:18 as permitted sender) client-ip=2620:137:e000::1:18;
Message-ID: <09fbcf0884ef5e03812566b9576979253e506ce9.camel@redhat.com>
Subject: Re: [PATCH 0/6] Drain remote per-cpu directly v3
From:   Nicolas Saenz Julienne <nsaenzju@redhat.com>
To:     Mel Gorman <mgorman@techsingularity.net>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     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>
Date:   Wed, 18 May 2022 20:01:07 +0200
In-Reply-To: <20220512085043.5234-1-mgorman@techsingularity.net>
References: <20220512085043.5234-1-mgorman@techsingularity.net>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
User-Agent: Evolution 3.44.1 (3.44.1-1.fc36) 
MIME-Version: 1.0
Precedence: bulk

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
>=20
> Changelog since v1
> o Fix unsafe RT locking scheme
> o Use spin_trylock on UP PREEMPT_RT
>=20
> 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.
>=20
> Currently an IRQ-safe local_lock protects the page allocator per-cpu list=
s.
> The local_lock on its own prevents migration and the IRQ disabling protec=
ts
> 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.
>=20
> 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.
>=20
> 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 comment=
s
> 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@redhat.com>

Thanks,

--=20
Nicol=C3=A1s S=C3=A1enz