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Message-ID: <9de48555e9139c91289a48e9296c0af1a62811f3.camel@redhat.com>
Subject: Re: [PATCH 7/7] mm/page_alloc: Replace local_lock with normal
 spinlock
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>,
        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>
Date:   Mon, 04 Jul 2022 18:33:53 +0200
In-Reply-To: <20220624125423.6126-8-mgorman@techsingularity.net>
References: <20220624125423.6126-1-mgorman@techsingularity.net>
         <20220624125423.6126-8-mgorman@techsingularity.net>
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On Fri, 2022-06-24 at 13:54 +0100, Mel Gorman wrote:
> struct per_cpu_pages is no longer strictly local as PCP lists can be
> drained remotely using a lock for protection. While the use of local_lock
> works, it goes against the intent of local_lock which is for "pure
> CPU local concurrency control mechanisms and not suited for inter-CPU
> concurrency control" (Documentation/locking/locktypes.rst)
>=20
> local_lock protects against migration between when the percpu pointer is
> accessed and the pcp->lock acquired. The lock acquisition is a preemption
> point so in the worst case, a task could migrate to another NUMA node
> and accidentally allocate remote memory. The main requirement is to pin
> the task to a CPU that is suitable for PREEMPT_RT and !PREEMPT_RT.
>=20
> Replace local_lock with helpers that pin a task to a CPU, lookup the
> per-cpu structure and acquire the embedded lock. It's similar to local_lo=
ck
> without breaking the intent behind the API. It is not a complete API
> as only the parts needed for PCP-alloc are implemented but in theory,
> the generic helpers could be promoted to a general API if there was
> demand for an embedded lock within a per-cpu struct with a guarantee
> that the per-cpu structure locked matches the running CPU and cannot use
> get_cpu_var due to RT concerns. PCP requires these semantics to avoid
> accidentally allocating remote memory.
>=20
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> ---

Reviewed-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Tested-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>

Thanks!

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