linux-kernel - Re: [PATCH 7/7] mm/page_alloc: Replace local

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Message-ID: <ae15b692-bf68-51a8-1796-7ba15e156d2d@suse.cz>
Date:   Mon, 4 Jul 2022 16:39:57 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Hugh Dickins <hughd@...gle.com>, Yu Zhao <yuzhao@...gle.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 7/7] mm/page_alloc: Replace local_lock with normal
 spinlock

On 6/24/22 14:54, 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)
> 
> 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.
> 
> 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_lock
> 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.
> 
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

-fix looks OK too