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Message-ID: <aba43427-0f51-7eb9-fa73-6e55237c8ddb@redhat.com>
Date: Thu, 25 Mar 2021 10:56:38 +0100
From: David Hildenbrand <david@...hat.com>
To: Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Cc: Roman Gushchin <guro@...com>, Michal Hocko <mhocko@...e.com>,
Shakeel Butt <shakeelb@...gle.com>,
Oscar Salvador <osalvador@...e.de>,
Muchun Song <songmuchun@...edance.com>,
David Rientjes <rientjes@...gle.com>,
Miaohe Lin <linmiaohe@...wei.com>,
Peter Zijlstra <peterz@...radead.org>,
Matthew Wilcox <willy@...radead.org>,
HORIGUCHI NAOYA <naoya.horiguchi@....com>,
"Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
Waiman Long <longman@...hat.com>, Peter Xu <peterx@...hat.com>,
Mina Almasry <almasrymina@...gle.com>,
Hillf Danton <hdanton@...a.com>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/8] mm: cma: introduce cma_release_nowait()
On 25.03.21 01:28, Mike Kravetz wrote:
> From: Roman Gushchin <guro@...com>
>
> cma_release() has to lock the cma_lock mutex to clear the cma bitmap.
> It makes it a blocking function, which complicates its usage from
> non-blocking contexts. For instance, hugetlbfs code is temporarily
> dropping the hugetlb_lock spinlock to call cma_release().
>
> This patch introduces a non-blocking cma_release_nowait(), which
> postpones the cma bitmap clearance. It's done later from a work
> context. The first page in the cma allocation is used to store
> the work struct. Because CMA allocations and de-allocations are
> usually not that frequent, a single global workqueue is used.
>
> To make sure that subsequent cma_alloc() call will pass, cma_alloc()
> flushes the cma_release_wq workqueue. To avoid a performance
> regression in the case when only cma_release() is used, gate it
> by a per-cma area flag, which is set by the first call
> of cma_release_nowait().
>
> Signed-off-by: Roman Gushchin <guro@...com>
> [mike.kravetz@...cle.com: rebased to v5.12-rc3-mmotm-2021-03-17-22-24]
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
> ---
1. Is there a real reason this is a mutex and not a spin lock? It seems
to only protect the bitmap. Are bitmaps that huge that we spend a
significant amount of time in there?
Because I also read "Because CMA allocations and de-allocations are
usually not that frequent".
With a spinlock, you would no longer be sleeping, but obviously you
might end up waiting for the lock ;) Not sure if that would help.
2. IIUC, if we would do the clearing completely lockless and use atomic
bitmap ops instead, only cma_debug_show_areas() would see slight
inconsistencies. As long as the setting code (-> allocation code) holds
the lock, I think this should be fine (-> no double allocations).
(sorry if that has already been discussed)
--
Thanks,
David / dhildenb
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