linux-kernel - Re: [PATCH 1/8] mm: cma: introduce cma_release

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Date:   Mon, 29 Mar 2021 09:46:42 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Roman Gushchin <guro@...com>, Minchan Kim <minchan@...nel.org>,
        David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, 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>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH 1/8] mm: cma: introduce cma_release_nowait()

On Fri 26-03-21 14:32:01, Mike Kravetz wrote:
[...]
> - Just change the mutex to an irq safe spinlock.

Yes please.

>   AFAICT, the potential
>   downsides could be:
>   - Interrupts disabled during long bitmap scans

How large those bitmaps are in practice?

>   - Wasted cpu cycles (spinning) if there is much contention on lock
>   Both of these would be more of an issue on small/embedded systems. I
>   took a quick look at the callers of cma_alloc/cma_release and nothing
>   stood out that could lead to high degrees of contention.  However, I
>   could have missed something.

If this is really a practical concern then we can try a more complex
solution based on some data.

-- 
Michal Hocko
SUSE Labs