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From:   Michel Lespinasse <walken@google.com>
Date:   Thu, 13 Aug 2020 02:53:20 -0700
Message-ID: <CANN689G0DkL-wpxMha=nyysPYG6LM3Aw7060k2xQTxTA4PAf-w@mail.gmail.com>
Subject: Re: [PATCH v2 0/2] Try to release mmap_lock temporarily in smaps_rollup
To:     Chinwen Chang <chinwen.chang@mediatek.com>
Cc:     Matthias Brugger <matthias.bgg@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Daniel Jordan <daniel.m.jordan@oracle.com>,
        Davidlohr Bueso <dbueso@suse.de>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Steven Price <steven.price@arm.com>,
        Song Liu <songliubraving@fb.com>,
        Jimmy Assarsson <jimmyassarsson@gmail.com>,
        Huang Ying <ying.huang@intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org, linux-fsdevel@vger.kernel.org,
        wsd_upstream@mediatek.com
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On Wed, Aug 12, 2020 at 7:14 PM Chinwen Chang
<chinwen.chang@mediatek.com> wrote:
> Recently, we have observed some janky issues caused by unpleasantly long
> contention on mmap_lock which is held by smaps_rollup when probing large
> processes. To address the problem, we let smaps_rollup detect if anyone
> wants to acquire mmap_lock for write attempts. If yes, just release the
> lock temporarily to ease the contention.
>
> smaps_rollup is a procfs interface which allows users to summarize the
> process's memory usage without the overhead of seq_* calls. Android uses it
> to sample the memory usage of various processes to balance its memory pool
> sizes. If no one wants to take the lock for write requests, smaps_rollup
> with this patch will behave like the original one.
>
> Although there are on-going mmap_lock optimizations like range-based locks,
> the lock applied to smaps_rollup would be the coarse one, which is hard to
> avoid the occurrence of aforementioned issues. So the detection and
> temporary release for write attempts on mmap_lock in smaps_rollup is still
> necessary.

I do not mind extending the mmap lock API as needed. However, in the
past I have tried adding rwsem_is_contended to mlock(), and later to
mm_populate() paths, and IIRC gotten pushback on it both times. I
don't feel strongly on this, but would prefer if someone else approved
the rwsem_is_contended() use case.

Couple related questions, how many VMAs are we looking at here ? Would
need_resched() be workable too ?

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.