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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 5 Nov 2018 10:35:31 -0800
Message-ID: <CAHk-=whr2Aio3R49TVWqW3es6heyxXDuxGHcv8Bcc=_kw4vDeQ@mail.gmail.com>
Subject: Re: [LKP] [mm] 9bc8039e71: will-it-scale.per_thread_ops -64.1% regression
To:     yang.shi@linux.alibaba.com
Cc:     rong.a.chen@intel.com, vbabka@suse.cz,
        kirill.shutemov@linux.intel.com, mhocko@kernel.org,
        Matthew Wilcox <willy@infradead.org>,
        ldufour@linux.vnet.ibm.com,
        Andrew Morton <akpm@linux-foundation.org>,
        Colin King <colin.king@canonical.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        lkp@01.org
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On Mon, Nov 5, 2018 at 10:28 AM Yang Shi <yang.shi@linux.alibaba.com> wrote:
>
> Actually, the commit is mainly for optimizing the long stall time caused
> by holding mmap_sem by write when unmapping or shrinking large mapping.
> It downgrades write mmap_sem to read when zapping pages. So, it looks
> the downgrade incurs more context switches. This is kind of expected.
>
> However, the test looks just shrink the mapping with one normal 4K page
> size. It sounds the overhead of context switches outpace the gain in
> this case at the first glance.

I'm not seeing why there should be a context switch in the first place.

Even if you have lots of concurrent brk() users, they should all block
exactly the same way as before (a write lock blocks against a write
lock, but it *also* blocks against a downgraded read lock).

So no, I don't want just some limit to hide this problem for that
particular test. There's something else going on.

                 Linus