Re: [LKP] [mm] 9bc8039e71: will-it-scale.per_thread_ops -64.1% regression

[Linus Torvalds]

On Mon, Nov 5, 2018 at 10:28 AM Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> 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