Re: [PATCH 00/10] Latest numa/core release, v18

[Linus Torvalds]

On Fri, Nov 30, 2012 at 11:58 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> When pushed hard enough via threaded workloads (for example via the
> numa02 test) then the upstream page migration code in mm/migration.c
> becomes unscalable, resulting in lot of scheduling on the anon vma
> mutex and a subsequent drop in performance.

Ugh.

I wonder if migration really needs that thing to be a mutex? I may be
wrong, but the anon_vma lock only protects the actual rmap chains, and
migration only ever changes the pte *contents*, not the actual chains
of pte's themselves, right?

So if this is a migration-specific scalability issue, then it might be
possible to solve by making the mutex be a rwsem instead, and have
migration only take it for reading.

Of course, I'm quite possibly wrong, and the code depends on full
mutual exclusion.

Just a thought, in case it makes somebody go "Hmm.."

               Linus
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