Re: [PATCH 00/13] mm: preemptibility -v2

Nick Piggin
2010-04-09T04:14:27Z

On Thu, Apr 08, 2010 at 09:17:37PM +0200, Peter Zijlstra wrote:
> Hi,
>
> This (still incomplete) patch-set makes part of the mm a lot more preemptible.
> It converts i_mmap_lock and anon_vma->lock to mutexes. On the way there it
> also makes mmu_gather preemptible.
>
> The main motivation was making mm_take_all_locks() preemptible, since it
> appears people are nesting hundreds of spinlocks there.
>
> The side-effects are that we can finally make mmu_gather preemptible, something
> which lots of people have wanted to do for a long time.

What's the straight-line performance impact of all this? And how about
concurrency, I wonder. mutexes of course are double the atomics, and
you've added a refcount which is two more again for those paths using
it.

Page faults are very important. We unfortunately have some databases
doing a significant amount of mmap/munmap activity too. I'd like to
see microbenchmark numbers for each of those (both anon and file backed
for page faults).

kbuild does quite a few pages faults, that would be an easy thing to
test. Not sure what reasonable kinds of cases exercise parallelism.

> What kind of performance tests would people have me run on this to satisfy
> their need for numbers? I've done a kernel build on x86_64 and if anything that
> was slightly faster with these patches, but it was well within the noise
> levels so it might be heat noise I'm looking at ;-)

Is it because you're reducing the number of TLB flushes, or what
(kbuild isn't multi threaded so on x86 TLB flushes should be really
fast anyway).