Re: [PATCH v2 2/4] mm: vmalloc: use rwsem, mutex for vmap_area_lock and vmap_block->lock

Lorenzo Stoakes
2023-03-20T08:25:41Z

On Mon, Mar 20, 2023 at 08:54:33AM +0100, Uladzislau Rezki wrote:
> > vmalloc() is, by design, not permitted to be used in atomic context and
> > already contains components which may sleep, so avoiding spin locks is not
> > a problem from the perspective of atomic context.
> >
> > The global vmap_area_lock is held when the red/black tree rooted in
> > vmap_are_root is accessed and thus is rather long-held and under
> > potentially high contention. It is likely to be under contention for reads
> > rather than write, so replace it with a rwsem.
> >
> > Each individual vmap_block->lock is likely to be held for less time but
> > under low contention, so a mutex is not an outrageous choice here.
> >
> > A subset of test_vmalloc.sh performance results:-
> >
> > fix_size_alloc_test 0.40%
> > full_fit_alloc_test 2.08%
> > long_busy_list_alloc_test 0.34%
> > random_size_alloc_test -0.25%
> > random_size_align_alloc_test 0.06%
> > ...
> > all tests cycles 0.2%
> >
> > This represents a tiny reduction in performance that sits barely above
> > noise.
> >
> How important to have many simultaneous users of vread()? I do not see a
> big reason to switch into mutexes due to performance impact and making it
> less atomic.

It's less about simultaneous users of vread() and more about being able to write
direct to user memory rather than via a bounce buffer and not hold a spinlock
over possible page faults.

The performance impact is barely above noise (I got fairly widely varying
results), so I don't think it's really much of a cost at all. I can't imagine
there are many users critically dependent on a sub-single digit % reduction in
speed in vmalloc() allocation.

As I was saying to Willy, the code is already not atomic, or rather needs rework
to become atomic-safe (there are a smattering of might_sleep()'s throughout)

However, given your objection alongside Willy's, let me examine Willy's
suggestion that we instead of doing this, prefault the user memory in advance of
the vread call.

>
> So, how important for you to have this change?
>

Personally, always very important :)

> --
> Uladzislau Rezki