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

[Uladzislau Rezki <urezki@gmail.com>]

> 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.

So, how important for you to have this change?

--
Uladzislau Rezki