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
