On Mon, Mar 20, 2023 at 08:35:11AM +0000, Lorenzo Stoakes wrote: > On Mon, Mar 20, 2023 at 09:32:06AM +0100, Uladzislau Rezki wrote: > > On Mon, Mar 20, 2023 at 08:25:32AM +0000, Lorenzo Stoakes wrote: > > > 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. > > > > > Just a quick perf tests shows regression around 6%. 10 workers test_mask is 31: > > > > # default > > [ 140.349731] All test took worker0=485061693537 cycles > > [ 140.386065] All test took worker1=486504572954 cycles > > [ 140.418452] All test took worker2=467204082542 cycles > > [ 140.435895] All test took worker3=512591010219 cycles > > [ 140.458316] All test took worker4=448583324125 cycles > > [ 140.494244] All test took worker5=501018129647 cycles > > [ 140.518144] All test took worker6=516224787767 cycles > > [ 140.535472] All test took worker7=442025617137 cycles > > [ 140.558249] All test took worker8=503337286539 cycles > > [ 140.590571] All test took worker9=494369561574 cycles > > > > # patch > > [ 144.464916] All test took worker0=530373399067 cycles > > [ 144.492904] All test took worker1=522641540924 cycles > > [ 144.528999] All test took worker2=529711158267 cycles > > [ 144.552963] All test took worker3=527389011775 cycles > > [ 144.592951] All test took worker4=529583252449 cycles > > [ 144.610286] All test took worker5=523605706016 cycles > > [ 144.627690] All test took worker6=531494777011 cycles > > [ 144.653046] All test took worker7=527150114726 cycles > > [ 144.669818] All test took worker8=526599712235 cycles > > [ 144.693428] All test took worker9=526057490851 cycles > > > > OK ouch, that's worse than I observed! Let me try this prefault approach and > then we can revert back to spinlocks. > > > > > > > > > So, how important for you to have this change? > > > > > > > > > > Personally, always very important :) > > > > > This is good. Personal opinion always wins :) > > > > The heart always wins ;) well, an adaption here can make everybody's hearts > happy I think. > Totally agree :) -- Uladzislau Rezki
