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

Uladzislau Rezki
2023-03-20T08:32:17Z

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

> >
> > So, how important for you to have this change?
> >
>
> Personally, always very important :)
>
This is good. Personal opinion always wins :)

--
Uladzislau Rezki