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

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Mar 21, 2023 at 07:45:56AM +0000, Lorenzo Stoakes wrote:
> On Tue, Mar 21, 2023 at 06:23:39AM +0100, Uladzislau Rezki wrote:
> > On Tue, Mar 21, 2023 at 12:09:12PM +1100, Dave Chinner wrote:
> > > On Sun, Mar 19, 2023 at 07:09:31AM +0000, Lorenzo Stoakes 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.
> > >
> > > I'm travelling right now, but give me a few days and I'll test this
> > > against the XFS workloads that hammer the global vmalloc spin lock
> > > really, really badly. XFS can use vm_map_ram and vmalloc really
> > > heavily for metadata buffers and hit the global spin lock from every
> > > CPU in the system at the same time (i.e. highly concurrent
> > > workloads). vmalloc is also heavily used in the hottest path
> > > throught the journal where we process and calculate delta changes to
> > > several million items every second, again spread across every CPU in
> > > the system at the same time.
> > >
> > > We really need the global spinlock to go away completely, but in the
> > > mean time a shared read lock should help a little bit....
> > >
> 
> Hugely appreciated Dave, however I must disappoint on the rwsem as I have now
> reworked my patch set to use the original locks in order to satisfy Willy's
> desire to make vmalloc atomic in future, and Uladzislau's desire to not have a
> ~6% performance hit -
> https://lore.kernel.org/all/cover.1679354384.git.lstoakes@gmail.com/
> 
> > I am working on it. I submitted a proposal how to eliminate it:
> >
> >
> > <snip>
> > Hello, LSF.
> >
> > Title: Introduce a per-cpu-vmap-cache to eliminate a vmap lock contention
> >
> > Description:
> >  Currently the vmap code is not scaled to number of CPU cores in a system
> >  because a global vmap space is protected by a single spinlock. Such approach
> >  has a clear bottleneck if many CPUs simultaneously access to one resource.
> >
> >  In this talk i would like to describe a drawback, show some data related
> >  to contentions and places where those occur in a code. Apart of that i
> >  would like to share ideas how to eliminate it providing a few approaches
> >  and compare them.
> >
> > Requirements:
> >  * It should be a per-cpu approach;
> >  * Search of freed ptrs should not interfere with other freeing(as much as we can);
> >  *   - offload allocated areas(buzy ones) per-cpu;
> >  * Cache ready sized objects or merge them into one big per-cpu-space(split on demand);
> >  * Lazily-freed areas either drained per-cpu individually or by one CPU for all;
> >  * Prefetch a fixed size in front and allocate per-cpu
> >
> > Goals:
> >  * Implement a per-cpu way of allocation to eliminate a contention.
> >
> > Thanks!
> > <snip>
> >
> > --
> > Uladzislau Rezki
> >
> 
> That's really awesome! I will come to that talk at LSF/MM :) being able to
> sustain the lock in atomic context seems to be an aspect that is important going
> forward also.
>
Uhh... So i need to prepare then :)))

--
Uladzislau Rezki