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

Uladzislau Rezki
2023-03-21T10:25:13Z

On Tue, Mar 21, 2023 at 09:05:26PM +1100, Dave Chinner wrote:
> 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/
>
> Yeah, I'd already read that.
>
> What I want to do, though, is to determine whether the problem
> shared access contention or exclusive access contention. If it's
> exclusive access contention, then an rwsem will do nothing to
> alleviate the problem, and that's kinda critical to know before any
> fix for the contention problems are worked out...
>
> > > 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.
>
> If you want data about contention problems with vmalloc
>
> > > Requirements:
> > > * It should be a per-cpu approach;
>
> Hmmmm. My 2c worth on this: That is not a requirement.
>
> That's a -solution-.
>
> The requirement is that independent concurrent vmalloc/vfree
> operations do not severely contend with each other.
>
> Yes, the solution will probably involve sharding the resource space
> across mulitple independent structures (as we do in filesystems with
> block groups, allocations groups, etc) but that does not necessarily
> need the structures to be per-cpu.
>
> e.g per-node vmalloc arenas might be sufficient and allow more
> expensive but more efficient indexing structures to be used because
> we don't have to care about the explosion of memory that
> fine-grained per-cpu indexing generally entails. This may also fit
> in to the existing per-node structure of the memory reclaim
> infrastructure to manage things like compaction, balancing, etc of
> vmalloc space assigned to the given node.
>
> Hence I think saying "per-cpu is a requirement" kinda prevents
> exploration of other novel solutions that may have advantages other
> than "just solves the concurrency problem"...
>
> > > * 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
>
> I'd call these desired traits and/or potential optimisations, not
> hard requirements.
>
> > > Goals:
> > > * Implement a per-cpu way of allocation to eliminate a contention.
>
> The goal should be to "allow contention-free vmalloc operations", not
> that we implement a specific solution.
>
I think we are on the same page. I do not see that we go apart in anything.
Probably i was a bit more specific in requirements but this is how i see
personally on it based on different kind of experiments with it.

Thank you for your 2c!

--
Uladzislau Rezki