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

Uladzislau Rezki
2023-03-21T05:24:22Z

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....
>
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