Re: [PATCH 10/11] io_uring/rsrc: cache struct io_rsrc_node

[Pavel Begunkov]

On 4/4/23 17:53, Gabriel Krisman Bertazi wrote:
> Jens Axboe <axboe@xxxxxxxxx> writes:
>
> > On 4/4/23 9:48?AM, Gabriel Krisman Bertazi wrote:
> > > Pavel Begunkov <asml.silence@xxxxxxxxx> writes:
> > >
> > > > On 4/1/23 01:04, Gabriel Krisman Bertazi wrote:
> > > > > Pavel Begunkov <asml.silence@xxxxxxxxx> writes:
> > >
> > > > > > I didn't try it, but kmem_cache vs kmalloc, IIRC, doesn't bring us
> > > > > > much, definitely doesn't spare from locking, and the overhead
> > > > > > definitely wasn't satisfactory for requests before.
> > > > > There is no locks in the fast path of slub, as far as I know.  it has
> > > > > a
> > > > > per-cpu cache that is refilled once empty, quite similar to the fastpath
> > > > > of this cache.  I imagine the performance hit in slub comes from the
> > > > > barrier and atomic operations?
> > > >
> > > > Yeah, I mean all kinds of synchronisation. And I don't think
> > > > that's the main offender here, the test is single threaded without
> > > > contention and the system was mostly idle.
> > > >
> > > > > kmem_cache works fine for most hot paths of the kernel.  I think this
> > > >
> > > > It doesn't for io_uring. There are caches for the net side and now
> > > > in the block layer as well. I wouldn't say it necessarily halves
> > > > performance but definitely takes a share of CPU.
> > >
> > > Right.  My point is that all these caches (block, io_uring) duplicate
> > > what the slab cache is meant to do.  Since slab became a bottleneck, I'm
> > > looking at how to improve the situation on their side, to see if we can
> > > drop the caching here and in block/.
> >
> > That would certainly be a worthy goal, and I do agree that these caches
> > are (largely) working around deficiencies. One important point that you
> > may miss is that most of this caching gets its performance from both
> > avoiding atomics in slub, but also because we can guarantee that both
> > alloc and free happen from process context. The block IRQ bits are a bit
> > different, but apart from that, it's true elsewhere. Caching that needs
> > to even disable IRQs locally generally doesn't beat out slub by much,
> > the big wins are the cases where we know free+alloc is done in process
> > context.
>
> Yes, I noticed that.  I was thinking of exposing a flag at kmem_cache
> creation-time to tell slab the user promises not to use it in IRQ
> context, so it doesn't need to worry about nested invocation in the
> allocation/free path.  Then, for those caches, have a
> kmem_cache_alloc_locked variant, where the synchronization is maintained
> by the caller (i.e. by ->uring_lock here), so it can manipulate the
> cache without atomics.
>
> I was looking at your implementation of the block cache for inspiration
> and saw how you kept a second list for IRQ.  I'm thinking how to fit a

It works fine, but one problem I had while implementing it is
local_irq_save() in bio_put_percpu_cache() not being so cheap even when
interrupts are already disabled. Apparently, raw_local_save_flags() is
not as free as I wished it to be, and we can't easily pass the current
context. Would be nice to do something about it at some point.


> similar change inside slub.  But for now, I want to get the simpler
> case, which is all io_uring need.
>
> I'll try to get a prototype before lsfmm and see if I get the MM folks
> input there.

--
Pavel Begunkov