Re: [RFC PATCH] Introducing lockless cache built on top of slab allocator

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Mon, Sep 20, 2021 at 11:07:36AM +0200, Vlastimil Babka wrote:
> On 9/20/21 03:53, Matthew Wilcox wrote:
> > On Mon, Sep 20, 2021 at 01:09:38AM +0000, Hyeonggon Yoo wrote:
> >> Hello Matthew, Thanks to give me a comment! I appreciate it.
> >> Yeah, we can implement lockless cache using kmem_cache_alloc_{bulk, free}
> >> but kmem_cache_alloc_{free,bulk} is not enough.
> >> 
> >> > I'd rather see this be part of the slab allocator than a separate API.
> >> 
> >> And I disagree on this. for because most of situation, we cannot
> >> allocate without lock, it is special case for IO polling.
> >> 
> >> To make it as part of slab allocator, we need to modify existing data
> >> structure. But making it part of slab allocator will be waste of memory
> >> because most of them are not using this.
> > 
> > Oh, it would have to be an option.  Maybe as a new slab_flags_t flag.
> > Or maybe a kmem_cache_alloc_percpu_lockless().
> 
> I've recently found out that similar attempts (introduce queueing to SLUB)
> have been done around 2010. See e.g. [1] but there will be other threads to
> search at lore too. Haven't checked yet while it wasn't ultimately merged, 
> I guess Christoph and David could remember (this was before my time).

There was attempt on SLUB with queueing as you said.
I searched a bit and found [2] and [3].

- SLUB with queueing (V2) beats SLAB netperf TCP_RR, 2010-07
[2] https://lore.kernel.org/lkml/alpine.DEB.2.00.1007121010420.14328@router.home/T/#m5a31c7caa28b93a00de3af6d547b79273449f5ba

- The Unified slab allocator (V4), 2010-10
[3] https://linux-mm.kvack.narkive.com/e595iCuz/unifiedv4-00-16-the-unified-slab-allocator-v4#post47

Looking at [3], There was still some regression comparing "SLUB with queueing"
with SLAB. And I couldn't find patch series after [3] yet. I'll add link
if I find.

> I guess making it opt-in only for caches where performance improvement was
> measured would make it easier to add, as for some caches it would mean no
> improvement, but increased memory usage. But of course it makes the API more
> harder to use.

Do you mean "lockless cache" it should be separate from slab because some caches
doesn't benefit at all?

> I'd be careful about the name "lockless", as that's ambiguous. Is it "mostly
> lockless" therefore fast, but if the cache is empty, it will still take
> locks as part of refill?

It is actually "mostly lockless" so it is ambiguous.
Can you suggest a name? like try_lockless or anything?

> Or is it lockless always, therefore useful in
> contexts that can take no locks, but then the caller has to have fallbacks
> in case the cache is empty and nothing is allocated?
> 
> [1] https://lore.kernel.org/linux-mm/20100804024531.914852850@linux.com/T/#u