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

[Hyeonggon Yoo]

On Mon, Sep 20, 2021 at 02:53:34AM +0100, 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.
> > 
> > On Sun, Sep 19, 2021 at 08:17:44PM +0100, Matthew Wilcox wrote:
> > > On Sun, Sep 19, 2021 at 04:42:39PM +0000, Hyeonggon Yoo wrote:
> > > > It is just simple proof of concept, and not ready for submission yet.
> > > > There can be wrong code (like wrong gfp flags, or wrong error handling,
> > > > etc) it is just simple proof of concept. I want comment from you.
> > > 
> > > Have you read:
> > > 
> > > https://www.usenix.org/legacy/event/usenix01/full_papers/bonwick/bonwick_html/
> > > The relevant part of that paper is section 3, magazines.  We should have
> > > low and high water marks for number of objects
> > 
> > I haven't read that before, but after reading it seems not different from
> > SLAB's percpu queuing.
> >  
> > > and we should allocate
> > > from / free to the slab allocator in batches.  Slab has bulk alloc/free
> > > APIs already.
> > > 
> > 
> > There's kmem_cache_alloc_{bulk,free} functions for bulk
> > allocation. But it's designed for large number of allocation
> > to reduce locking cost, not for percpu lockless allocation.
> 
> What I'm saying is that rather than a linked list of objects, we should
> have an array of, say, 15 pointers per CPU (and a count of how many
> allocations we have).  If we are trying to allocate and have no objects,
> call kmem_cache_alloc_bulk() for 8 objects.  If we are trying to free
> and have 15 objects already, call kmem_cache_free_bulk() for the last
> 8 objects and set the number of allocated objects to 7.
> 
> (maybe 8 and 15 are the wrong numbers.  this is just an example)
>

Ah, Okay. it seems better to use array. Using cache for list is
unnecessary cost. array is simpler.

> > 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().

Oh, Now I got what you mean. That is a good improvement!

For example,
there is a slab_flags_t flag like SLAB_LOCKLESS.
and a cache created with SLAB_LOCKLESS flag can allocate 
using both kmem_cache_alloc, or kmem_cache_alloc_percpu_lockless
depending on situation? (I suggest kmem_cache_alloc_lockless is better name)

it seems MUCH better. (because it prevents duplicating a cache)

I'll send RFC v2 soon.
Thank you so much Matthew.

If there's misunderstanding from me, please let me know.

Thanks,
Hyeonggon Yoo