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

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Date:   Mon, 20 Sep 2021 02:53:34 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>
Subject: Re: [RFC PATCH] Introducing lockless cache built on top of slab
 allocator

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)

> 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().