netdev - Re: Experiences with slub bulk use-case for network stack

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Date:	Thu, 17 Sep 2015 18:57:17 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
cc:	linux-mm@...ck.org, netdev@...r.kernel.org,
	akpm@...ux-foundation.org,
	Alexander Duyck <alexander.duyck@...il.com>,
	iamjoonsoo.kim@....com
Subject: Re: Experiences with slub bulk use-case for network stack

On Thu, 17 Sep 2015, Jesper Dangaard Brouer wrote:

> What I'm proposing is keeping interrupts on, and then simply cmpxchg
> e.g 2 slab-pages out of the SLUB allocator (which the SLUB code calls
> freelist's). The bulk call now owns these freelists, and returns them
> to the caller.  The API caller gets some helpers/macros to access
> objects, to shield him from the details (of SLUB freelist's).
>
> The pitfall with this API is we don't know how many objects are on a
> SLUB freelist.  And we cannot walk the freelist and count them, because
> then we hit the problem of memory/cache stalls (that we are trying so
> hard to avoid).

If you get a fresh page from the page allocator then you know how many
objects are available in a slab page.

There is also a counter in each slab page for the objects allocated. The
number of free object is page->objects - page->inuse.

This is only true for a lockec cmpxchg. The unlocked cmpxchg used for the
per cpu freelist does not use the counters in the page struct.


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