netdev - Re: [RFC PATCH 1/3] net: introduce kfree_skb_bulk() user of kmem_cache_free

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Date:	Mon, 7 Sep 2015 09:25:49 -0700
From:	Tom Herbert <tom@...bertland.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
Cc:	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	akpm@...ux-foundation.org, linux-mm@...ck.org,
	aravinda@...ux.vnet.ibm.com, Christoph Lameter <cl@...ux.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	iamjoonsoo.kim@....com
Subject: Re: [RFC PATCH 1/3] net: introduce kfree_skb_bulk() user of kmem_cache_free_bulk()

>> What not pass a list of skbs (e.g. using skb->next)?
>
> Because the next layer, the slab API needs an array:
>   kmem_cache_free_bulk(struct kmem_cache *s, size_t size, void **p)
>

I suppose we could ask the same question of that function. IMO
encouraging drivers to define arrays of pointers on the stack like
you're doing in the ixgbe patch is a bad direction.

In any case I believe this would be simpler in the networking side
just to maintain a list of skb's to free. Then the dev_free_waitlist
structure might not be needed then since we could just use a
skb_buf_head for that.


Tom

> Look at the patch:
>  [PATCH V2 3/3] slub: build detached freelist with look-ahead
>  http://thread.gmane.org/gmane.linux.kernel.mm/137469/focus=137472
>
> Where I use this array to progressively scan for objects belonging to
> the same page.  (A subtle detail is I manage to zero out the array,
> which is good from a security/error-handling point of view, as pointers
> to the objects are not left dangling on the stack).
>
>
> I cannot argue that, writing skb->next comes as an additional cost,
> because the slUb free also writes into this cacheline.  Perhaps the
> slAb allocator does not?
>
> [...]
>> > +       if (likely(cnt)) {
>> > +               kmem_cache_free_bulk(skbuff_head_cache, cnt, (void **) skbs);
>> > +       }
>> > +}
>> > +EXPORT_SYMBOL(kfree_skb_bulk);
>
> --
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Sr. Network Kernel Developer at Red Hat
>   Author of http://www.iptv-analyzer.org
>   LinkedIn: http://www.linkedin.com/in/brouer
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