netdev - Re: [RFC PATCH 1/3] net: Split netdev_alloc_frag into __alloc_page_frag and add __napi_alloc

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Date:	Wed, 26 Nov 2014 21:29:17 -0800
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Alexander Duyck <alexander.h.duyck@...hat.com>
Cc:	Network Development <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [RFC PATCH 1/3] net: Split netdev_alloc_frag into
 __alloc_page_frag and add __napi_alloc_frag

On Wed, Nov 26, 2014 at 4:05 PM, Alexander Duyck
<alexander.h.duyck@...hat.com> wrote:
> This patch splits the netdev_alloc_frag function up so that it can be used
> on one of two page frag pools instead of being fixed on the
> netdev_alloc_cache.  By doing this we can add a NAPI specific function
> __napi_alloc_frag that accesses a pool that is only used from softirq
> context.  The advantage to this is that we do not need to call
> local_irq_save/restore which can be a significant savings.
>
> I also took the opportunity to refactor the core bits that were placed in
> __alloc_page_frag.  First I updated the allocation to do either a 32K
> allocation or an order 0 page.  Then I also rewrote the logic to work from
> the end of the page to the start.  By doing this the size value doesn't
> have to be used unless we have run out of space for page fragments.
> Finally I cleaned up the atomic bits so that we just do an
> atomic_sub_return and if that returns 0 then we set the page->_count via an
> atomic_set.  This way we can remove the extra conditional for the
> atomic_read since it would have led to an atomic_inc in the case of success
> anyway.

Nice simplification. Complicated, but looks good to me.
I think only replacement of loop with 32k+page begs
better explanation in the commit log. I'm guessing you're
killing intermediate sizes to simplify the code?

Thank you for doing it. It's a great improvement.
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