netdev - Re: irq disable in __netdev_alloc

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Date:	Wed, 22 Oct 2014 19:22:29 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Eric Dumazet <edumazet@...gle.com>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: irq disable in __netdev_alloc_frag() ?

On Wed, Oct 22, 2014 at 6:52 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> On Wed, 2014-10-22 at 17:15 -0700, Alexei Starovoitov wrote:
>> Hi Eric,
>>
>> in the commit 6f532612cc24 ("net: introduce netdev_alloc_frag()")
>> you mentioned that the reason to disable interrupts
>> in __netdev_alloc_frag() is:
>> "- Must be IRQ safe (non NAPI drivers can use it)"
>>
>> Is there a way to do this conditionally?
>>
>> Without it I see 10% performance gain for my RX tests
>> (from 6.9Mpps to 7.7Mpps) and __netdev_alloc_frag()
>> itself goes from 6.6% to 2.1%
>> (popf seems to be quite costly)
>
> Well, your driver is probably a NAPI one, so you need to
> mask irqs, or to remove all non NAPI drivers from linux.

yeah, the 10G+ nics I care about are all napi :)

> __netdev_alloc_frag() (__netdev_alloc_skb()) is used by all.
>
> Problem is __netdev_alloc_frag() is generally deep inside caller
> chain, so using a private pool might have quite an overhead.

yes. I was thinking, since dev is already passed
into __netdev_alloc_skb(), we can check whether
dev registered with napi via dev->napi_list and if so,
tell inner __netdev_alloc_frag() to skip irq disabling...

don't know about skb_queue_head() and friends.
I'm only looking at pure rx now. One challenge at a time.
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