netdev - Re: irq disable in __netdev_alloc

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Date:	Mon, 27 Oct 2014 21:35:23 +0100
From:	Jesper Dangaard Brouer <brouer@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	brouer@...hat.com, Alexander Duyck <alexander.duyck@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Network Development <netdev@...r.kernel.org>,
	Christoph Lameter <cl@...ux.com>
Subject: Re: irq disable in __netdev_alloc_frag() ?

On Wed, 22 Oct 2014 20:51:16 -0700
Eric Dumazet <eric.dumazet@...il.com> wrote:

> On my hosts, this hard irq masking is pure noise.

On my hosts I can measure a significant difference between using
local_irq_disable() vs. local_irq_save(flags)

 *  2.860 ns cost for local_irq_{disable,enable} 
 * 14.840 ns cost for local_irq_save()+local_irq_restore() 

This is quite significant in my nanosec world ;-)

 
> What CPU are you using Alexander ?

I'm using a E5-2695 (Ivy-bridge)

You can easily reproduce my results on your own system with my
time_bench_sample module here:
 https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/lib/time_bench_sample.c#L173

> Same could be done with some kmem_cache_alloc() : SLAB uses hard irq
> masking while some caches are never used from hard irq context.

Sounds interesting.

-- 
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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