linux-kernel - Re: e1000_netpoll(): disable_irq() triggers might

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Message-ID: <alpine.DEB.2.11.1410292201530.5308@nanos>
Date:	Wed, 29 Oct 2014 22:03:20 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Sabrina Dubroca <sd@...asysnail.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, jeffrey.t.kirsher@...el.com
Subject: Re: e1000_netpoll(): disable_irq() triggers might_sleep() on
 linux-next

On Wed, 29 Oct 2014, Peter Zijlstra wrote:

> On Wed, Oct 29, 2014 at 09:23:42PM +0100, Thomas Gleixner wrote:
> > But at least it allows to mitigate the impact by making it conditional
> > at a central point.
> > 
> > static inline void netpoll_lock(struct net_device *nd)
> > {
> > 	if (netpoll_active(nd))
> > 		spin_lock(&nd->netpoll_lock);
> > }
> 
> branch fail vs lock might be a toss on most machines, but if we're
> hitting cold cachelines we loose big.

Well, if the net_device is not cache hot on irq entry you have lost
already. The extra branch/lock is not going to add much to that.
 
Thanks,

	tglx
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