netdev - Re: [PATCH] net: fix softnet

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Date:	Thu, 15 Apr 2010 06:14:06 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Changli Gao <xiaosuo@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Tom Herbert <therbert@...gle.com>, netdev@...r.kernel.org
Subject: Re: [PATCH] net: fix softnet_stat

Le jeudi 15 avril 2010 à 05:40 +0200, Eric Dumazet a écrit :
> Le jeudi 15 avril 2010 à 05:28 +0200, Eric Dumazet a écrit :
> > >  
> > > +	per_cpu(netdev_rx_stat, cpu).dropped++;
> > 
> > This is a bit expensive, and could be a queue->rx_stat.dropped++
> > if netdev_rx_stat is moved inside queue structure.
> 
> Hmm, this is not really needed, this event is the slow path and should
> almost never happen. This part of your fix is fine.
> 
> 
> -

Oh well, I revert this comment, we need a mutual exclusion between cpus
if we dont want to miss some increments, even in a slow path.

__get_cpu_var(netdev_rx_stat).field++; is safe because updated only by
this cpu.

Once you use per_cpu(netdev_rx_stat, cpu).field++; you need a
synchronization since this is not atomic.

so queue->dropped++ is safer and faster too (inside the rps_lock
section)


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