netdev - Re: [PATCH net-next-2.6] loopback: Implement 64bit stats on 32bit arches

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Date:	Tue, 15 Jun 2010 09:23:04 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	bhutchings@...arflare.com
Subject: Re: [PATCH net-next-2.6] loopback: Implement 64bit stats on 32bit
 arches

Le mardi 15 juin 2010 à 16:49 +1000, Nick Piggin a écrit :
> On Mon, Jun 14, 2010 at 11:14:12PM -0700, David Miller wrote:
> > From: Eric Dumazet <eric.dumazet@...il.com>
> > Date: Mon, 14 Jun 2010 17:59:22 +0200
> > 
> > > Uses a seqcount_t to synchronize stat producer and consumer, for packets
> > > and bytes counter, now u64 types.
> > > 
> > > (dropped counter being rarely used, stay a native "unsigned long" type)
> > > 
> > > No noticeable performance impact on x86, as it only adds two increments
> > > per frame. It might be more expensive on arches where smp_wmb() is not
> > > free.
> > > 
> > > Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> > 
> > Applied, but I suspect we might end up eventually needing to
> > abstract this kind of technique in a common place so other
> > spots can use it.
> 

David, I was planning adding similar stuff for SNMP, xt_RATEEST and txq
accounting, so yes, I'll probably move helpers to a common include file.

This first patch was a first shot to introduce the ground.

> Check i_size stuff in include/linux/fs.h if you consider doing this.

Yes, I was aware of this stuff.

> And keep preempt in mind too. I assume you can't be preempted at this
> point, but if you're prone to change the locking, it might be worth
> the (small) cost of doing explicit preempt_disable() (and maybe to
> help the sanity of the -rt guys too).
> 

Yes, we cannot be preempted at this point, as ndo_start_xmit() handlers
are called with BH disabled (there is a comment for this assertion in
front of loopback_xmit())

dev_queue_xmit() 
  rcu_read_lock_bh();
  ...
  ndo_start_xmit();

I'll take care of preempt status for following patches, but I suspect
its more a BH enable/disable question in network stack anyway.

Thanks !


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