netdev - RE: [PATCH net-next] r8169: Add 64bit statistics

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Message-ID: <1321534700.2751.27.camel@edumazet-HP-Compaq-6005-Pro-SFF-PC>
Date:	Thu, 17 Nov 2011 13:58:20 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	Junchang Wang <junchangwang@...il.com>,
	Stephen Hemminger <stephen.hemminger@...tta.com>,
	netdev@...r.kernel.org, romieu@...zoreil.com,
	nic swsd <nic_swsd@...ltek.com>
Subject: RE: [PATCH net-next] r8169: Add 64bit statistics

Le jeudi 17 novembre 2011 à 11:13 +0000, David Laight a écrit :
> The 64bit stats update sequence used to get valid
> counts on 32bit systems (that can't do locked 64bit
> memory access) seems to be:
> 
> >     u64_stats_update_begin(&sky2->tx_stats.syncp);
> >     ++sky2->tx_stats.packets;
> >     sky2->tx_stats.bytes += skb->len;
> >     u64_stats_update_end(&sky2->tx_stats.syncp);
> 
> I'm not sure what the begin/end markers do, but
> they need to hold off the readers during updates
> and the writers during reads - this is probably
> expensive on the update path.
> 
> A thought that might work is for the writer to
> write the middle bits of the 64 bit walue to
> another location, eg:
>        count = sky2->tx_stats.bytes + skb->len;
>        sky2->tx_stats.bytes = count;
>        sky2->tx_stats.bytes_check = count >> 16;
> The reader then loops until the two value are
> consistent.
> 
> I think this doesn't even require a memory barrier
> in the ISR since the order of the reads an writes
> doesn't matter at all.
> 
> 	David
> 
> 

Oh well...

Before claiming all this, you really should read 
include/linux/u64_stats_sync.h

This should answer all your questions.



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