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

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Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6D8AEC9@saturn3.aculab.com>
Date:	Thu, 17 Nov 2011 11:13:12 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Junchang Wang" <junchangwang@...il.com>,
	"Eric Dumazet" <eric.dumazet@...il.com>
Cc:	"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

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


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