netdev - Re: [PATCH v2 net] bonding: don't use stale speed and duplex information

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Message-ID: <11163.1456407235@nyx>
Date:	Thu, 25 Feb 2016 05:33:55 -0800
From:	Jay Vosburgh <jay.vosburgh@...onical.com>
To:	zhuyj <zyjzyj2000@...il.com>
cc:	netdev@...r.kernel.org,
	"Tantilov, Emil S" <emil.s.tantilov@...el.com>,
	Veaceslav Falico <vfalico@...il.com>,
	dingtianhong <dingtianhong@...wei.com>,
	Andy Gospodarek <gospo@...ulusnetworks.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH v2 net] bonding: don't use stale speed and duplex information

zhuyj <zyjzyj2000@...il.com> wrote:
[...]
>I delved into the source code and Emil's tests. I think that the problem
>that this patch expects to fix occurs very unusually.
>
>Do you agree with me?
>
>If so, maybe the following patch can reduce the performance loss.
>Please comment on it. Thanks a lot.
>
>
>diff --git a/drivers/net/bonding/bond_main.c
>b/drivers/net/bonding/bond_main.c
>index b7f1a99..c4c511a 100644
>--- a/drivers/net/bonding/bond_main.c
>+++ b/drivers/net/bonding/bond_main.c
>@@ -2129,7 +2129,9 @@ static void bond_miimon_commit(struct bonding *bond)
>                        continue;
>
>                case BOND_LINK_UP:
>-                       bond_update_speed_duplex(slave);
>+                       if (slave->speed == SPEED_UNKNOWN)
>+                               bond_update_speed_duplex(slave);
>+
>                        bond_set_slave_link_state(slave, BOND_LINK_UP,
>BOND_SLAVE_NOTIFY_NOW);
>                        slave->last_link_up = jiffies;

	I don't believe the speed is necessarily SPEED_UNKNOWN coming in
here.  If the race occurs at a time later than the initial enslavement,
speed may already be set (and the race manifests if the new speed
changes, i.e., the link changes from 1 Gb/sec to 10 Gb/sec), so I don't
think this is functionally correct.

	Also, the call to bond_miimon_commit itself is already gated by
bond_miimon_inspect finding a link state change.  The performance impact
here should be minimal.

	-J

---
	-Jay Vosburgh, jay.vosburgh@...onical.com