linux-kernel - RE: [PATCH RFC] sctp: Update HEARTBEAT timer immediately after user changed HB.interval

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Date:	Tue, 18 Feb 2014 15:39:50 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Vlad Yasevich' <vyasevich@...il.com>,
	Xufeng Zhang <xufeng.zhang@...driver.com>,
	"nhorman@...driver.com" <nhorman@...driver.com>,
	"davem@...emloft.net" <davem@...emloft.net>
CC:	"linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH RFC] sctp: Update HEARTBEAT timer immediately after user
 changed HB.interval

> > +
> > +				/* Update the heartbeat timer immediately. */
> > +				if (!mod_timer(&trans->hb_timer,
> > +					    sctp_transport_timeout(trans)))
> > +					sctp_transport_hold(trans);
> 
> This is causes a rather inconsistent behavior in that it doesn't
> account of the amount of time left on the hb timer.  Thus it doesn't
> always do what commit log implies.  If the remaining time is shorter
> then the new timeout value, it will take longer till the next HB
> the application expects.

Being able to stop heartbeats being sent by repeatedly configuring
the interval is certainly not a good idea!

> If the application has very strict timing requirement on the HBs, it
> should trigger the HB manually.
> 
> We could rework the code to allow the combination of
> SPP_HB_DEMAND | SPP_HB_ENABLE to work as follows:
>   1) update the timer to the new value
>   2) Send an immediate HB
>      a) Update the hb timeout.
> 
> 2a should probably be done regardless since it can cause 2 HB to be
> outstanding at the same time.

Sending a heartbeat 'on demand' might be problematic if one
has also just been sent (from the timer).

I'd have thought that you wouldn't want to send a heartbeat while
still expecting a response from the previous one (this might require
splitting the time interval in two).

	David



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