linux-kernel - Re: [PREEMPT-RT] [patch 4 14/22] timer: Switch to a non cascading wheel

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Date:	Wed, 17 Aug 2016 11:23:26 +0200
From:	rcochran@...utronix.de
To:	Jouni Malinen <j@...fi>
Cc:	Richard Cochran <richardcochran@...il.com>,
	Jouni Malinen <jkmalinen@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rik van Riel <riel@...hat.com>, Len Brown <lenb@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	George Spelvin <linux@...encehorizons.net>,
	Josh Triplett <josh@...htriplett.org>,
	Chris Mason <clm@...com>, Eric Dumazet <edumazet@...gle.com>,
	rt@...utronix.de, Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PREEMPT-RT] [patch 4 14/22] timer: Switch to a non cascading
 wheel

On Wed, Aug 17, 2016 at 12:05:22PM +0300, Jouni Malinen wrote:
> I had not realized this previously due to the test case passing, but the
> same retransmit SYN case was happening with older kernels, it just was
> done a tiny bit faster to escape that 1.0 second timeout limit.. That
> about 1.03 sec value after this kernel commit is 1.0 sec before this
> kernel commit. In other words, something in this specific kernel commit
> seems to add about 0.03 sec delay to the TCP SYN retransmission.

Yes, and this 3% increase in the timeout expiration interval is an
expected result using the new timer wheel.  See the large comment at
the top of kernel/time/timer.c and the lkml discussion about this
series.

> That
> said, I realize that this is quite unlikely timeout to use for connect()
> in real world and as such, it looks simply as a side effect of a test
> case that was using way too close timing requirement in the first place.

Right, and user space should not rely on the exact timing of TCP SYN
re-transmission!

Thanks,
Richard