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Date: Wed, 17 Aug 2016 11:23:26 +0200
From: rcochran@linutronix.de
To: Jouni Malinen <j@w1.fi>
Cc: Richard Cochran <richardcochran@gmail.com>,
        Jouni Malinen <jkmalinen@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>, Rik van Riel <riel@redhat.com>,
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        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@kernel.org>, Arjan van de Ven <arjan@infradead.org>
Subject: Re: [PREEMPT-RT] [patch 4 14/22] timer: Switch to a non cascading
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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