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

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Message-ID: <1471024216.32433.5.camel@redhat.com>
Date:	Fri, 12 Aug 2016 13:50:16 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Jouni Malinen <jkmalinen@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Chris Mason <clm@...com>,
	Arjan van de Ven <arjan@...radead.org>, rt@...utronix.de,
	George Spelvin <linux@...encehorizons.net>,
	Len Brown <lenb@...nel.org>,
	Josh Triplett <josh@...htriplett.org>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [patch 4 14/22] timer: Switch to a non cascading wheel

On Thu, 2016-08-11 at 18:21 +0300, Jouni Malinen wrote:
> On Mon, Jul 4, 2016 at 12:50 PM, Thomas Gleixner <tglx@...utronix.de>
> wrote:
> > The current timer wheel has some drawbacks:
> ...
> 
> It looks like this change (commit
> 500462a9de657f86edaa102f8ab6bff7f7e43fc2 in linux.git) breaks one of
> the automated test cases I'm using to test hostapd and wpa_supplicant
> with mac80211_hwsim from the kernel. I'm not sure what exactly causes
> this (did not really expect git bisect to point to timers..), but
> this
> seems to be very reproducible for me under kvm (though, this
> apparently did not happen on another device, so I'm not completely
> sure what it is needed to reproduce) with the ap_wps_er_http_proto
> test cases failing to connect 20 TCP stream sockets to a server on
> the
> localhost. The client side is a python test script and the server is
> hostapd. The failure shows up with about the 13th of those socket
> connects failing while all others (both before and after this failed
> one) going through.
> 
> Would you happen to have any idea why this commit has such a
> difference in behavior? 

I have a vague hypothesis, more of a question actually.

How does the new timer wheel code handle lost timer ticks?

If a KVM guest does not run for a while, because the host
is scheduling something else, the guest generally only gets
one timer tick after the guest is scheduled back in.

If there are multiple lost ticks, they will remain lost.

Could that cause the new timer wheel code to skip over
timer buckets occasionally, or is this hypothesis bunk?

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