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From: Rik van Riel <riel@redhat.com>
To: Jouni Malinen <jkmalinen@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Paul McKenney <paulmck@linux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Chris Mason <clm@fb.com>, Arjan van de Ven <arjan@infradead.org>,
	rt@linutronix.de, George Spelvin <linux@sciencehorizons.net>,
	Len Brown <lenb@kernel.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Eric Dumazet <edumazet@google.com>
Subject: Re: [patch 4 14/22] timer: Switch to a non cascading wheel
Date: Fri, 12 Aug 2016 13:50:16 -0400	[thread overview]
Message-ID: <1471024216.32433.5.camel@redhat.com> (raw)
In-Reply-To: <CANe27j+-sm3nVZZg0U2LWCduKMVw6WJJwMa6aSaxB5M-d53kPw@mail.gmail.com>

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On Thu, 2016-08-11 at 18:21 +0300, Jouni Malinen wrote:
> On Mon, Jul 4, 2016 at 12:50 PM, Thomas Gleixner <tglx@linutronix.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?

-- 

All Rights Reversed.

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  parent reply	other threads:[~2016-08-12 17:50 UTC|newest]

Thread overview: 60+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-07-04  9:50 [patch 4 00/22] timer: Refactor the timer wheel Thomas Gleixner
2016-07-04  9:50 ` [patch 4 01/22] timer: Make pinned a timer property Thomas Gleixner
2016-07-07  8:39   ` [tip:timers/core] timers: Make 'pinned' " tip-bot for Thomas Gleixner
2016-07-04  9:50 ` [patch 4 02/22] x86/apic/uv: Initialize timer as pinned Thomas Gleixner
2016-07-07  8:40   ` [tip:timers/core] timers, x86/apic/uv: Initialize the UV heartbeat " tip-bot for Thomas Gleixner
2016-07-04  9:50 ` [patch 4 03/22] x86/mce: Initialize " Thomas Gleixner
2016-07-07  8:40   ` [tip:timers/core] timers, x86/mce: Initialize MCE restart " tip-bot for Thomas Gleixner
2016-07-04  9:50 ` [patch 4 04/22] cpufreq/powernv: Initialize " Thomas Gleixner
2016-07-07  8:41   ` [tip:timers/core] timers, cpufreq/powernv: Initialize the gpstate " tip-bot for Thomas Gleixner
2016-07-04  9:50 ` [patch 4 05/22] driver/net/ethernet/tile: Initialize " Thomas Gleixner
2016-07-07  8:41   ` [tip:timers/core] timers, driver/net/ethernet/tile: Initialize the egress " tip-bot for Thomas Gleixner
2016-07-04  9:50 ` [patch 4 06/22] drivers/tty/metag_da: Initialize " Thomas Gleixner
2016-07-07  8:42   ` [tip:timers/core] timers, drivers/tty/metag_da: Initialize the poll " tip-bot for Thomas Gleixner
2016-07-04  9:50 ` [patch 4 07/22] drivers/tty/mips_ejtag: Initialize " Thomas Gleixner
2016-07-07  8:42   ` [tip:timers/core] timers, drivers/tty/mips_ejtag: Initialize the poll " tip-bot for Thomas Gleixner
2016-07-04  9:50 ` [patch 4 08/22] net/ipv4/inet: Initialize timers " Thomas Gleixner
2016-07-07  8:43   ` [tip:timers/core] timers, net/ipv4/inet: Initialize connection request " tip-bot for Thomas Gleixner
2016-07-04  9:50 ` [patch 4 09/22] timer: Remove mod_timer_pinned Thomas Gleixner
2016-07-07  8:43   ` [tip:timers/core] timers: Remove the deprecated mod_timer_pinned() API tip-bot for Thomas Gleixner
2016-07-04  9:50 ` [patch 4 10/22] signal: Use hrtimer for sigtimedwait Thomas Gleixner
2016-07-07  8:43   ` [tip:timers/core] signals: Use hrtimer for sigtimedwait() tip-bot for Thomas Gleixner
2016-07-04  9:50 ` [patch 4 11/22] hlist: Add hlist_is_singular_node() helper Thomas Gleixner
2016-07-07  8:44   ` [tip:timers/core] " tip-bot for Thomas Gleixner
2016-07-04  9:50 ` [patch 4 12/22] timer: Give a few structs and members proper names Thomas Gleixner
2016-07-07  8:44   ` [tip:timers/core] timers: " tip-bot for Thomas Gleixner
2016-07-04  9:50 ` [patch 4 13/22] timer: Reduce the CPU index space to 256k Thomas Gleixner
2016-07-07  8:45   ` [tip:timers/core] timers: " tip-bot for Thomas Gleixner
2016-07-04  9:50 ` [patch 4 14/22] timer: Switch to a non cascading wheel Thomas Gleixner
2016-07-07  8:45   ` [tip:timers/core] timers: Switch to a non-cascading wheel tip-bot for Thomas Gleixner
2016-08-11 15:21   ` [patch 4 14/22] timer: Switch to a non cascading wheel Jouni Malinen
2016-08-11 20:25     ` [PREEMPT-RT] " rcochran
2016-08-13  9:12       ` Jouni Malinen
2016-08-16  9:46         ` Richard Cochran
2016-08-16 14:35           ` Eric Dumazet
2016-08-17  9:05           ` Jouni Malinen
2016-08-17  9:23             ` rcochran
2016-08-12 17:50     ` Rik van Riel [this message]
2016-08-12 19:14       ` Paul E. McKenney
2016-08-16  8:55         ` Richard Cochran
2016-08-16  7:57       ` Richard Cochran
2016-07-04  9:50 ` [patch 4 15/22] timer: Remove slack leftovers Thomas Gleixner
2016-07-07  8:46   ` [tip:timers/core] timers: Remove set_timer_slack() leftovers tip-bot for Thomas Gleixner
2016-07-22 11:31   ` [patch 4 15/22] timer: Remove slack leftovers Jason A. Donenfeld
2016-07-22 13:04     ` Thomas Gleixner
2016-07-22 15:18       ` Jason A. Donenfeld
2016-07-22 22:54         ` Jason A. Donenfeld
2016-07-04  9:50 ` [patch 4 16/22] timer: Move __run_timers() function Thomas Gleixner
2016-07-07  8:46   ` [tip:timers/core] timers: " tip-bot for Anna-Maria Gleixner
2016-07-04  9:50 ` [patch 4 17/22] timer: Optimize collect timers for NOHZ Thomas Gleixner
2016-07-07  8:47   ` [tip:timers/core] timers: Optimize collect_expired_timers() " tip-bot for Anna-Maria Gleixner
2016-07-04  9:50 ` [patch 4 18/22] tick/sched: Remove pointless empty function Thomas Gleixner
2016-07-07  8:47   ` [tip:timers/core] timers/nohz: Remove pointless tick_nohz_kick_tick() function tip-bot for Thomas Gleixner
2016-07-04  9:50 ` [patch 4 19/22] timer: Forward wheel clock whenever possible Thomas Gleixner
2016-07-07  8:48   ` [tip:timers/core] timers: Forward the " tip-bot for Thomas Gleixner
2016-07-04  9:50 ` [patch 4 20/22] timer: Only wake softirq if necessary Thomas Gleixner
2016-07-07  8:48   ` [tip:timers/core] timers: " tip-bot for Thomas Gleixner
2016-07-04  9:50 ` [patch 4 21/22] timer: Split out index calculation Thomas Gleixner
2016-07-07  8:48   ` [tip:timers/core] timers: " tip-bot for Anna-Maria Gleixner
2016-07-04  9:50 ` [patch 4 22/22] timer: Optimization for same expiry time in mod_timer() Thomas Gleixner
2016-07-07  8:49   ` [tip:timers/core] timers: Implement optimization " tip-bot for Anna-Maria Gleixner

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