linux-kernel - Re: [patch V2 00/20] timer: Refactor the timer wheel

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Message-Id: <20160620150555.GR3923@linux.vnet.ibm.com>
Date:	Mon, 20 Jun 2016 08:05:55 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Chris Mason <clm@...com>,
	Arjan van de Ven <arjan@...radead.org>, rt@...utronix.de,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	George Spelvin <linux@...encehorizons.net>,
	Len Brown <lenb@...nel.org>
Subject: Re: [patch V2 00/20] timer: Refactor the timer wheel

On Fri, Jun 17, 2016 at 01:26:28PM -0000, Thomas Gleixner wrote:
> This is the second version of the timer wheel rework series. The first series
> can be found here:
> 
>    http://lkml.kernel.org/r/20160613070440.950649741@linutronix.de
> 
> The series is also available in git:
> 
>    git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git WIP.timers

Ran some longer rcutorture tests, and the scripting complained about
hangs.  This turned out to be due to the 12.5% uncertainty, so I fixed
this by switching the rcutorture stop-test timer to hrtimers.  Things are
now working as well as before, with the exception of SRCU, for which I
am getting lots of grace-period stall complaints.  This came as a bit
of a surprise.  Anyway, I will be reviewing SRCU for timing dependencies.

							Thanx, Paul

> Changes vs. V1:
> 
>  - Addressed the review comments of V1
> 
>      - Fixed the fallout in tty/metag (noticed by Arjan)
>      - Renamed the hlist helper (noticed by Paolo/George)
>      - Used the proper mask in get_timer_base() (noticed by Richard)
>      - Fixed the inverse state check in internal_add_timer() (noticed by Richard)
>      - Simplified the macro maze, removed wrapper (noticed by George)
>      - Reordered data retrieval in run_timer() (noticed by George)
> 
>  - Removed cascading completely
> 
>    We have a hard cutoff of expiry times at the capacity of the last wheel
>    level now. Timers which insist on timeouts longer than that, i.e. ~6days,
>    will expire at the cutoff, i.e. ~6 days. From our data gathering the
>    largest timeouts are 5 days (networking contrack), which are well in the
>    capacity.
> 
>    To achieve this capacity with HZ=1000 without increasing the storage size
>    by another level, we reduced the granularity of the first wheel level from
>    1ms to 4ms. According to our data, there is no user which relies on that
>    1ms granularity and 99% of those timers are canceled before expiry.
> 
>    As a side effect there is the benefit of better batching in the first level
>    which helps networking to avoid rearming timers in the hotpath.
> 
> We gathered more data about performance and batching. Compared to mainline the
> following changes have been observed:
> 
>    - The bad outliers in mainline when the timer wheel needs to be forwarded
>      after a long idle sleep are completely gone.
> 
>    - The total cpu time used for timer softirq processing is significantly
>      reduced. Depending on the HZ setting and workload this ranges from factor
>      2 to 6.
> 
>    - The average invocation period of the timer softirq on an idle system
>      increases significantly. Depending on the HZ settings and workload this
>      ranges from factor 1.5 to 5. That means that the residency in deep
>      c-states should be improved. Have not yet have time to verify this with
>      the power tools.
> 
> Thanks,
> 
> 	tglx
> 
> ---
>  arch/x86/kernel/apic/x2apic_uv_x.c  |    4 
>  arch/x86/kernel/cpu/mcheck/mce.c    |    4 
>  block/genhd.c                       |    5 
>  drivers/cpufreq/powernv-cpufreq.c   |    5 
>  drivers/mmc/host/jz4740_mmc.c       |    2 
>  drivers/net/ethernet/tile/tilepro.c |    4 
>  drivers/power/bq27xxx_battery.c     |    5 
>  drivers/tty/metag_da.c              |    4 
>  drivers/tty/mips_ejtag_fdc.c        |    4 
>  drivers/usb/host/ohci-hcd.c         |    1 
>  drivers/usb/host/xhci.c             |    2 
>  include/linux/list.h                |   10 
>  include/linux/timer.h               |   30 
>  kernel/time/tick-internal.h         |    1 
>  kernel/time/tick-sched.c            |   46 -
>  kernel/time/timer.c                 | 1099 +++++++++++++++++++++---------------
>  lib/random32.c                      |    1 
>  net/ipv4/inet_connection_sock.c     |    7 
>  net/ipv4/inet_timewait_sock.c       |    5 
>  19 files changed, 725 insertions(+), 514 deletions(-)
> 
>