From: Andy Lutomirski <luto@amacapital.net>
To: Frederic Weisbecker <fweisbec@gmail.com>, Alex Shi <alex.shi@linaro.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Steven Rostedt <rostedt@goodmis.org>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
John Stultz <john.stultz@linaro.org>,
Kevin Hilman <khilman@linaro.org>
Subject: Re: [RFC PATCH 00/13] nohz: Use sysidle detection to let the timekeeper sleep
Date: Wed, 18 Dec 2013 13:29:53 -0800 [thread overview]
Message-ID: <52B213D1.1000104@mit.edu> (raw)
In-Reply-To: <20131218174355.GA19788@localhost.localdomain>
On 12/18/2013 09:43 AM, Frederic Weisbecker wrote:
> On Wed, Dec 18, 2013 at 10:04:43AM +0800, Alex Shi wrote:
>> On 12/18/2013 06:51 AM, Frederic Weisbecker wrote:
>>> So this is what this series brings, more details following:
>>>
>>> * Some code, naming and whitespace cleanups
>>>
>>> * Allow all CPUs outside the nohz_full range to handle the timekeeping
>>> duty, not just CPU 0. Balancing the timekeeping duty should improve
>>> powersavings.
>>
>> If the system just has one nohz_full cpu running, it will need another
>> cpu to do timerkeeper job. Then the system roughly needs 2 cpu living.
>> From powersaving POV, that is not good compare to normal nohz idle.
>
> Sure, but everything has a tradeoff :)
>
> We could theoretically run with the timekeeper purely idle if the other
> CPU in full dynticks mode runs in userspace for a long while and seldom
> do syscalls and faults. Timekeeping could be updated on kernel/user
> boundaries in this case without much impact on performances.
>
> But then there is one strict condition for that: it can't read the timeofday
> through the vdso but only through a syscall.
Where's your ambition? :)
If the vdso timing functions could see that it's been too long since a
real timekeeping update, they could fall back to a syscall. Otherwise,
they could using rdtsc or whatever is in use.
--Andy
next prev parent reply other threads:[~2013-12-18 21:30 UTC|newest]
Thread overview: 57+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-12-17 22:51 [RFC PATCH 00/13] nohz: Use sysidle detection to let the timekeeper sleep Frederic Weisbecker
2013-12-17 22:51 ` [PATCH 01/13] tick: Rename tick_check_idle() to tick_irq_enter() Frederic Weisbecker
2014-01-25 14:22 ` [tip:timers/urgent] " tip-bot for Frederic Weisbecker
2013-12-17 22:51 ` [PATCH 02/13] time: New helper to check CPU eligibility to handle timekeeping Frederic Weisbecker
2013-12-17 22:51 ` [PATCH 03/13] rcu: Exclude all potential timekeepers from sysidle detection Frederic Weisbecker
2013-12-17 23:27 ` Paul E. McKenney
2013-12-17 23:49 ` Frederic Weisbecker
2013-12-18 11:43 ` Peter Zijlstra
2013-12-18 11:46 ` Peter Zijlstra
2013-12-18 14:15 ` Paul E. McKenney
2013-12-18 16:24 ` Frederic Weisbecker
2013-12-17 22:51 ` [PATCH 04/13] tick: Use timekeeping_cpu() to elect the CPU handling timekeeping duty Frederic Weisbecker
2013-12-17 23:55 ` Paul E. McKenney
2013-12-17 22:51 ` [PATCH 05/13] rcu: Fix unraised IPI to timekeeping CPU Frederic Weisbecker
2013-12-17 23:21 ` Paul E. McKenney
2013-12-18 14:13 ` Frederic Weisbecker
2013-12-18 14:22 ` Paul E. McKenney
2013-12-18 14:56 ` Frederic Weisbecker
2013-12-18 15:11 ` Peter Zijlstra
2013-12-18 15:58 ` Frederic Weisbecker
2013-12-18 12:12 ` Peter Zijlstra
2013-12-18 15:38 ` Frederic Weisbecker
2013-12-18 15:45 ` Christoph Hellwig
2013-12-18 17:10 ` Peter Zijlstra
2013-12-17 22:51 ` [PATCH 06/13] nohz: Introduce full dynticks' default timekeeping target Frederic Weisbecker
2013-12-17 23:54 ` Paul E. McKenney
2013-12-17 22:51 ` [PATCH 07/13] sched: Enable IPI reception on timekeeper under nohz full system Frederic Weisbecker
2013-12-17 23:52 ` Paul E. McKenney
2013-12-18 14:49 ` Frederic Weisbecker
2013-12-18 15:50 ` Paul E. McKenney
2013-12-18 10:06 ` Peter Zijlstra
2013-12-17 22:51 ` [PATCH 08/13] nohz: Get timekeeping max deferment outside jiffies_lock Frederic Weisbecker
2014-01-25 14:22 ` [tip:timers/urgent] " tip-bot for Frederic Weisbecker
2013-12-17 22:51 ` [PATCH 09/13] nohz: Allow timekeeper's tick to stop when all full dynticks CPUs are idle Frederic Weisbecker
2013-12-17 23:51 ` Paul E. McKenney
2013-12-18 14:36 ` Frederic Weisbecker
2013-12-18 15:29 ` Paul E. McKenney
2013-12-17 22:51 ` [PATCH 10/13] nohz: Hand over timekeeping duty on cpu offlining Frederic Weisbecker
2013-12-17 23:40 ` Paul E. McKenney
2013-12-18 14:19 ` Frederic Weisbecker
2013-12-18 12:30 ` Peter Zijlstra
2013-12-18 16:43 ` Frederic Weisbecker
2013-12-17 22:51 ` [PATCH 11/13] nohz: Wake up timekeeper on exit from sysidle state Frederic Weisbecker
2013-12-17 23:34 ` Paul E. McKenney
2013-12-17 23:52 ` Frederic Weisbecker
2013-12-17 22:51 ` [PATCH 12/13] nohz: Allow all CPUs outside nohz_full range to do timekeeping Frederic Weisbecker
2013-12-17 23:32 ` Paul E. McKenney
2013-12-17 22:51 ` [PATCH 13/13] nohz_full: fix code style issue of tick_nohz_full_stop_tick Frederic Weisbecker
2013-12-18 2:04 ` [RFC PATCH 00/13] nohz: Use sysidle detection to let the timekeeper sleep Alex Shi
2013-12-18 10:19 ` Peter Zijlstra
2013-12-18 14:18 ` Paul E. McKenney
2013-12-18 17:43 ` Frederic Weisbecker
2013-12-18 21:29 ` Andy Lutomirski [this message]
2013-12-18 21:49 ` Paul E. McKenney
2013-12-18 21:53 ` Andy Lutomirski
2013-12-18 21:57 ` Paul E. McKenney
2013-12-18 22:55 ` Andy Lutomirski
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