linux-kernel - Re: [PATCH] nohz1: Documentation

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Message-ID: <CAFTL4hzFmrHPHhi3gBj05eJgxb5H7bACEWwCm7jp6Vb0XU15bA@mail.gmail.com>
Date:	Mon, 25 Mar 2013 15:37:30 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Rob Landley <rob@...dley.net>, linux-kernel@...r.kernel.org,
	josh@...htriplett.org, zhong@...ux.vnet.ibm.com,
	khilman@...aro.org, geoff@...radead.org, tglx@...utronix.de
Subject: Re: [PATCH] nohz1: Documentation

2013/3/25 Christoph Lameter <cl@...ux.com>:
> On Fri, 22 Mar 2013, Paul E. McKenney wrote:
>
>> On Fri, Mar 22, 2013 at 02:38:58PM +0000, Christoph Lameter wrote:
>> > On Thu, 21 Mar 2013, Paul E. McKenney wrote:
>> >
>> > > So, how long of busy periods are you contemplating for your SCHED_FIFO
>> > > threads?  Is it possible to tune/adjust the offending per-CPU ktheads
>> > > to wake up less frequently than that time?
>> >
>> > Test programs right now run 10 seconds. 30 seconds would definitely be
>> > enough for the worst case.
>>
>> OK, that might be doable for some workloads.  What happens when you
>> try tuning the 2-second wakeup interval to (say) 45 seconds?
>
> The vm kernel threads do no useful work if no system calls are being done.
> If there is no kernel action then they can be deferred indefinitely.
>

We can certainly add some user deferrable timer_list. But that's going
to be for extreme usecases (those who require pure isolation) because
we'll need to settle that with a timer reprogramming into user/kernel
boundaries. That won't be free.
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