Re: [PATCH 11/32] nohz/cpuset: Don't turn off the tick if rcu needs it

[Gilad Ben-Yossef]

On Thu, Mar 22, 2012 at 6:18 PM, Christoph Lameter <cl@xxxxxxxxx> wrote:
> On Thu, 22 Mar 2012, Gilad Ben-Yossef wrote:
>
>> > Is there any way for userspace to know that the tick is not off yet due to
>> > this? It would make sense for us to have busy loop in user space that
>> > waits until the OS has completed all processing if that avoids future
>> > latencies for the application.
>> >
>>
>> I previously suggested having the user register to receive a signal
>> when the tick
>> is turned off. Since the tick is always turned off the user task is
>> the current task
>> by design, *I think* you can simply mark the signal pending when you
>> turn the tick off.
>
> Ok that sounds good. You would define a new signal for this?
>

My gut instinct is to let the process register with a specific signal
(properly the RT range)
it wants to receive when the tick goes off and/or on.

> So we would startup the application. App will do all prep work (memory
> allocation, device setup etc etc) and then wait for the signal to be
> received. After that it would enter the low latency processing phase.
>
> Could we also get a signal if something disrupts the peace and switches
> the timer interrupt on again?
>

I think you'll have to since once you have the tick turned off there
is no guarantee that
it wont get turned on by a timer scheduling an task or an IPI.


-- 
Gilad Ben-Yossef
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gilad@xxxxxxxxxxxxx
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 -- Jean-Baptiste Queru
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