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

[Gilad Ben-Yossef]

On Wed, Mar 21, 2012 at 4:54 PM, Christoph Lameter <cl@xxxxxxxxx> wrote:
> On Wed, 21 Mar 2012, Frederic Weisbecker wrote:
>
>> If RCU is waiting for the current CPU to complete a grace
>> period, don't turn off the tick. Unlike dynctik-idle, we
>> are not necessarily going to enter into rcu extended quiescent
>> state, so we may need to keep the tick to note current CPU's
>> quiescent states.
>
> 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.

The user would register a signal handler to set a flag when it is
called and then busy
loop waiting for a flag to clear.

Checking this approach is on my todo list, but alas I had no progress
with it yet :-)

Gilad



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