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

[Christoph Lameter]

On Tue, 27 Mar 2012, Frederic Weisbecker 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.
>
> What is the usecase you have in mind? Is it for realtime purpose?

Please do not use "realtime" since I am not sure what you mean by that.
Its for a low latency applications that cannot use "realtime" because that
implies high latencies.

> The "tick stopped" is a volatile and relative state.

The use case is an application that cannot tolerate the latencies
introduced by timer tick processing. It will only start running when the
system is in a sufficiently quiet state.

> Relative because if a timer list is enqueued to fire 1 second later,
> the tick will be stopped until that happens. How do we consider this (common)
> case?
>
> Also as Chris noted it is volatile because the tick can be restarted anytime
> for random reasons: the CPU receives an IPI which makes it restart the
> periodic tick.

Ok some sort of notification would be good for that case. If a timer tick
happens and that was unavoidable then it would be good to log the reason
why this occured so that the system can be configured in such a way that
these interruptions are minimized.

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