Re: [PATCH tip/core/rcu 6/7] rcu: Drive quiescent-state-forcing delay from HZ

[Peter Zijlstra <peterz@infradead.org>]

On Wed, May 15, 2013 at 09:37:00AM -0700, Paul E. McKenney wrote:
> The need is to detect that an idle CPU is idle without making it do
> anything.  To do otherwise would kill battery lifetime and introduce
> OS jitter.

Not anything isn't leaving us much room to wriggle, we could maybe try and do a
wee bit without people shooting us :-) In fact, looking at rcu_idle_enter()
its very much not an empty function.

> This other CPU must be able to correctly detect idle CPUs regardless of
> how long they have been idle.  In particular, it is necessary to detect
> CPUs that were idle at the start of the current grace period and have
> remained idle throughout the entirety of the current grace period.

OK, so continuing this hypothetical merry go round :-) 

Since RCU is a global endeavour, I'm assuming there is a global GP sequence
number. Could we not stamp the CPU with the current GP# in rcu_idle_enter().

> A CPU might transition between idle and non-idle states at any time.
> Therefore, if RCU collects a given CPU's idleness state during a given
> grace period, it must be very careful to avoid relying on that state
> during some other grace period.

However, if we know during which GP it became idle, we know we can ignore it
for all GPs thereafter, right?

> Therefore, from what I can see, unless all CPUs explicitly report a
> quiescent state in a timely fashion during a given grace period (in
> which case each CPU was non-idle at some point during that grace period),
> there is no alternative to polling RCU's per-CPU rcu_dynticks structures
> during that grace period.  In particular, if at least one CPU remained
> idle throughout that grace period, it will be necessary to poll.

Agreed.. 

> Of course, during boot time, there are often long time periods during
> which at least one CPU remains idle.  Therefore, we can expect many
> boot-time grace periods to delay for at least one FQS time period.
> 
> OK, so how much delay does this cause? 

Oh, I'm so way past that, it is a neat puzzle by now ;-)