[RFC] jiffies_till_first_fqs off by 1

[Joel Fernandes <joelagnelf@nvidia.com>]

Hello,

During studying some synchronize_rcu() latencies, I found that the
jiffies_till_first_fqs value passed to the timer tick subsystem does is always
off by one. This is natural due to calc_index() rounding up.

For example, jiffies_till_first_fqs=3 means the "Jiffies till first FQS" delay
is actually 4ms. And same for the next FQS. In fact, in testing it shows it can
never ever be 3ms for HZ=1000. And in rare cases, it will go to 5ms probably due
to interrupts.

Considering this, I think it is better to reduce the jiffies_till_first_fqs by 1
before passing it to the wait APIs.

But before I wanted to send a patch, I wanted to get everyone's thoughts.
Considering this the RFC.

The other place I found this was when call_rcu_hurry() is called, but the GP
thread takes a tick to wake up, but this isn't related to the timer per-se, it
is just that we don't want to wake the GP thread too often. So we just wait for
the next tick to notice callbacks before doing a wakeup.

Heh, and this means synchronize_rcu() latencies will multiply when HZ < 1000. I
wonder if this is also what caused Uladzislau to investigate it for mobile devices.

 - Joel