Re: [RFC] jiffies_till_first_fqs off by 1

[Joel Fernandes <joelagnelf@nvidia.com>]

Hi Paul,

On Tue, Dec 23, 2025 at 03:53:23PM -0800, Paul E. McKenney wrote:
> On Tue, Dec 23, 2025 at 12:38:19PM -0500, Joel Fernandes wrote:
> > 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.
> 
> Inadvertent passing of the value zero?

This should not be an issue because at the moment, even a value of
jiffies_till_first_fqs == 0 waits for ~1 jiffie due to schedule_timeout(0).

But you raise a good point, we should cap the minimum allowed jiffie value
for the fqs parameters to 1 so that we don't pass schedule_timeout() with
negative values when/if we do the reduce-by-one approach.

thanks,

 - Joel