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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