On Fri, Jan 02, 2026 at 12:58:08PM -0500, Joel Fernandes wrote:
>
>
> > On Jan 1, 2026, at 10:41 PM, Paul E. McKenney <[email protected]> wrote:
> >
> > On Thu, Jan 01, 2026 at 09:59:27PM -0500, Joel Fernandes wrote:
> >>
> >>
> >>> On 1/1/2026 5:24 PM, Paul E. McKenney wrote:
> >>> On Thu, Dec 25, 2025 at 09:15:59PM -0500, Joel Fernandes wrote:
> >>>> On Thu, Dec 25, 2025 at 10:54:20AM -0800, Paul E. McKenney wrote:
> >>>>> On Tue, Dec 23, 2025 at 09:06:19PM -0500, Joel Fernandes wrote:
> >>>>>> 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.
> >>>>>
> >>>>> There is a potential use case for jiffies_till_first_fqs=0 and no wait,
> >>>>> which would be systems that want to scan for idle CPUs immediately after
> >>>>> the grace period has been initialized. Note the word "potential". ;-)
> >>>>
> >>>> Sure, we could add support for that but that would be new behavior that
> >>>> is
> >>>> not in the existing code.
> >>>>
> >>>> So jiffies_till_first_fqs=0 today, I think it is not 'working as
> >>>> intended'
> >>>> because it will never not wait I think.
> >>>
> >>> Agreed.
> >>>>> So we should fix that too? Or maybe it can be a patch separate from this
> >>>> (that I can work on). I think no harming in allowing that mode, at least
> >>>> it
> >>>> will be more in line with the expected outcome.
> >>>
> >>> Makes sense! However, given that no one has complained, care is required.
> >>> Someone might be relying on the old behavior. (In which case an easy
> >>> fix would be to make -1 be no waiting, though one might hope for a
> >>> better fix.)
> >> Some further investigations revealed that the "1 jiffie error" is actually
> >> worst
> >> case. In the best case, it could still be closer to a jiffie. It is just
> >> the
> >> nature of the timer wheel, since it snaps to numerical TICK_NS boundary,
> >> the
> >> rounding error is intentionally added depending on how far along in the
> >> boundary
> >> was the timer for the wait enqueued. If we took probability distributions,
> >> we
> >> should be landing with a 1/2 jiffie error, though in practice I've seen it
> >> to be
> >> 3/4 jiffie error on average.
> >>
> >> Given this, it would probably not make sense for us to do the -1 to adjust
> >> for
> >> the error (since we don't clearly have bounds on the minimum error). We
> >> just
> >> have to accept that we'd lose 1-2 extra jiffie per FQS loop iteration wait,
> >> which is amplified if a grace period is already in progress. I've seen
> >> this add
> >> upto 4 jiffies to back-to-back synchronize_rcu() latency even when there
> >> are no
> >> readers in progress.
> > .
> >> But I had to go down the rabbit hole and check... ;-)
> >
> > I was thinking in terms of special-casing -1 to skip the sleep, but I
> > guess that there are as many ways to skin a rabbit as a cat. ;-)
>
> Sure I am happy to do that. One of my fears though is no one will know to use
> it that way making it not that useful.
>
> Do let me know if anyone sets it to 0 though. Perhaps for testing even to
> make the GP cycle shorter?
I do not know of anyone doing that, hence the non-urgency. The "-1"
would be just in case someone actually is setting it to zero, and
complains about us breaking userspace. :-/
Thanx, Paul
