linux-kernel - Re: [RFC] jiffies_till_first

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Message-ID: <20251224020619.GA1297269@joelbox2>
Date: Tue, 23 Dec 2025 21:06:19 -0500
From: Joel Fernandes <joelagnelf@...dia.com>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: rcu <rcu@...r.kernel.org>, Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
	Josh Triplett <josh@...htriplett.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Uladzislau Rezki <urezki@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang@...ux.dev>
Subject: Re: [RFC] jiffies_till_first_fqs off by 1

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