linux-kernel - Re: [RFC] jiffies_till_first

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Message-ID: <b6bd973d-3cc6-4174-aaae-f3d67d8ccca1@paulmck-laptop>
Date: Thu, 25 Dec 2025 10:54:20 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Joel Fernandes <joelagnelf@...dia.com>
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

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".  ;-)

If we want to support this, then perhaps we would need to avoid that
schedule_timeout(0).  Or rcu_gp_fqs_check_wake(), as the case may be.

							Thanx, Paul