linux-kernel - Re: [PATCH v3] clocksource: Scale the max retry number of watchdog read according to CPU numbers

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Message-ID: <0b7833a4-75f3-43ba-9d87-6f83cf4faa5a@paulmck-laptop>
Date: Tue, 20 Feb 2024 07:24:27 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Waiman Long <longman@...hat.com>
Cc: Feng Tang <feng.tang@...el.com>, Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <jstultz@...gle.com>, Stephen Boyd <sboyd@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org,
	Jin Wang <jin1.wang@...el.com>
Subject: Re: [PATCH v3] clocksource: Scale the max retry number of watchdog
 read according to CPU numbers

On Mon, Feb 19, 2024 at 09:20:31PM -0500, Waiman Long wrote:
> 
> On 2/19/24 09:37, Feng Tang wrote:
> > Hi Thomas,
> > 
> > On Mon, Feb 19, 2024 at 12:32:05PM +0100, Thomas Gleixner wrote:
> > > On Mon, Jan 29 2024 at 21:45, Feng Tang wrote:
> > > > +static inline long clocksource_max_watchdog_read_retries(void)
> > > > +{
> > > > +	long max_retries = max_cswd_read_retries;
> > > > +
> > > > +	if (max_cswd_read_retries <= 0) {
> > > > +		/* santity check for user input value */
> > > > +		if (max_cswd_read_retries != -1)
> > > > +			pr_warn_once("max_cswd_read_retries was set with an invalid number: %ld\n",
> > > > +				max_cswd_read_retries);
> > > > +
> > > > +		max_retries = ilog2(num_online_cpus()) + 1;
> > > I'm getting tired of these knobs and the horrors behind them. Why not
> > > simply doing the obvious:
> > > 
> > >         retries = ilog2(num_online_cpus()) + 1;
> > > 
> > > and remove the knob alltogether?
> > Thanks for the suggestion! Yes, this makes sense to me. IIUC, the
> > 'max_cswd_read_retries' was introduced mainly to cover different
> > platforms' requirement, which could now be covered by the new
> > self-adaptive number.
> > 
> > If there is no concern from other developers, I will send a new
> > version in this direction.
> 
> I see no problem simplifying it.

My guess is that we will eventually end up with something like this:

	retries = ilog2(num_online_cpus()) / 2 + 1;

but I am not at all opposed to starting without the division by 2.

							Thanx, Paul