linux-kernel - Re: [PATCH v9 09/17] arm: tegra20: cpuidle: Handle case where secondary CPU hangs on entering LP2

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Date:   Fri, 21 Feb 2020 18:36:50 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Peter De Schrijver <pdeschrijver@...dia.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        Jasper Korten <jja2000@...il.com>,
        David Heidelberg <david@...t.cz>,
        Peter Geis <pgwipeout@...il.com>, linux-pm@...r.kernel.org,
        linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 09/17] arm: tegra20: cpuidle: Handle case where
 secondary CPU hangs on entering LP2

On Fri, Feb 21, 2020 at 07:56:51PM +0300, Dmitry Osipenko wrote:
> Hello Daniel,
> 
> 21.02.2020 18:43, Daniel Lezcano пишет:
> > On Thu, Feb 13, 2020 at 02:51:26AM +0300, Dmitry Osipenko wrote:
> >> It is possible that something may go wrong with the secondary CPU, in that
> >> case it is much nicer to get a dump of the flow-controller state before
> >> hanging machine.
> >>
> >> Acked-by: Peter De Schrijver <pdeschrijver@...dia.com>
> >> Tested-by: Peter Geis <pgwipeout@...il.com>
> >> Tested-by: Jasper Korten <jja2000@...il.com>
> >> Tested-by: David Heidelberg <david@...t.cz>
> >> Signed-off-by: Dmitry Osipenko <digetx@...il.com>
> >> ---

[ ... ]

> >> +static int tegra20_wait_for_secondary_cpu_parking(void)
> >> +{
> >> +	unsigned int retries = 3;
> >> +
> >> +	while (retries--) {
> >> +		ktime_t timeout = ktime_add_ms(ktime_get(), 500);
> > 
> > Oops I missed this one. Do not use ktime_get() in this code path, use jiffies.
> 
> Could you please explain what benefits jiffies have over the ktime_get()?

ktime_get() is very slow, jiffies is updated every tick.

> >> +
> >> +		/*
> >> +		 * The primary CPU0 core shall wait for the secondaries
> >> +		 * shutdown in order to power-off CPU's cluster safely.
> >> +		 * The timeout value depends on the current CPU frequency,
> >> +		 * it takes about 40-150us  in average and over 1000us in
> >> +		 * a worst case scenario.
> >> +		 */
> >> +		do {
> >> +			if (tegra_cpu_rail_off_ready())
> >> +				return 0;
> >> +
> >> +		} while (ktime_before(ktime_get(), timeout));
> > 
> > So this loop will aggresively call tegra_cpu_rail_off_ready() and retry 3
> > times. The tegra_cpu_rail_off_ready() function can be called thoushand of times
> > here but the function will hang 1.5s :/
> > 
> > I suggest something like:
> > 
> > 	while (retries--i && !tegra_cpu_rail_off_ready()) 
> > 		udelay(100);
> > 
> > So <retries> calls to tegra_cpu_rail_off_ready() and 100us x <retries> maximum
> > impact.
> But udelay() also results into CPU spinning in a busy-loop, and thus,
> what's the difference?

busy looping instead of register reads with all the hardware things involved behind.

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