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 23:54:03 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
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

21.02.2020 23:48, Daniel Lezcano пишет:
> On 21/02/2020 21:21, Dmitry Osipenko wrote:
>> 21.02.2020 23:02, Daniel Lezcano пишет:
> 
> [ ... ]
> 
>>>>>>>> +
>>>>>>>> +		/*
>>>>>>>> +		 * 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.
>>>>
>>>> Please notice that this code runs only on an older Cortex-A9/A15, which
>>>> doesn't support WFE for the delaying, and thus, CPU always busy-loops
>>>> inside udelay().
>>>>
>>>> What about if I'll add cpu_relax() to the loop? Do you think it it could
>>>> have any positive effect?
>>>
>>> I think udelay() has a call to cpu_relax().
>>
>> Yes, my point is that udelay() doesn't bring much benefit for us here
>> because:
>>
>> 1. we want to enter into power-gated state as quick as possible and
>> udelay() just adds an unnecessary delay
>>
>> 2. udelay() spins in a busy-loop until delay is expired, just like we're
>> doing it in this function already
> 
> In this case why not remove ktime_get() and increase the number of retries?

Because the busy-loop performance depends on CPU's frequency, so we
can't rely on a bare number of the retries.