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

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Message-ID: <f27e7974-f102-f9dc-6b48-9814b88465bf@linaro.org>
Date:   Fri, 21 Feb 2020 22:11:10 +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 21/02/2020 21:54, Dmitry Osipenko wrote:
> 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.

Why not if computed in the worst case scenario?

Anyway, I'll let you give a try.

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