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: <239a2b66-8da8-2e6c-d19d-9ed207ad0a64@gmail.com>
Date:   Fri, 21 Feb 2020 19:56:51 +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

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>
>> ---
>>  arch/arm/mach-tegra/cpuidle-tegra20.c | 47 +++++++++++++++++++++++++--
>>  1 file changed, 45 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm/mach-tegra/cpuidle-tegra20.c b/arch/arm/mach-tegra/cpuidle-tegra20.c
>> index 9672c619f4bc..bcc158b72e67 100644
>> --- a/arch/arm/mach-tegra/cpuidle-tegra20.c
>> +++ b/arch/arm/mach-tegra/cpuidle-tegra20.c
>> @@ -83,14 +83,57 @@ static inline void tegra20_wake_cpu1_from_reset(void)
>>  }
>>  #endif
>>  
>> +static void tegra20_report_cpus_state(void)
>> +{
>> +	unsigned long cpu, lcpu, csr;
>> +
>> +	for_each_cpu(lcpu, cpu_possible_mask) {
>> +		cpu = cpu_logical_map(lcpu);
>> +		csr = flowctrl_read_cpu_csr(cpu);
>> +
>> +		pr_err("cpu%lu: online=%d flowctrl_csr=0x%08lx\n",
>> +		       cpu, cpu_online(lcpu), csr);
>> +	}
>> +}
>> +
>> +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()?

>> +
>> +		/*
>> +		 * 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?