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Subject: Re: [PATCH 2/2] PM / devfreq: Add devfreq_transitions debugfs file
To:     Dmitry Osipenko <digetx@gmail.com>, linux-pm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Cc:     leonard.crestez@nxp.com, lukasz.luba@arm.com, a.swigon@samsung.com,
        m.szyprowski@samsung.com, enric.balletbo@collabora.com,
        hl@rock-chips.com, bjorn.andersson@linaro.org,
        jcrouse@codeaurora.org, chanwoo@kernel.org,
        myungjoo.ham@samsung.com, kyungmin.park@samsung.com
From:   Chanwoo Choi <cw00.choi@samsung.com>
Organization: Samsung Electronics
Message-ID: <e9895970-7f73-0ac4-c9cd-4ad85d464e11@samsung.com>
Date:   Thu, 9 Jan 2020 17:14:05 +0900
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On 1/8/20 11:10 PM, Dmitry Osipenko wrote:
> 08.01.2020 14:22, Chanwoo Choi пишет:
>> On 1/8/20 6:56 AM, Dmitry Osipenko wrote:
>>> 07.01.2020 12:05, Chanwoo Choi пишет:
>>>> Add new devfreq_transitions debugfs file to track the frequency transitions
>>>> of all devfreq devices for the simple profiling as following:
>>>> - /sys/kernel/debug/devfreq/devfreq_transitions
>>>>
>>>> And the user can decide the storage size (CONFIG_NR_DEVFREQ_TRANSITIONS)
>>>> in Kconfig in order to save the transition history.
>>>>
>>>> [Detailed description of each field of 'devfreq_transitions' debugfs file]
>>>> - time_ms	: Change time of frequency transition. (unit: millisecond)
>>>> - dev_name	: Device name of h/w.
>>>> - dev		: Device name made by devfreq core.
>>>> - parent_dev	: If devfreq device uses the passive governor,
>>>> 		  show parent devfreq device name.
>>>> - load_%	: If devfreq device uses the simple_ondemand governor,
>>>> 		  load is used by governor whene deciding the new frequency.
>>>> 		  (unit: percentage)
>>>> - old_freq_hz	: Frequency before changing. (unit: hz)
>>>> - new_freq_hz	: Frequency after changed. (unit: hz)
>>>>
>>>> [For example on Exynos5422-based Odroid-XU3 board]
>>>> $ cat /sys/kernel/debug/devfreq/devfreq_transitions
>>>> time_ms    dev_name                       dev        parent_dev load_% old_freq_hz  new_freq_hz
>>>> ---------- ------------------------------ ---------- ---------- ---------- ------------ ------------
>>>> 14600      soc:bus_noc                    devfreq2   devfreq1   0      100000000    67000000
>>>> 14600      soc:bus_fsys_apb               devfreq3   devfreq1   0      200000000    100000000
>>>> 14600      soc:bus_fsys                   devfreq4   devfreq1   0      200000000    100000000
>>>> 14600      soc:bus_fsys2                  devfreq5   devfreq1   0      150000000    75000000
>>>> 14602      soc:bus_mfc                    devfreq6   devfreq1   0      222000000    96000000
>>>> 14602      soc:bus_gen                    devfreq7   devfreq1   0      267000000    89000000
>>>> 14602      soc:bus_g2d                    devfreq9   devfreq1   0      300000000    84000000
>>>> 14602      soc:bus_g2d_acp                devfreq10  devfreq1   0      267000000    67000000
>>>> 14602      soc:bus_jpeg                   devfreq11  devfreq1   0      300000000    75000000
>>>> 14602      soc:bus_jpeg_apb               devfreq12  devfreq1   0      167000000    84000000
>>>> 14603      soc:bus_disp1_fimd             devfreq13  devfreq1   0      200000000    120000000
>>>> 14603      soc:bus_disp1                  devfreq14  devfreq1   0      300000000    120000000
>>>> 14606      soc:bus_gscl_scaler            devfreq15  devfreq1   0      300000000    150000000
>>>> 14606      soc:bus_mscl                   devfreq16  devfreq1   0      333000000    84000000
>>>> 14608      soc:bus_wcore                  devfreq1              9      333000000    84000000
>>>> 14783      10c20000.memory-controller     devfreq0              35     825000000    633000000
>>>> 15873      soc:bus_wcore                  devfreq1              41     84000000     400000000
>>>> 15873      soc:bus_noc                    devfreq2   devfreq1   0      67000000     100000000
>>>> [snip]
>>>>
>>>> Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
>>>> ---
>>>>  drivers/devfreq/Kconfig            |  13 +++
>>>>  drivers/devfreq/devfreq.c          | 126 +++++++++++++++++++++++++++++
>>>>  drivers/devfreq/governor.h         |   3 +
>>>>  drivers/devfreq/governor_passive.c |   2 +
>>>>  include/linux/devfreq.h            |   1 +
>>>>  5 files changed, 145 insertions(+)
>>>>
>>>> diff --git a/drivers/devfreq/Kconfig b/drivers/devfreq/Kconfig
>>>> index 0b1df12e0f21..84936eec0ef9 100644
>>>> --- a/drivers/devfreq/Kconfig
>>>> +++ b/drivers/devfreq/Kconfig
>>>> @@ -74,6 +74,19 @@ config DEVFREQ_GOV_PASSIVE
>>>>  	  through sysfs entries. The passive governor recommends that
>>>>  	  devfreq device uses the OPP table to get the frequency/voltage.
>>>>  
>>>> +comment "DEVFREQ Debugging"
>>>> +
>>>> +config NR_DEVFREQ_TRANSITIONS
>>>> +	int "Maximum storage size to save DEVFREQ Transitions (10-1000)"
>>>> +	depends on DEBUG_FS
>>>> +	range 10 1000
>>>> +	default "100"
>>>> +	help
>>>> +	  Show the frequency transitions of all devfreq devices via
>>>> +	  '/sys/kernel/debug/devfreq/devfreq_transitions' for the simple
>>>> +	  profiling. It needs to decide the storage size to save transition
>>>> +	  history of all devfreq devices.
>>>> +
>>>>  comment "DEVFREQ Drivers"
>>>>  
>>>>  config ARM_EXYNOS_BUS_DEVFREQ
>>>> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
>>>> index c7f5e4e06420..7abaae06fa65 100644
>>>> --- a/drivers/devfreq/devfreq.c
>>>> +++ b/drivers/devfreq/devfreq.c
>>>> @@ -268,6 +268,57 @@ int devfreq_update_status(struct devfreq *devfreq, unsigned long freq)
>>>>  }
>>>>  EXPORT_SYMBOL(devfreq_update_status);
>>>>  
>>>> +/**
>>>> + * devfreq_update_transitions() - Update frequency transitions for debugfs file
>>>> + * @devfreq:	the devfreq instance
>>>> + * @old_freq:	the previous frequency before changing the frequency
>>>> + * @new_freq:	the new frequency after frequency is changed
>>>> + */
>>>> +struct devfreq_transitions {
>>>> +	struct devfreq *devfreq;
>>>> +	struct devfreq_freqs freqs;
>>>> +	unsigned long load;
>>>> +} debugfs_transitions[CONFIG_NR_DEVFREQ_TRANSITIONS];
>>>> +
>>>> +static spinlock_t devfreq_debugfs_lock;
>>>> +static int debugfs_transitions_index;
>>>> +
>>>> +void devfreq_update_transitions(struct devfreq *devfreq,
>>>> +			unsigned long old_freq, unsigned long new_freq,
>>>> +			unsigned long busy_time, unsigned long total_time)
>>>> +{
>>>> +	unsigned long load;
>>>> +	int i;
>>>> +
>>>> +	if (!devfreq_debugfs || !devfreq || (old_freq == new_freq))
>>>> +		return;
>>>> +
>>>> +	spin_lock_nested(&devfreq_debugfs_lock, SINGLE_DEPTH_NESTING);
>>>> +
>>>> +	i = debugfs_transitions_index;
>>>> +
>>>> +	/*
>>>> +	 * Calculate the load and if load is larger than 100,
>>>> +	 * initialize to 100 because the unit of load is percentage.
>>>> +	 */
>>>> +	load = (total_time == 0 ? 0 : (100 * busy_time) / total_time);
>>>> +	if (load > 100)
>>>> +		load = 100;
>>>> +
>>>> +	debugfs_transitions[i].devfreq = devfreq;
>>>> +	debugfs_transitions[i].freqs.time = ktime_to_ms(ktime_get());
>>>> +	debugfs_transitions[i].freqs.old = old_freq;
>>>> +	debugfs_transitions[i].freqs.new = new_freq;
>>>> +	debugfs_transitions[i].load = load;
>>>> +
>>>> +	if (++i == CONFIG_NR_DEVFREQ_TRANSITIONS)
>>>> +		i = 0;
>>>> +	debugfs_transitions_index = i;
>>>> +
>>>> +	spin_unlock(&devfreq_debugfs_lock);
>>>> +}
>>>> +EXPORT_SYMBOL(devfreq_update_transitions);
>>>> +
>>>>  /**
>>>>   * find_devfreq_governor() - Find devfreq governor from name
>>>>   * @name:	name of the governor
>>>> @@ -401,6 +452,10 @@ static int set_target(struct devfreq *devfreq,
>>>>  		return err;
>>>>  	}
>>>>  
>>>> +	devfreq_update_transitions(devfreq, cur_freq, new_freq,
>>>> +					devfreq->last_status.busy_time,
>>>> +					devfreq->last_status.total_time);
>>>> +
>>>>  	freqs.new = new_freq;
>>>>  	notify_transition(devfreq, &freqs, DEVFREQ_POSTCHANGE);
>>>>  
>>>> @@ -1787,6 +1842,72 @@ static int devfreq_summary_show(struct seq_file *s, void *data)
>>>>  }
>>>>  DEFINE_SHOW_ATTRIBUTE(devfreq_summary);
>>>>  
>>>> +/**
>>>> + * devfreq_transitions_show() - Show the frequency transitions of the registered
>>>> + *			devfreq devices via 'devfreq_transitions' debugfs file.
>>>> + */
>>>> +static int devfreq_transitions_show(struct seq_file *s, void *data)
>>>> +{
>>>> +	struct devfreq *devfreq = NULL;
>>>> +	struct devfreq *p_devfreq = NULL;
>>>> +	struct devfreq_freqs *freqs = NULL;
>>>> +	unsigned long load;
>>>> +	int i = debugfs_transitions_index;
>>>> +	int count;
>>>> +
>>>> +	seq_printf(s, "%-10s %-30s %-10s %-10s %-6s %-12s %-12s\n",
>>>> +			"time_ms",
>>>> +			"dev_name",
>>>> +			"dev",
>>>> +			"parent_dev",
>>>> +			"load_%",
>>>> +			"old_freq_hz",
>>>> +			"new_freq_hz");
>>>> +	seq_printf(s, "%-10s %-30s %-10s %-10s %-6s %-12s %-12s\n",
>>>> +			"----------",
>>>> +			"------------------------------",
>>>> +			"----------",
>>>> +			"----------",
>>>> +			"----------",
>>>> +			"------------",
>>>> +			"------------");
>>>
>>> Isn't this needed here?
>>>
>>> mutex_lock(&devfreq_list_lock);
>>
>> It doesn't touch the devfreq instance of devfreq_list.
>> So, it is not necessary locked of devfreq_list_lock.
> 
> What stops devfreq device to be removed by another CPU thread while this
> function is in a process of execution?
> 
> This condition is unlikely to happen in practice ever, but technically
> it should be possible to happen.
> 
>>>> +	spin_lock(&devfreq_debugfs_lock);
>>>> +	for (count = 0; count < CONFIG_NR_DEVFREQ_TRANSITIONS; count++) {
>>>> +		devfreq = debugfs_transitions[i].devfreq;
>>>> +		freqs = &debugfs_transitions[i].freqs;
>>>> +		load = debugfs_transitions[i].load;
>>>> +
>>>> +		i = (CONFIG_NR_DEVFREQ_TRANSITIONS == ++i) ? 0 : i;
>>>> +		if (!devfreq)
>>>> +			continue;
>>>
>>> I suppose debugfs_transitions[i].devfreq should be set to NULL when
>>> devfreq device is removed, but I don't see it happening anywhere in this
>>> patch.
>>
>> When debugfs_transitions[] array is not fully filled out
>> by devfreq_update_transitions(), debugfs_transitions[i].devfreq is NULL.
>> In this case, if user execute 'cat /sys/kernel/debug/devfreq/devfreq_transitions',
>> devfreq_transitions_show() need to check the debugfs_transitions[i].devfreq
>> is NULL or not.
>>
>> After filled out the debugfs_transitions[] array,
>> actually, 'if(!devfreq)' is not necessary. Maybe, this style is inefficient
>> It need to rework. I'll think again.
> 
> Imagine this situation:
> 
> 1. there is a devfreq device, let's name it defreq123
> 
> 2. the debugfs_transitions array is getting filled and now it has this
> entry:
> 
> 	debugfs_transitions[0].devfreq = defreq123
> 
> 3. user removes defreq123 driver module
> 
> 	# rmmod defreq123
> 
> 4. the defreq123 is released now
> 
> 5. at what memory location debugfs_transitions[0].devfreq is pointing now?
> 
> 

You're right. It is my missing point.
Instead of storing the devfreq pointer into debugfs_transitions[].devfreq,
It is better to copy the necessary information to debugfs_transitions[]
for preventing the mentioned situation.

-- 
Best Regards,
Chanwoo Choi
Samsung Electronics