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Subject: Re: [PATCH 2/2] PM / devfreq: Add devfreq_transitions debugfs file
To:     Chanwoo Choi <cw00.choi@samsung.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
References: <20200107090519.3231-1-cw00.choi@samsung.com>
 <CGME20200107085812epcas1p4670ae2265573d887aa75cab36c04b1ea@epcas1p4.samsung.com>
 <20200107090519.3231-3-cw00.choi@samsung.com>
 <b64bf3d4-5b46-243b-495a-e1060af7a266@gmail.com>
 <9cfbdb8a-1326-e7e3-3a65-c3f8c45eaf19@samsung.com>
From:   Dmitry Osipenko <digetx@gmail.com>
Message-ID: <e93f6e16-e18d-bafd-5761-ffc8a2642149@gmail.com>
Date:   Wed, 8 Jan 2020 17:10:41 +0300
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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?