[PATCH] PM / wakeup: use seq_open() to show wakeup stats

[PATCH] PM / wakeup: use seq_open() to show wakeup stats

[Ganesh Mahendran]

single_open() interface requires that the whole output must
fit into a single buffer. This will lead to timeout when
system memory is not in a good situation.

This patch use seq_open() to show wakeup stats. This method
need only one page, so timeout will not be observed.

Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
---
 drivers/base/power/wakeup.c | 71 +++++++++++++++++++++++++++++++++++----------
 1 file changed, 56 insertions(+), 15 deletions(-)

diff --git a/drivers/base/power/wakeup.c b/drivers/base/power/wakeup.c
index ea01621..c64609a 100644
--- a/drivers/base/power/wakeup.c
+++ b/drivers/base/power/wakeup.c
@@ -1029,32 +1029,73 @@ static int print_wakeup_source_stats(struct seq_file *m,
 	return 0;
 }
 
+static void *wakeup_sources_stats_seq_start(struct seq_file *m,
+					loff_t *pos)
+{
+	struct wakeup_source *ws;
+	loff_t n = *pos;
+
+	if (n == 0) {
+		seq_puts(m, "name\t\tactive_count\tevent_count\twakeup_count\t"
+			"expire_count\tactive_since\ttotal_time\tmax_time\t"
+			"last_change\tprevent_suspend_time\n");
+	}
+
+	rcu_read_lock();
+	list_for_each_entry_rcu(ws, &wakeup_sources, entry) {
+		if (n-- > 0)
+			continue;
+		goto out;
+	}
+	ws = NULL;
+out:
+	return ws;
+}
+
+static void *wakeup_sources_stats_seq_next(struct seq_file *m,
+					void *v, loff_t *pos)
+{
+	struct wakeup_source *ws = v;
+	struct wakeup_source *next_ws = NULL;
+
+	++(*pos);
+
+	list_for_each_entry_continue_rcu(ws, &wakeup_sources, entry) {
+		next_ws = ws;
+		break;
+	}
+
+	return next_ws;
+}
+
+static void wakeup_sources_stats_seq_stop(struct seq_file *m, void *v)
+{
+	rcu_read_unlock();
+}
+
 /**
  * wakeup_sources_stats_show - Print wakeup sources statistics information.
  * @m: seq_file to print the statistics into.
  */
-static int wakeup_sources_stats_show(struct seq_file *m, void *unused)
+static int wakeup_sources_stats_seq_show(struct seq_file *m, void *v)
 {
-	struct wakeup_source *ws;
-	int srcuidx;
+	struct wakeup_source *ws = v;
 
-	seq_puts(m, "name\t\tactive_count\tevent_count\twakeup_count\t"
-		"expire_count\tactive_since\ttotal_time\tmax_time\t"
-		"last_change\tprevent_suspend_time\n");
-
-	srcuidx = srcu_read_lock(&wakeup_srcu);
-	list_for_each_entry_rcu(ws, &wakeup_sources, entry)
-		print_wakeup_source_stats(m, ws);
-	srcu_read_unlock(&wakeup_srcu, srcuidx);
-
-	print_wakeup_source_stats(m, &deleted_ws);
+	print_wakeup_source_stats(m, ws);
 
 	return 0;
 }
 
+static const struct seq_operations wakeup_sources_stats_seq_ops = {
+	.start = wakeup_sources_stats_seq_start,
+	.next  = wakeup_sources_stats_seq_next,
+	.stop  = wakeup_sources_stats_seq_stop,
+	.show  = wakeup_sources_stats_seq_show,
+};
+
 static int wakeup_sources_stats_open(struct inode *inode, struct file *file)
 {
-	return single_open(file, wakeup_sources_stats_show, NULL);
+	return seq_open(file, &wakeup_sources_stats_seq_ops);
 }
 
 static const struct file_operations wakeup_sources_stats_fops = {
@@ -1062,7 +1103,7 @@ static int wakeup_sources_stats_open(struct inode *inode, struct file *file)
 	.open = wakeup_sources_stats_open,
 	.read = seq_read,
 	.llseek = seq_lseek,
-	.release = single_release,
+	.release = seq_release,
 };
 
 static int __init wakeup_sources_debugfs_init(void)
-- 
1.9.1

Re: [PATCH] PM / wakeup: use seq_open() to show wakeup stats

[Rafael J. Wysocki]

On Fri, Mar 2, 2018 at 6:01 AM, Ganesh Mahendran
<opensource.ganesh@gmail.com> wrote:
> single_open() interface requires that the whole output must
> fit into a single buffer. This will lead to timeout when
> system memory is not in a good situation.

Did you actually see this problem with this particular file or is it
theoretical?

> This patch use seq_open() to show wakeup stats. This method
> need only one page, so timeout will not be observed.
>
> Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
> ---
>  drivers/base/power/wakeup.c | 71 +++++++++++++++++++++++++++++++++++----------
>  1 file changed, 56 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/base/power/wakeup.c b/drivers/base/power/wakeup.c
> index ea01621..c64609a 100644
> --- a/drivers/base/power/wakeup.c
> +++ b/drivers/base/power/wakeup.c
> @@ -1029,32 +1029,73 @@ static int print_wakeup_source_stats(struct seq_file *m,
>         return 0;
>  }
>
> +static void *wakeup_sources_stats_seq_start(struct seq_file *m,
> +                                       loff_t *pos)
> +{
> +       struct wakeup_source *ws;
> +       loff_t n = *pos;
> +
> +       if (n == 0) {
> +               seq_puts(m, "name\t\tactive_count\tevent_count\twakeup_count\t"
> +                       "expire_count\tactive_since\ttotal_time\tmax_time\t"
> +                       "last_change\tprevent_suspend_time\n");
> +       }
> +
> +       rcu_read_lock();

The code running after this cannot sleep.  Use
srcu_read_lock(&wakeup_srcu) instead.

> +       list_for_each_entry_rcu(ws, &wakeup_sources, entry) {
> +               if (n-- > 0)
> +                       continue;
> +               goto out;
> +       }
> +       ws = NULL;
> +out:
> +       return ws;
> +}
> +
> +static void *wakeup_sources_stats_seq_next(struct seq_file *m,
> +                                       void *v, loff_t *pos)
> +{
> +       struct wakeup_source *ws = v;
> +       struct wakeup_source *next_ws = NULL;
> +
> +       ++(*pos);
> +
> +       list_for_each_entry_continue_rcu(ws, &wakeup_sources, entry) {
> +               next_ws = ws;
> +               break;
> +       }
> +
> +       return next_ws;
> +}
> +
> +static void wakeup_sources_stats_seq_stop(struct seq_file *m, void *v)
> +{
> +       rcu_read_unlock();
> +}
> +
>  /**
>   * wakeup_sources_stats_show - Print wakeup sources statistics information.
>   * @m: seq_file to print the statistics into.
>   */
> -static int wakeup_sources_stats_show(struct seq_file *m, void *unused)
> +static int wakeup_sources_stats_seq_show(struct seq_file *m, void *v)
>  {
> -       struct wakeup_source *ws;
> -       int srcuidx;
> +       struct wakeup_source *ws = v;
>
> -       seq_puts(m, "name\t\tactive_count\tevent_count\twakeup_count\t"
> -               "expire_count\tactive_since\ttotal_time\tmax_time\t"
> -               "last_change\tprevent_suspend_time\n");
> -
> -       srcuidx = srcu_read_lock(&wakeup_srcu);
> -       list_for_each_entry_rcu(ws, &wakeup_sources, entry)
> -               print_wakeup_source_stats(m, ws);
> -       srcu_read_unlock(&wakeup_srcu, srcuidx);
> -
> -       print_wakeup_source_stats(m, &deleted_ws);
> +       print_wakeup_source_stats(m, ws);
>
>         return 0;
>  }
>
> +static const struct seq_operations wakeup_sources_stats_seq_ops = {
> +       .start = wakeup_sources_stats_seq_start,
> +       .next  = wakeup_sources_stats_seq_next,
> +       .stop  = wakeup_sources_stats_seq_stop,
> +       .show  = wakeup_sources_stats_seq_show,
> +};
> +
>  static int wakeup_sources_stats_open(struct inode *inode, struct file *file)
>  {
> -       return single_open(file, wakeup_sources_stats_show, NULL);
> +       return seq_open(file, &wakeup_sources_stats_seq_ops);
>  }
>
>  static const struct file_operations wakeup_sources_stats_fops = {
> @@ -1062,7 +1103,7 @@ static int wakeup_sources_stats_open(struct inode *inode, struct file *file)
>         .open = wakeup_sources_stats_open,
>         .read = seq_read,
>         .llseek = seq_lseek,
> -       .release = single_release,
> +       .release = seq_release,
>  };
>
>  static int __init wakeup_sources_debugfs_init(void)
> --
> 1.9.1
>

Re: [PATCH] PM / wakeup: use seq_open() to show wakeup stats

[Pavel Machek]

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On Fri 2018-03-02 13:01:00, Ganesh Mahendran wrote:
> single_open() interface requires that the whole output must
> fit into a single buffer. This will lead to timeout when
> system memory is not in a good situation.
> 
> This patch use seq_open() to show wakeup stats. This method
> need only one page, so timeout will not be observed.

Can the rcu_read_unlock be delayed arbitrarily by userland code?

Is that ok with RCU?
								Pavel
								
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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Re: [PATCH] PM / wakeup: use seq_open() to show wakeup stats

[Rafael J. Wysocki]

On Fri, Mar 2, 2018 at 10:05 AM, Pavel Machek <pavel@ucw.cz> wrote:
> On Fri 2018-03-02 13:01:00, Ganesh Mahendran wrote:
>> single_open() interface requires that the whole output must
>> fit into a single buffer. This will lead to timeout when
>> system memory is not in a good situation.
>>
>> This patch use seq_open() to show wakeup stats. This method
>> need only one page, so timeout will not be observed.
>
> Can the rcu_read_unlock be delayed arbitrarily by userland code?
>
> Is that ok with RCU?

No, it isn't. :-)

Re: [PATCH] PM / wakeup: use seq_open() to show wakeup stats

[Rafael J. Wysocki]

On Fri, Mar 2, 2018 at 9:58 AM, Rafael J. Wysocki <rafael@kernel.org> wrote:
> On Fri, Mar 2, 2018 at 6:01 AM, Ganesh Mahendran
> <opensource.ganesh@gmail.com> wrote:
>> single_open() interface requires that the whole output must
>> fit into a single buffer. This will lead to timeout when
>> system memory is not in a good situation.
>
> Did you actually see this problem with this particular file or is it
> theoretical?
>
>> This patch use seq_open() to show wakeup stats. This method
>> need only one page, so timeout will not be observed.
>>
>> Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
>> ---
>>  drivers/base/power/wakeup.c | 71 +++++++++++++++++++++++++++++++++++----------
>>  1 file changed, 56 insertions(+), 15 deletions(-)
>>
>> diff --git a/drivers/base/power/wakeup.c b/drivers/base/power/wakeup.c
>> index ea01621..c64609a 100644
>> --- a/drivers/base/power/wakeup.c
>> +++ b/drivers/base/power/wakeup.c
>> @@ -1029,32 +1029,73 @@ static int print_wakeup_source_stats(struct seq_file *m,
>>         return 0;
>>  }
>>
>> +static void *wakeup_sources_stats_seq_start(struct seq_file *m,
>> +                                       loff_t *pos)
>> +{
>> +       struct wakeup_source *ws;
>> +       loff_t n = *pos;
>> +
>> +       if (n == 0) {
>> +               seq_puts(m, "name\t\tactive_count\tevent_count\twakeup_count\t"
>> +                       "expire_count\tactive_since\ttotal_time\tmax_time\t"
>> +                       "last_change\tprevent_suspend_time\n");
>> +       }
>> +
>> +       rcu_read_lock();
>
> The code running after this cannot sleep.  Use
> srcu_read_lock(&wakeup_srcu) instead.

But generally, as Pavel points it out, even under srcu_read_lock()
things cannot be delayed indefinitely (at least for actions other than
freeing up more memory), so the entire approach appears to be flawed.

Re: [PATCH] PM / wakeup: use seq_open() to show wakeup stats

[Ganesh Mahendran]

Hi, Rafael:

2018-03-02 16:58 GMT+08:00 Rafael J. Wysocki <rafael@kernel.org>:
> On Fri, Mar 2, 2018 at 6:01 AM, Ganesh Mahendran
> <opensource.ganesh@gmail.com> wrote:
>> single_open() interface requires that the whole output must
>> fit into a single buffer. This will lead to timeout when
>> system memory is not in a good situation.
>
> Did you actually see this problem with this particular file or is it
> theoretical?

We got report of android watchdog timeout when memory situation
is bad.

>
>> This patch use seq_open() to show wakeup stats. This method
>> need only one page, so timeout will not be observed.
>>
>> Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
>> ---
>>  drivers/base/power/wakeup.c | 71 +++++++++++++++++++++++++++++++++++----------
>>  1 file changed, 56 insertions(+), 15 deletions(-)
>>
>> diff --git a/drivers/base/power/wakeup.c b/drivers/base/power/wakeup.c
>> index ea01621..c64609a 100644
>> --- a/drivers/base/power/wakeup.c
>> +++ b/drivers/base/power/wakeup.c
>> @@ -1029,32 +1029,73 @@ static int print_wakeup_source_stats(struct seq_file *m,
>>         return 0;
>>  }
>>
>> +static void *wakeup_sources_stats_seq_start(struct seq_file *m,
>> +                                       loff_t *pos)
>> +{
>> +       struct wakeup_source *ws;
>> +       loff_t n = *pos;
>> +
>> +       if (n == 0) {
>> +               seq_puts(m, "name\t\tactive_count\tevent_count\twakeup_count\t"
>> +                       "expire_count\tactive_since\ttotal_time\tmax_time\t"
>> +                       "last_change\tprevent_suspend_time\n");
>> +       }
>> +
>> +       rcu_read_lock();
>
> The code running after this cannot sleep.  Use
> srcu_read_lock(&wakeup_srcu) instead.

wakeup_sources_stats_seq_[start | end] are called in seq_read().
So rcu_read_unlock() will soon be called  in seq_read().

I am not familar with rcu. I refered to kmemleak.c which use seq_open()
to show the stats.

Thanks for your review.

>
>> +       list_for_each_entry_rcu(ws, &wakeup_sources, entry) {
>> +               if (n-- > 0)
>> +                       continue;
>> +               goto out;
>> +       }
>> +       ws = NULL;
>> +out:
>> +       return ws;
>> +}
>> +
>> +static void *wakeup_sources_stats_seq_next(struct seq_file *m,
>> +                                       void *v, loff_t *pos)
>> +{
>> +       struct wakeup_source *ws = v;
>> +       struct wakeup_source *next_ws = NULL;
>> +
>> +       ++(*pos);
>> +
>> +       list_for_each_entry_continue_rcu(ws, &wakeup_sources, entry) {
>> +               next_ws = ws;
>> +               break;
>> +       }
>> +
>> +       return next_ws;
>> +}
>> +
>> +static void wakeup_sources_stats_seq_stop(struct seq_file *m, void *v)
>> +{
>> +       rcu_read_unlock();
>> +}
>> +
>>  /**
>>   * wakeup_sources_stats_show - Print wakeup sources statistics information.
>>   * @m: seq_file to print the statistics into.
>>   */
>> -static int wakeup_sources_stats_show(struct seq_file *m, void *unused)
>> +static int wakeup_sources_stats_seq_show(struct seq_file *m, void *v)
>>  {
>> -       struct wakeup_source *ws;
>> -       int srcuidx;
>> +       struct wakeup_source *ws = v;
>>
>> -       seq_puts(m, "name\t\tactive_count\tevent_count\twakeup_count\t"
>> -               "expire_count\tactive_since\ttotal_time\tmax_time\t"
>> -               "last_change\tprevent_suspend_time\n");
>> -
>> -       srcuidx = srcu_read_lock(&wakeup_srcu);
>> -       list_for_each_entry_rcu(ws, &wakeup_sources, entry)
>> -               print_wakeup_source_stats(m, ws);
>> -       srcu_read_unlock(&wakeup_srcu, srcuidx);
>> -
>> -       print_wakeup_source_stats(m, &deleted_ws);
>> +       print_wakeup_source_stats(m, ws);
>>
>>         return 0;
>>  }
>>
>> +static const struct seq_operations wakeup_sources_stats_seq_ops = {
>> +       .start = wakeup_sources_stats_seq_start,
>> +       .next  = wakeup_sources_stats_seq_next,
>> +       .stop  = wakeup_sources_stats_seq_stop,
>> +       .show  = wakeup_sources_stats_seq_show,
>> +};
>> +
>>  static int wakeup_sources_stats_open(struct inode *inode, struct file *file)
>>  {
>> -       return single_open(file, wakeup_sources_stats_show, NULL);
>> +       return seq_open(file, &wakeup_sources_stats_seq_ops);
>>  }
>>
>>  static const struct file_operations wakeup_sources_stats_fops = {
>> @@ -1062,7 +1103,7 @@ static int wakeup_sources_stats_open(struct inode *inode, struct file *file)
>>         .open = wakeup_sources_stats_open,
>>         .read = seq_read,
>>         .llseek = seq_lseek,
>> -       .release = single_release,
>> +       .release = seq_release,
>>  };
>>
>>  static int __init wakeup_sources_debugfs_init(void)
>> --
>> 1.9.1
>>

Re: [PATCH] PM / wakeup: use seq_open() to show wakeup stats

[Rafael J. Wysocki]

On Fri, Mar 2, 2018 at 12:34 PM, Ganesh Mahendran
<opensource.ganesh@gmail.com> wrote:
> Hi, Rafael:
>
> 2018-03-02 16:58 GMT+08:00 Rafael J. Wysocki <rafael@kernel.org>:
>> On Fri, Mar 2, 2018 at 6:01 AM, Ganesh Mahendran
>> <opensource.ganesh@gmail.com> wrote:
>>> single_open() interface requires that the whole output must
>>> fit into a single buffer. This will lead to timeout when
>>> system memory is not in a good situation.
>>
>> Did you actually see this problem with this particular file or is it
>> theoretical?
>
> We got report of android watchdog timeout when memory situation
> is bad.

I see.

>>
>>> This patch use seq_open() to show wakeup stats. This method
>>> need only one page, so timeout will not be observed.
>>>
>>> Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
>>> ---
>>>  drivers/base/power/wakeup.c | 71 +++++++++++++++++++++++++++++++++++----------
>>>  1 file changed, 56 insertions(+), 15 deletions(-)
>>>
>>> diff --git a/drivers/base/power/wakeup.c b/drivers/base/power/wakeup.c
>>> index ea01621..c64609a 100644
>>> --- a/drivers/base/power/wakeup.c
>>> +++ b/drivers/base/power/wakeup.c
>>> @@ -1029,32 +1029,73 @@ static int print_wakeup_source_stats(struct seq_file *m,
>>>         return 0;
>>>  }
>>>
>>> +static void *wakeup_sources_stats_seq_start(struct seq_file *m,
>>> +                                       loff_t *pos)
>>> +{
>>> +       struct wakeup_source *ws;
>>> +       loff_t n = *pos;
>>> +
>>> +       if (n == 0) {
>>> +               seq_puts(m, "name\t\tactive_count\tevent_count\twakeup_count\t"
>>> +                       "expire_count\tactive_since\ttotal_time\tmax_time\t"
>>> +                       "last_change\tprevent_suspend_time\n");
>>> +       }
>>> +
>>> +       rcu_read_lock();
>>
>> The code running after this cannot sleep.  Use
>> srcu_read_lock(&wakeup_srcu) instead.
>
> wakeup_sources_stats_seq_[start | end] are called in seq_read().
> So rcu_read_unlock() will soon be called  in seq_read().

But you have to guarantee that the code between rcu_read_lock() and
rcu_read_unlock() will *never* sleep.

> I am not familar with rcu.

So you need to get familiar with it to make changes involving it.
Otherwise you may fall a victim of the Wizard's Second Rule.

> I refered to kmemleak.c which use seq_open()
> to show the stats.

I see.

You need to use srcu_read_lock(&wakeup_srcu) in this file anyway.