Re: [PATCH v3] PM / wakeup: use seq_open() to show wakeup stats

Ganesh Mahendran
2018-05-02T02:26:41Z

Hi, Pavel

Thanks for your review.

2018-04-29 22:30 GMT+08:00 Pavel Machek <pavel@ucw.cz>:
> On Wed 2018-04-25 18:59:31, 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.
>
> Sounds like magic.

I did not explain clearly here.

If we use single_open() to open the file, a single buffer(physical
continious) will be allocated
to store the whole data of file /sys/kernel/debug/wakeup_sources.
When memory situation is not good(fragments...), long time may be used
to allocate
such buffer which may cause android watchdog timeout.

>
>> -static int wakeup_sources_stats_show(struct seq_file *m, void *unused)
>> +static void *wakeup_sources_stats_seq_start(struct seq_file *m,
>> + loff_t *pos)
>> {
> ...
>> - 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);
>> + *srcuidx = srcu_read_lock(&wakeup_srcu);
>> + list_for_each_entry_rcu(ws, &wakeup_sources, entry) {
>> + if (n-- <= 0)
>> + return ws;
>> + }
>> +
>> + return NULL;
>> +}
> ...
>> +static void wakeup_sources_stats_seq_stop(struct seq_file *m, void *v)
>> +{
>> + int *srcuidx = m->private;
>> +
>> + srcu_read_unlock(&wakeup_srcu, *srcuidx);
>> +}
>
> But you are holding srcu_lock over return to userspace, and somehow I
> don't think that's permitted?

In seq_read(), the m->op->[start | stop] will be invoked as a pair.
So the srcu_lock will not be hold and return to userspace.

Thanks.

> Pavel
> --
> (english) http://www.livejournal.com/~pavelmachek
> (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html