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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Fri, 2 Mar 2018 09:58:22 +0100
Message-ID: <CAJZ5v0hmj873=2qXtV_PzGEv-dRp-1KVuoC0mYO0AcOVt0e3hg@mail.gmail.com>
Subject: Re: [PATCH] PM / wakeup: use seq_open() to show wakeup stats
To:     Ganesh Mahendran <opensource.ganesh@gmail.com>
Cc:     "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Pavel Machek <pavel@ucw.cz>, Len Brown <len.brown@intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Linux PM <linux-pm@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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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
>