Re: [PATCH] PM / wakeup: show wakeup sources stats in sysfs

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Jun 25, 2019 at 05:54:49PM -0700, Tri Vo wrote:
> Userspace can use wakeup_sources debugfs node to plot history of suspend
> blocking wakeup sources over device's boot cycle. This information can
> then be used (1) for power-specific bug reporting and (2) towards
> attributing battery consumption to specific processes over a period of
> time.
> 
> However, debugfs doesn't have stable ABI. For this reason, expose wakeup
> sources statistics in sysfs under /sys/power/wakeup_sources/<name>/
> 
> Add following attributes to each wakeup source. These attributes match
> the columns of /sys/kernel/debug/wakeup_sources.
> 
>   active_count
>   event_count
>   wakeup_count
>   expire_count
>   active_time (named "active_since" in debugfs)
>   total_time
>   max_time
>   last_change
>   prevent_suspend_time

Can you also add a Documentation/ABI/ update for your new sysfs files so
that we can properly review this?

> Embedding a struct kobject into struct wakeup_source changes lifetime
> requirements on the latter. To that end, change deallocation of struct
> wakeup_source using kfree to kobject_put().

Ick, are you sure you need a new kobject here?  Why wouldn't a named
attribute group work instead?  That should keep this patch much smaller
and simpler.

> +static ssize_t wakeup_source_count_show(struct wakeup_source *ws,
> +					struct wakeup_source_attribute *attr,
> +					char *buf)
> +{
> +	unsigned long flags;
> +	unsigned long var;
> +
> +	spin_lock_irqsave(&ws->lock, flags);
> +	if (strcmp(attr->attr.name, "active_count") == 0)
> +		var = ws->active_count;
> +	else if (strcmp(attr->attr.name, "event_count") == 0)
> +		var = ws->event_count;
> +	else if (strcmp(attr->attr.name, "wakeup_count") == 0)
> +		var = ws->wakeup_count;
> +	else
> +		var = ws->expire_count;
> +	spin_unlock_irqrestore(&ws->lock, flags);
> +
> +	return sprintf(buf, "%lu\n", var);
> +}

Why is this lock always needed to be grabbed?  You are just reading a
value, who cares if it changes inbetween reading it and returning the
buffer string as it can change at that point in time anyway?

thanks,

greg k-h