linux-kernel - Re: [PATCH v2] PM: fix dynamic debug within pm_pr

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Date:   Mon, 21 Mar 2022 18:09:12 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     David Cohen <dacohen@...me>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] PM: fix dynamic debug within pm_pr_debug()

On Mon, Mar 21, 2022 at 4:57 PM David Cohen <dacohen@...me> wrote:
>
> On Mon, Mar 21, 2022 at 04:12:24PM +0100, Rafael J. Wysocki wrote:
> > On Mon, Mar 21, 2022 at 3:51 PM David Cohen <dacohen@...me> wrote:
> > >
> > > On Mon, Mar 21, 2022 at 03:20:20PM +0100, Rafael J. Wysocki wrote:
> > > > On Sat, Mar 19, 2022 at 2:54 AM David Cohen <dacohen@...me> wrote:
> > > > >
> > > > > On Thu, Mar 17, 2022 at 02:45:11PM +0100, Rafael J. Wysocki wrote:
> > > > > > On Sat, Mar 12, 2022 at 5:37 AM David Cohen <dacohen@...me> wrote:
> > > > > > >
> > > > > > > Currently, pm_pr_debug() and pm_deferred_pr_debug() use __pm_pr_debug()
> > > > > > > to filter pm debug messages based on pm_debug_messages_on flag.
> > > > > > > According to __pm_pr_debug() implementation, pm_deferred_pr_debug()
> > > > > > > indirectly calls printk_deferred() within __pm_pr_debug() which doesn't
> > > > > > > support dynamic debug, but pm_pr_debug() indirectly calls pr_debug()
> > > > > >
> > > > > > I'm not sure what you mean by pm_pr_debug().  There's no such thing in
> > > > > > the kernel tree.
> > > > > >
> > > > > > Assuming that it means pm_pr_dbg(), it doesn't call pr_debug():
> > > > >
> > > > > Yeah, I apologize for the typo. I meant pm_pr_dbg(). I can fix that if
> > > > > you're ok with the patch as per comments below.
> > > > >
> > > > > >
> > > > > > #define pm_pr_dbg(fmt, ...) __pm_pr_dbg(false, fmt, ##__VA_ARGS__)
> > > > > >
> > > > > > and
> > > > > >
> > > > > > void __pm_pr_dbg(bool defer, const char *fmt, ...)
> > > > > > {
> > > > > > ...
> > > > > >         if (defer)
> > > > > >                printk_deferred(KERN_DEBUG "PM: %pV", &vaf);
> > > > > >         else
> > > > > >                printk(KERN_DEBUG "PM: %pV", &vaf);
> > > > > >
> > > > > > And as I said printk(KERN_DEBUG ...) is not equivalent to
> > > > > > pr_debug(...), because it is not dynamic printk().
> > > > >
> > > > > The problem is not about __pm_pr_dbg() calling printk(). The problem is
> > > > > the pm files that used to call pr_debug() were modified to call
> > > > > pm_pr_dbg() in order to be behing the pm_debug_messages_on flag, as per
> > > > > this commit:
> > > > > 8d8b2441db96 PM / sleep: Do not print debug messages by default
> > > >
> > > > So what's the problem with setting pm_debug_messages_on in addition to
> > > > enabling dynamic debug for a given file?
> > >
> > > Let me be a bit more detailed:
> > >
> > > Before "8d8b2441db96 PM / sleep: Do not print debug messages by default":
> > > - pr_debug() was used
> > > - The kernel pm files had dynamic debug support
> > > - All the instances using pr_debug() are visible on
> > >   /sys/kernel/debug/dynamic_debug/control
> > > - pm_debug_messages_on flag was not supported
> > >
> > > After "8d8b2441db96 PM / sleep: Do not print debug messages by default":
> > > - pr_debug() was replaced with pm_pr_dbg()
> >
> > Just for some PM-related messages, not in general.
>
> Yes, I'm only addressing the kernel PM cases.
>
> >
> > > - The kernel pm files where pm_pr_dbg() replaced pr_debug() *lost* dynamic
> > >   debug support and they are no longer visible on
> > >   /sys/kernel/debug/dynamic_debug/control
> >
> > Again, some of them.
> >
> > > - pm_debug_messages_on flag was introduced
> >
> > So what exactly is the problem you are trying to address?
> >
> > > What my patch is doing:
> > > - Reintroducing dynamic debug support to the same files who lost it
> > >   after the patch mentioned above
> >
> > Why is this necessary?
>
> I'm trying to have more granularity when enabling the debug messages.
> See below for more context.
>
> >
> > > - The instances using pr_pm_dbg() (which originally came from pr_debug())
> > >   are reintroduced to /sys/kernel/debug/dynamic_debug/control
> > > - pm_debug_messages_on flag is unaltered
> >
> > I think that you really want to turn off some PM messages by default
> > when pm_debug_messages_on is set.
> >
> > OTOH I want to be able to see pr_pm_dbg() messages in the log when
> > pm_debug_messages_on is set without having to turn on dynamic debug
> > for every file in which pr_pm_dbg() is used.  Your patch breaks this.
>
> When debugging an issue with s2idle on my laptop where it enters an
> infinite loop instead of suspending, the excess of PM messages were
> changing the behavior and making it more a lot difficult to reproduce
> the issue (and a less severe side effect, printing many debug info I
> didn't need).

I see.

> But as you pointed out, my patch is indeed changing the behavior for
> when the pm_debug_messages_on is enabled. Would you be open for this
> behavior instead:
>  - If dynamic debug support is disabled, pm_pr_dbg() is printed if
>    pm_debug_messages_on == 1
>  - If dynamic debug support is enabled, pm_pr_dbg() is printed if
>    pm_debug_messages_on == 1 or if pm_pr_dbg() was explicitly enabled on
>    /sys/kernel/debug/dynamic_debug/control

Yes, that would work.