linux-kernel - Re: [PATCH 2/2] cpufreq: Specify default governor on command line

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Message-ID: <20200616094802.GA139416@google.com>
Date:   Tue, 16 Jun 2020 10:48:02 +0100
From:   Quentin Perret <qperret@...gle.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     rjw@...ysocki.net, rafael@...nel.org, arnd@...db.de,
        mpe@...erman.id.au, benh@...nel.crashing.org, paulus@...ba.org,
        mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        kernel-team@...roid.com, tkjos@...gle.com, adharmap@...eaurora.org
Subject: Re: [PATCH 2/2] cpufreq: Specify default governor on command line

On Tuesday 16 Jun 2020 at 14:57:59 (+0530), Viresh Kumar wrote:
> There is another problem here which we need to look at. Any governor
> which is built as a module and isn't currently used, should be allowed
> to unload. And this needs to be tested by you as well, should be easy
> enough.
> 
> With the current implementation, you take a reference to the default
> governor when the driver is registered and drop it only when the
> driver goes away. Which means we won't be able to unload the module of
> the governor even if it isn't used. Which is wrong. The solution I
> proposed had the same issue as well.
> 
> You need to figure out a way where we don't need to keep holding the
> module hostage even when it isn't used. I see two ways at least for
> the same:
> 
> - Do that from the existing place: cpufreq_init_policy().
> 
> - And I think this can be done from governor-register/unregister as
>   well.
> 
> Second one sounds good, if it is feasible to do that.

Good point.

I'm thinking something along the lines of:

---8<---
diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 0f05caedc320..a9219404e07f 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -2340,6 +2340,11 @@ int cpufreq_register_governor(struct cpufreq_governor *governor)
 		list_add(&governor->governor_list, &cpufreq_governor_list);
 	}
 
+	if (!strncasecmp(cpufreq_param_governor, governor->name, CPUFREQ_NAME_LEN))
+		default_governor = governor;
+	else if (!default_governor && cpufreq_default_governor() == governor)
+		default_governor = cpufreq_default_governor();
+
 	mutex_unlock(&cpufreq_governor_mutex);
 	return err;
 }
@@ -2368,6 +2373,8 @@ void cpufreq_unregister_governor(struct cpufreq_governor *governor)
 
 	mutex_lock(&cpufreq_governor_mutex);
 	list_del(&governor->governor_list);
+	if (governor == default_governor)
+		default_governor = cpufreq_default_governor();
 	mutex_unlock(&cpufreq_governor_mutex);
 }
 EXPORT_SYMBOL_GPL(cpufreq_unregister_governor);
--->8---

should do the trick. That removes the unnecessary reference count, and
feels like a good place to hook things -- that is how cpuidle does it
too IIRC.

I'll double check the locking/synchronization, but that shouldn't be too
bad (famous last words).

Cheers,
Quentin