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From:   Valentin Schneider <vschneid@redhat.com>
To:     Shrikanth Hegde <sshegde@linux.vnet.ibm.com>
Cc:     dietmar.eggemann@arm.com, linux-kernel@vger.kernel.org,
        ionela.voinescu@arm.com, quentin.perret@arm.com,
        srikar@linux.vnet.ibm.com, mgorman@techsingularity.net,
        mingo@kernel.org, pierre.gondois@arm.com, yu.c.chen@intel.com,
        tim.c.chen@linux.intel.com, mingo@redhat.com, peterz@infradead.org,
        vincent.guittot@linaro.org
Subject: Re: [PATCH v3] sched/topology: remove sysctl_sched_energy_aware
 depending on the architecture
In-Reply-To: <7c6dd2ec-b9a9-b364-5a29-05336127e519@linux.vnet.ibm.com>
References: <20230913114807.665094-1-sshegde@linux.vnet.ibm.com>
 <xhsmhil8cvi7s.mognet@vschneid.remote.csb>
 <7c6dd2ec-b9a9-b364-5a29-05336127e519@linux.vnet.ibm.com>
Date:   Fri, 15 Sep 2023 14:00:15 +0200
Message-ID: <xhsmhfs3fve7k.mognet@vschneid.remote.csb>
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On 14/09/23 23:26, Shrikanth Hegde wrote:
> On 9/14/23 9:51 PM, Valentin Schneider wrote:
>> On 13/09/23 17:18, Shrikanth Hegde wrote:
>>> sysctl_sched_energy_aware is available for the admin to disable/enable
>>> energy aware scheduling(EAS). EAS is enabled only if few conditions are
>>> met by the platform. They are, asymmetric CPU capacity, no SMT,
>>> valid cpufreq policy, frequency invariant load tracking. It is possible
>>> platform when booting may not have EAS capability, but can do that after.
>>> For example, changing/registering the cpufreq policy.
>>>
>>> At present, though platform doesn't support EAS, this sysctl is still
>>> present and it ends up calling rebuild of sched domain on write to 1 and
>>> NOP when writing to 0. That is confusing and un-necessary.
>>>
>>
>
> Hi Valentin, Thanks for taking a look at this patch.
>
>> But why would you write to it in the first place? Or do you mean to use
>> this as an indicator for userspace that EAS is supported?
>>
>
> Since this sysctl is present and its value being 1, it gives the
> impression to the user that EAS is supported when it is not.
> So its an attempt to correct that part.
>

Ah, I see. Then how about just making the sysctl return 0 when EAS isn't
supported? And on top of it, prevent all writes when EAS isn't supported
(perf domains cannot be built, so there would be no point in forcing a
rebuild that will do nothing).

I can never remember how to properly use the sysctl API, so that's a very
crude implementation, but something like so?

---

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 05a5bc678c089..dadfc5afc4121 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -230,9 +230,28 @@ static int sched_energy_aware_handler(struct ctl_table *table, int write,
 	if (write && !capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
+	if (!sched_energy_enabled()) {
+		if (write)
+			return -EOPNOTSUPP;
+		else {
+			size_t len;
+
+			if (*ppos) {
+				*lenp = 0;
+				return 0;
+			}
+
+			len = snprintf((char *)buffer, 3, "0\n");
+
+			*lenp = len;
+			*ppos += len;
+			return 0;
+		}
+	}
+
 	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
 	if (!ret && write) {
-		state = static_branch_unlikely(&sched_energy_present);
+		state = sched_energy_enabled();
 		if (state != sysctl_sched_energy_aware)
 			rebuild_sched_domains_energy();
 	}