[PATCH v2] powerpc: Warn about use of smt_snooze_delay

[Joel Stanley]

It's not done anything for a long time. Save the percpu variable, and
emit a warning to remind users to not expect it to do anything.

Fixes: 3fa8cad82b94 ("powerpc/pseries/cpuidle: smt-snooze-delay cleanup.")
Cc: stable at vger.kernel.org # v3.14
Signed-off-by: Joel Stanley <joel at jms.id.au>
--
v2:
 Use pr_warn instead of WARN
 Reword and print proccess name with pid in message
 Leave CPU_FTR_SMT test in
 Add Fixes line

mpe, if you don't agree then feel free to drop the cc stable.

Testing 'ppc64_cpu --smt=off' on a 24 core / 4 SMT system it's quite noisy
as the online/offline loop that ppc64_cpu runs is slow.

This could be fixed by open coding pr_warn_ratelimit with the ratelimit
parameters tweaked if someone was concerned. I'll leave that to someone
else as a future enhancement.

[  237.642088][ T1197] ppc64_cpu (1197) used unsupported smt_snooze_delay, this has no effect
[  237.642175][ T1197] ppc64_cpu (1197) used unsupported smt_snooze_delay, this has no effect
[  237.642261][ T1197] ppc64_cpu (1197) used unsupported smt_snooze_delay, this has no effect
[  237.642345][ T1197] ppc64_cpu (1197) used unsupported smt_snooze_delay, this has no effect
[  237.642430][ T1197] ppc64_cpu (1197) used unsupported smt_snooze_delay, this has no effect
[  237.642516][ T1197] ppc64_cpu (1197) used unsupported smt_snooze_delay, this has no effect
[  237.642625][ T1197] ppc64_cpu (1197) used unsupported smt_snooze_delay, this has no effect
[  237.642709][ T1197] ppc64_cpu (1197) used unsupported smt_snooze_delay, this has no effect
[  237.642793][ T1197] ppc64_cpu (1197) used unsupported smt_snooze_delay, this has no effect
[  237.642878][ T1197] ppc64_cpu (1197) used unsupported smt_snooze_delay, this has no effect
[  254.264030][ T1197] store_smt_snooze_delay: 14 callbacks suppressed
[  254.264033][ T1197] ppc64_cpu (1197) used unsupported smt_snooze_delay, this has no effect
[  254.264048][ T1197] ppc64_cpu (1197) used unsupported smt_snooze_delay, this has no effect
[  254.264062][ T1197] ppc64_cpu (1197) used unsupported smt_snooze_delay, this has no effect
[  254.264075][ T1197] ppc64_cpu (1197) used unsupported smt_snooze_delay, this has no effect
[  254.264089][ T1197] ppc64_cpu (1197) used unsupported smt_snooze_delay, this has no effect
[  254.264103][ T1197] ppc64_cpu (1197) used unsupported smt_snooze_delay, this has no effect
[  254.264116][ T1197] ppc64_cpu (1197) used unsupported smt_snooze_delay, this has no effect
[  254.264130][ T1197] ppc64_cpu (1197) used unsupported smt_snooze_delay, this has no effect
[  254.264143][ T1197] ppc64_cpu (1197) used unsupported smt_snooze_delay, this has no effect
[  254.264157][ T1197] ppc64_cpu (1197) used unsupported smt_snooze_delay, this has no effect

Signed-off-by: Joel Stanley <joel at jms.id.au>
---
 arch/powerpc/kernel/sysfs.c | 41 +++++++++++++++----------------------
 1 file changed, 16 insertions(+), 25 deletions(-)

diff --git a/arch/powerpc/kernel/sysfs.c b/arch/powerpc/kernel/sysfs.c
index 571b3259697e..ba6d4cee19ef 100644
--- a/arch/powerpc/kernel/sysfs.c
+++ b/arch/powerpc/kernel/sysfs.c
@@ -32,29 +32,26 @@
 
 static DEFINE_PER_CPU(struct cpu, cpu_devices);
 
-/*
- * SMT snooze delay stuff, 64-bit only for now
- */
-
 #ifdef CONFIG_PPC64
 
-/* Time in microseconds we delay before sleeping in the idle loop */
-static DEFINE_PER_CPU(long, smt_snooze_delay) = { 100 };
+/*
+ * Snooze delay has not been hooked up since 3fa8cad82b94 ("powerpc/pseries/cpuidle:
+ * smt-snooze-delay cleanup.") and has been broken even longer. As was foretold in
+ * 2014:
+ *
+ *  "ppc64_util currently utilises it. Once we fix ppc64_util, propose to clean
+ *  up the kernel code."
+ *
+ * At some point in the future this code should be removed.
+ */
 
 static ssize_t store_smt_snooze_delay(struct device *dev,
 				      struct device_attribute *attr,
 				      const char *buf,
 				      size_t count)
 {
-	struct cpu *cpu = container_of(dev, struct cpu, dev);
-	ssize_t ret;
-	long snooze;
-
-	ret = sscanf(buf, "%ld", &snooze);
-	if (ret != 1)
-		return -EINVAL;
-
-	per_cpu(smt_snooze_delay, cpu->dev.id) = snooze;
+	pr_warn_ratelimited("%s (%d) used unsupported smt_snooze_delay, this has no effect\n",
+			    current->comm, current->pid);
 	return count;
 }
 
@@ -62,9 +59,9 @@ static ssize_t show_smt_snooze_delay(struct device *dev,
 				     struct device_attribute *attr,
 				     char *buf)
 {
-	struct cpu *cpu = container_of(dev, struct cpu, dev);
-
-	return sprintf(buf, "%ld\n", per_cpu(smt_snooze_delay, cpu->dev.id));
+	pr_warn_ratelimited("%s (%d) used unsupported smt_snooze_delay, this has no effect\n",
+			    current->comm, current->pid);
+	return sprintf(buf, "100\n");
 }
 
 static DEVICE_ATTR(smt_snooze_delay, 0644, show_smt_snooze_delay,
@@ -72,16 +69,10 @@ static DEVICE_ATTR(smt_snooze_delay, 0644, show_smt_snooze_delay,
 
 static int __init setup_smt_snooze_delay(char *str)
 {
-	unsigned int cpu;
-	long snooze;
-
 	if (!cpu_has_feature(CPU_FTR_SMT))
 		return 1;
 
-	snooze = simple_strtol(str, NULL, 10);
-	for_each_possible_cpu(cpu)
-		per_cpu(smt_snooze_delay, cpu) = snooze;
-
+	pr_warn("smt-snooze-delay command line option has no effect\n");
 	return 1;
 }
 __setup("smt-snooze-delay=", setup_smt_snooze_delay);
-- 
2.27.0