[PATCH 4.14 093/125] powerpc: Warn about use of smt_snooze_delay

Greg Kroah-Hartman
2020-11-03T21:15:29Z

From: Joel Stanley <joel@jms.id.au>

commit a02f6d42357acf6e5de6ffc728e6e77faf3ad217 upstream.

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.

This uses pr_warn_once instead of pr_warn_ratelimit as testing
'ppc64_cpu --smt=off' on a 24 core / 4 SMT system showed the warning
to be noisy, as the online/offline loop is slow.

Fixes: 3fa8cad82b94 ("powerpc/pseries/cpuidle: smt-snooze-delay cleanup.")
Cc: stable@vger.kernel.org # v3.14
Signed-off-by: Joel Stanley <joel@jms.id.au>
Acked-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200902000012.3440389-1-joel@jms.id.au
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/powerpc/kernel/sysfs.c | 42 +++++++++++++++++-------------------------
1 file changed, 17 insertions(+), 25 deletions(-)

--- a/arch/powerpc/kernel/sysfs.c
+++ b/arch/powerpc/kernel/sysfs.c
@@ -28,29 +28,27 @@

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."
+ *
+ * powerpc-utils stopped using it as of 1.3.8. 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_once("%s (%d) stored to unsupported smt_snooze_delay, which has no effect.\n",
+ current->comm, current->pid);
return count;
}

@@ -58,9 +56,9 @@ static ssize_t show_smt_snooze_delay(str
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_once("%s (%d) read from unsupported smt_snooze_delay\n",
+ current->comm, current->pid);
+ return sprintf(buf, "100\n");
}

static DEVICE_ATTR(smt_snooze_delay, 0644, show_smt_snooze_delay,
@@ -68,16 +66,10 @@ static DEVICE_ATTR(smt_snooze_delay, 064

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);