| Message ID | 20201103203247.174991659@linuxfoundation.org |
|---|---|
| State | Superseded |
| Headers | show |
| Series | None | expand |
Hi! > 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. I don't believe this is good idea for stable. It is in 5.9-rc2, and likely mainline users will get userspace fixed, but that warning is less useful for -stable users. (And besides, it does not fix any serious bug). Best regards, Pavel -- http://www.livejournal.com/~pavelmachek
On Thu, 5 Nov 2020 at 19:24, Pavel Machek <pavel@ucw.cz> wrote: > > Hi! > > > 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. > > I don't believe this is good idea for stable. It is in 5.9-rc2, and > likely mainline users will get userspace fixed, but that warning is > less useful for -stable users. The warning is about the existing behaviour of the kernel. It does let the user know that they won't see any difference in behaviour when tweaking the smt_snooze_delay variable, which was a real issue that Anton hit. I agree that the future commit that removes smt_snooze_delay from the kernel should not be backported. Cheers, Joel > > (And besides, it does not fix any serious bug). > > Best regards, > Pavel > > -- > http://www.livejournal.com/~pavelmachek
--- a/arch/powerpc/kernel/sysfs.c +++ b/arch/powerpc/kernel/sysfs.c @@ -29,29 +29,27 @@ 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; } @@ -59,9 +57,9 @@ 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, @@ -69,16 +67,10 @@ 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);