Re: [PATCH 6/9] cpu/SMT: Allow enabling partial SMT states via sysfs

Thomas Gleixner
2023-06-10T20:09:54Z

On Thu, May 25 2023 at 01:56, Michael Ellerman wrote:
> There is a hook which allows arch code to control how many threads per

Can you please write out architecture in changelogs and comments?

I know 'arch' is commonly used but while my brain parser tolerates
'arch_' prefixes it raises an exception on 'arch' in prose as 'arch' is
a regular word with a completely different meaning. Changelogs and
comments are not space constraint.

> @@ -2505,20 +2505,38 @@ __store_smt_control(struct device *dev, struct device_attribute *attr,
> if (cpu_smt_control == CPU_SMT_NOT_SUPPORTED)
> return -ENODEV;
>
> - if (sysfs_streq(buf, "on"))
> + if (sysfs_streq(buf, "on")) {
> ctrlval = CPU_SMT_ENABLED;
> - else if (sysfs_streq(buf, "off"))
> + num_threads = cpu_smt_max_threads;
> + } else if (sysfs_streq(buf, "off")) {
> ctrlval = CPU_SMT_DISABLED;
> - else if (sysfs_streq(buf, "forceoff"))
> + num_threads = 1;
> + } else if (sysfs_streq(buf, "forceoff")) {
> ctrlval = CPU_SMT_FORCE_DISABLED;
> - else
> + num_threads = 1;
> + } else if (kstrtoint(buf, 10, &num_threads) == 0) {
> + if (num_threads == 1)
> + ctrlval = CPU_SMT_DISABLED;
> + else if (num_threads > 1 && topology_smt_threads_supported(num_threads))
> + ctrlval = CPU_SMT_ENABLED;
> + else
> + return -EINVAL;
> + } else {
> return -EINVAL;
> + }
>
> ret = lock_device_hotplug_sysfs();
> if (ret)
> return ret;
>
> - if (ctrlval != cpu_smt_control) {
> + orig_threads = cpu_smt_num_threads;
> + cpu_smt_num_threads = num_threads;
> +
> + if (num_threads > orig_threads) {
> + ret = cpuhp_smt_enable();
> + } else if (num_threads < orig_threads) {
> + ret = cpuhp_smt_disable(ctrlval);
> + } else if (ctrlval != cpu_smt_control) {
> switch (ctrlval) {
> case CPU_SMT_ENABLED:
> ret = cpuhp_smt_enable();

This switch case does not make sense anymore.

The only situation which reaches this is when the control value goes
from CPU_SMT_DISABLED to CPU_SMT_FORCE_DISABLED because that's not
changing the number of threads.

So something like this is completely sufficient:

if (num_threads > orig_threads)
ret = cpuhp_smt_enable();
else if (num_threads < orig_threads || ctrval == CPU_SMT_FORCE_DISABLED)
ret = cpuhp_smt_disable(ctrlval);

No?

Thanks,

tglx