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

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Message-ID: <87r0qj84fr.ffs@tglx>
Date:   Sat, 10 Jun 2023 22:09:44 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Michael Ellerman <mpe@...erman.id.au>, linux-kernel@...r.kernel.org
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-arch@...r.kernel.org,
        ldufour@...ux.ibm.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        mingo@...hat.com, x86@...nel.org
Subject: Re: [PATCH 6/9] cpu/SMT: Allow enabling partial SMT states via sysfs

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