linux-kernel - Re: [RFC v2 9/9] [DEBUG] powerpc: add debug file for set/unset cpu avoid

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Message-ID: <aFx94BDKk_WJ48pK@yury>
Date: Wed, 25 Jun 2025 18:53:20 -0400
From: Yury Norov <yury.norov@...il.com>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
	vincent.guittot@...aro.org, tglx@...utronix.de, maddy@...ux.ibm.com,
	vschneid@...hat.com, dietmar.eggemann@....com, rostedt@...dmis.org,
	kprateek.nayak@....com, huschle@...ux.ibm.com, srikar@...ux.ibm.com,
	linux-kernel@...r.kernel.org, christophe.leroy@...roup.eu,
	linuxppc-dev@...ts.ozlabs.org, gregkh@...uxfoundation.org
Subject: Re: [RFC v2 9/9] [DEBUG] powerpc: add debug file for set/unset cpu
 avoid

On Thu, Jun 26, 2025 at 12:41:08AM +0530, Shrikanth Hegde wrote:
> Reference patch for how an architecture can make use of this infra. 
> 
> This is not meant to be merged. Instead the vp_manual_hint should either
> come from hardware or could be derived using steal time. 

If you don't add any code that manages the 'avoid' mask on the host
side, all this becomes a dead code.
 
> When the provided hint is less than the total CPUs in the system, it
> will enable the cpu avoid static key and set those CPUs as avoid. 
> 
> Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
> ---
>  arch/powerpc/include/asm/paravirt.h |  2 ++
>  arch/powerpc/kernel/smp.c           | 50 +++++++++++++++++++++++++++++
>  2 files changed, 52 insertions(+)
> 
> diff --git a/arch/powerpc/include/asm/paravirt.h b/arch/powerpc/include/asm/paravirt.h
> index b78b82d66057..b6497e0b60d8 100644
> --- a/arch/powerpc/include/asm/paravirt.h
> +++ b/arch/powerpc/include/asm/paravirt.h
> @@ -10,6 +10,8 @@
>  #include <asm/hvcall.h>
>  #endif
>  
> +DECLARE_STATIC_KEY_FALSE(paravirt_cpu_avoid_enabled);
> +
>  #ifdef CONFIG_PPC_SPLPAR
>  #include <linux/smp.h>
>  #include <asm/kvm_guest.h>
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index 5ac7084eebc0..e00cdc4de441 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -64,6 +64,7 @@
>  #include <asm/systemcfg.h>
>  
>  #include <trace/events/ipi.h>
> +#include <linux/debugfs.h>
>  
>  #ifdef DEBUG
>  #include <asm/udbg.h>
> @@ -82,6 +83,7 @@ bool has_big_cores __ro_after_init;
>  bool coregroup_enabled __ro_after_init;
>  bool thread_group_shares_l2 __ro_after_init;
>  bool thread_group_shares_l3 __ro_after_init;
> +static int vp_manual_hint = NR_CPUS;
>  
>  DEFINE_PER_CPU(cpumask_var_t, cpu_sibling_map);
>  DEFINE_PER_CPU(cpumask_var_t, cpu_smallcore_map);
> @@ -1727,6 +1729,7 @@ static void __init build_sched_topology(void)
>  	BUG_ON(i >= ARRAY_SIZE(powerpc_topology) - 1);
>  
>  	set_sched_topology(powerpc_topology);
> +	vp_manual_hint = num_present_cpus();
>  }
>  
>  void __init smp_cpus_done(unsigned int max_cpus)
> @@ -1807,4 +1810,51 @@ void __noreturn arch_cpu_idle_dead(void)
>  	start_secondary_resume();
>  }
>  
> +/*
> + * sysfs hint to mark CPUs as Avoid. This will help in restricting
> + * the workload to specified number of CPUs.
> + * For example 40 > vp_manual_hint means, workload will run on
> + * 0-39 CPUs.
> + */
> +
> +static int pv_vp_manual_hint_set(void *data, u64 val)
> +{
> +	int cpu;
> +
> +	if (val == 0 || vp_manual_hint > num_present_cpus())
> +		vp_manual_hint = num_present_cpus();
> +
> +	if (val != vp_manual_hint)
> +		vp_manual_hint = val;

This all is effectively just:

	vp_manual_hint = val;

Isn't?

> +	if (vp_manual_hint < num_present_cpus())
> +		static_branch_enable(&paravirt_cpu_avoid_enabled);
> +	else
> +		static_branch_disable(&paravirt_cpu_avoid_enabled);
> +
> +	for_each_present_cpu(cpu) {
> +		if (cpu >= vp_manual_hint)
> +			set_cpu_avoid(cpu, true);
> +		else
> +			set_cpu_avoid(cpu, false);
> +	}
> +	return 0;
> +}
> +
> +static int pv_vp_manual_hint_get(void *data, u64 *val)
> +{
> +	*val = vp_manual_hint;
> +	return 0;
> +}
> +
> +DEFINE_SIMPLE_ATTRIBUTE(fops_pv_vp_manual_hint, pv_vp_manual_hint_get, pv_vp_manual_hint_set, "%llu\n");
> +
> +static __init int paravirt_debugfs_init(void)
> +{
> +	if (is_shared_processor())
> +		debugfs_create_file("vp_manual_hint", 0600, arch_debugfs_dir, NULL, &fops_pv_vp_manual_hint);
> +	return 0;
> +}
> +
> +device_initcall(paravirt_debugfs_init)
>  #endif
> -- 
> 2.43.0