linux-kernel - Re: [PATCH] KVM: Consider SMT idle status when halt polling

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Date:   Thu, 22 Jul 2021 07:15:16 +0300
From:   Mika Penttilä <mika.penttila@...tfour.com>
To:     Li RongQing <lirongqing@...du.com>, pbonzini@...hat.com,
        mingo@...hat.com, peterz@...radead.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: Consider SMT idle status when halt polling



On 22.7.2021 6.58, Li RongQing wrote:
> SMT siblings share caches and other hardware, halt polling
> will degrade its sibling performance if its sibling is busy
>
> Signed-off-by: Li RongQing <lirongqing@...du.com>
> ---
>   include/linux/kvm_host.h |  5 ++++-
>   include/linux/sched.h    | 17 +++++++++++++++++
>   kernel/sched/fair.c      | 17 -----------------
>   3 files changed, 21 insertions(+), 18 deletions(-)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index ae7735b..15b3ef4 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -269,7 +269,10 @@ static inline bool kvm_vcpu_mapped(struct kvm_host_map *map)
>   
>   static inline bool kvm_vcpu_can_poll(ktime_t cur, ktime_t stop)
>   {
> -	return single_task_running() && !need_resched() && ktime_before(cur, stop);
> +	return single_task_running() &&
> +		   !need_resched() &&
> +		   ktime_before(cur, stop) &&
> +		   is_core_idle(raw_smp_processor_id());
>   }
>   
>   /*
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index ec8d07d..c333218 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -34,6 +34,7 @@
>   #include <linux/rseq.h>
>   #include <linux/seqlock.h>
>   #include <linux/kcsan.h>
> +#include <linux/topology.h>
>   #include <asm/kmap_size.h>
>   
>   /* task_struct member predeclarations (sorted alphabetically): */
> @@ -2191,6 +2192,22 @@ int sched_trace_rq_nr_running(struct rq *rq);
>   
>   const struct cpumask *sched_trace_rd_span(struct root_domain *rd);
>   
> +static inline bool is_core_idle(int cpu)
> +{
> +#ifdef CONFIG_SCHED_SMT
> +	int sibling;
> +
> +	for_each_cpu(sibling, cpu_smt_mask(cpu)) {
> +		if (cpu == sibling)
> +			continue;
> +
> +		if (!idle_cpu(cpu))
> +			return false;

if (!idle_cpu(sibling))  instead, now it returns always false.



> +	}
> +#endif
> +	return true;
> +}
> +
>   #ifdef CONFIG_SCHED_CORE
>   extern void sched_core_free(struct task_struct *tsk);
>   extern void sched_core_fork(struct task_struct *p);
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 44c4520..5b0259c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1477,23 +1477,6 @@ struct numa_stats {
>   	int idle_cpu;
>   };
>   
> -static inline bool is_core_idle(int cpu)
> -{
> -#ifdef CONFIG_SCHED_SMT
> -	int sibling;
> -
> -	for_each_cpu(sibling, cpu_smt_mask(cpu)) {
> -		if (cpu == sibling)
> -			continue;
> -
> -		if (!idle_cpu(cpu))
> -			return false;
> -	}
> -#endif
> -
> -	return true;
> -}
> -
>   struct task_numa_env {
>   	struct task_struct *p;
>