On Wed, Sep 10, 2025 at 11:12:03PM +0530, Shrikanth Hegde wrote:
> CPUs are marked paravirt when there is contention for underlying
> physical CPU.
>
> The push mechanism and check for paravirt CPUs are in sched tick
> and wakeup. It should be close to no-op when there is no need for it.
> Achieve that using static key.
>
> Architecture needs to enable this key when it decides there are
> paravirt CPUs. Disable it when there are no paravirt CPUs.
Testing a bit is quite close to a no-op, isn't it? Have you measured
the performance impact that would advocate the static key? Please
share some numbers then. I believe I asked you about it on the previous
round.
> Signed-off-by: Shrikanth Hegde <sshe...@linux.ibm.com>
> ---
> kernel/sched/core.c | 1 +
> kernel/sched/sched.h | 17 +++++++++++++++++
> 2 files changed, 18 insertions(+)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 0f1e36bb5779..b8a84e4691c8 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -10967,4 +10967,5 @@ void sched_enq_and_set_task(struct
> sched_enq_and_set_ctx *ctx)
> #ifdef CONFIG_PARAVIRT
> struct cpumask __cpu_paravirt_mask __read_mostly;
> EXPORT_SYMBOL(__cpu_paravirt_mask);
> +DEFINE_STATIC_KEY_FALSE(cpu_paravirt_push_tasks);
> #endif
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index b5367c514c14..8f9991453d36 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -3880,4 +3880,21 @@ void sched_enq_and_set_task(struct
> sched_enq_and_set_ctx *ctx);
>
> #include "ext.h"
>
> +#ifdef CONFIG_PARAVIRT
> +DECLARE_STATIC_KEY_FALSE(cpu_paravirt_push_tasks);
> +
> +static inline bool is_cpu_paravirt(int cpu)
> +{
> + if (static_branch_unlikely(&cpu_paravirt_push_tasks))
> + return cpu_paravirt(cpu);
> +
> + return false;
> +}
So is_cpu_paravirt and cpu_paravirt are exactly the same in terms of
functionality. If you really believe that static branch benefits the
performance, it should go straight to the cpu_paravirt().
> +#else /* !CONFIG_PARAVIRT */
> +static inline bool is_cpu_paravirt(int cpu)
> +{
> + return false;
> +}
> +#endif /* !CONFIG_PARAVIRT */
> +
> #endif /* _KERNEL_SCHED_SCHED_H */
> --
> 2.47.3
