Re: [PATCH v8 4/5] tick/nohz_full: Ensure quiet_vmstat() is called on exit to user-mode when the idle tick is stopped

[Rafael Folco <rfolco@redhat.com>]

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Tested this patch w/ nohz_full setup and oslat 8h run on isolated cpus, max
latency is 7us versus 15us without the patch.
Thanks.

On Mon, Sep 26, 2022 at 8:02 PM Aaron Tomlin <atomlin@redhat.com> wrote:

> This patch ensures CPU-specific vmstat differentials do not remain
> when the scheduling-tick is stopped and before exiting to user-mode
> in the context of nohz_full only.
>
> A trivial test program was used to determine the impact of the proposed
> changes and under vanilla. The mlock(2) and munlock(2) system calls was
> used solely to modify vmstat item 'NR_MLOCK'. The following is an average
> count of CPU-cycles across the aforementioned system calls:
>
>                                   Vanilla                 Modified
>
>   Cycles per syscall              8461                    8690    (+2.6%)
>
> Signed-off-by: Aaron Tomlin <atomlin@redhat.com>
> ---
>  include/linux/tick.h     |  5 +++--
>  kernel/time/tick-sched.c | 15 +++++++++++++++
>  2 files changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index bfd571f18cfd..a2bbd6d32e33 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -11,7 +11,6 @@
>  #include <linux/context_tracking_state.h>
>  #include <linux/cpumask.h>
>  #include <linux/sched.h>
> -#include <linux/rcupdate.h>
>
>  #ifdef CONFIG_GENERIC_CLOCKEVENTS
>  extern void __init tick_init(void);
> @@ -272,6 +271,7 @@ static inline void tick_dep_clear_signal(struct
> signal_struct *signal,
>
>  extern void tick_nohz_full_kick_cpu(int cpu);
>  extern void __tick_nohz_task_switch(void);
> +void __tick_nohz_user_enter_prepare(void);
>  extern void __init tick_nohz_full_setup(cpumask_var_t cpumask);
>  #else
>  static inline bool tick_nohz_full_enabled(void) { return false; }
> @@ -296,6 +296,7 @@ static inline void tick_dep_clear_signal(struct
> signal_struct *signal,
>
>  static inline void tick_nohz_full_kick_cpu(int cpu) { }
>  static inline void __tick_nohz_task_switch(void) { }
> +static inline void __tick_nohz_user_enter_prepare(void) { }
>  static inline void tick_nohz_full_setup(cpumask_var_t cpumask) { }
>  #endif
>
> @@ -308,7 +309,7 @@ static inline void tick_nohz_task_switch(void)
>  static inline void tick_nohz_user_enter_prepare(void)
>  {
>         if (tick_nohz_full_cpu(smp_processor_id()))
> -               rcu_nocb_flush_deferred_wakeup();
> +               __tick_nohz_user_enter_prepare();
>  }
>
>  #endif
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index b0e3c9205946..634cd0fac267 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -26,6 +26,7 @@
>  #include <linux/posix-timers.h>
>  #include <linux/context_tracking.h>
>  #include <linux/mm.h>
> +#include <linux/rcupdate.h>
>
>  #include <asm/irq_regs.h>
>
> @@ -519,6 +520,20 @@ void __tick_nohz_task_switch(void)
>         }
>  }
>
> +void __tick_nohz_user_enter_prepare(void)
> +{
> +       struct tick_sched *ts;
> +
> +       if (tick_nohz_full_cpu(smp_processor_id())) {
> +               ts = this_cpu_ptr(&tick_cpu_sched);
> +
> +               if (ts->tick_stopped)
> +                       quiet_vmstat();
> +               rcu_nocb_flush_deferred_wakeup();
> +       }
> +}
> +EXPORT_SYMBOL_GPL(__tick_nohz_user_enter_prepare);
> +
>  /* Get the boot-time nohz CPU list from the kernel parameters. */
>  void __init tick_nohz_full_setup(cpumask_var_t cpumask)
>  {
> --
> 2.37.1
>
>
>

-- 
Folco

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