linux-kernel - Re: [PATCH v3 2/2] sched: Add a check for cpu unbound deferrable timers

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Date:   Wed, 06 May 2020 16:03:34 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Prasad Sodagudi <psodagud@...eaurora.org>, john.stultz@...aro.org,
        sboyd@...nel.org, tj@...nel.org
Cc:     linux-kernel@...r.kernel.org, saravanak@...gle.com,
        psodagud@...eaurora.org, pkondeti@...eaurora.org
Subject: Re: [PATCH v3 2/2] sched: Add a check for cpu unbound deferrable timers

Prasad Sodagudi <psodagud@...eaurora.org> writes:

> Subject: sched: Add...

How is this related to the scheduler?

> Add a check to find expired unbound deferrable timers
> and trigger softirq for handling timers. This way a CPU
> can process all the expired deferrable timers whenever
> it is out off idle state due to an interrupt.

A little bit more context would be useful.

> Signed-off-by: Prasad Sodagudi <psodagud@...eaurora.org>
> ---
>  include/linux/timer.h    |  3 +++
>  kernel/time/tick-sched.c |  8 +++++++-
>  kernel/time/timer.c      | 29 ++++++++++++++++++++++++++++-
>  3 files changed, 38 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/timer.h b/include/linux/timer.h
> index 0dc19a8..e85dd2d 100644
> --- a/include/linux/timer.h
> +++ b/include/linux/timer.h
> @@ -172,6 +172,9 @@ extern int del_timer(struct timer_list * timer);
>  extern int mod_timer(struct timer_list *timer, unsigned long expires);
>  extern int mod_timer_pending(struct timer_list *timer, unsigned long expires);
>  extern int timer_reduce(struct timer_list *timer, unsigned long expires);
> +#ifdef CONFIG_SMP

This #ifdef is useless because ...

> +extern bool check_pending_deferrable_timers(int cpu);
> +#endif
>  
>  /*
>   * The jiffies value which is added to now, when there is no timer
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 3e2dc9b..16aec80 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -23,6 +23,7 @@
>  #include <linux/module.h>
>  #include <linux/irq_work.h>
>  #include <linux/posix-timers.h>
> +#include <linux/timer.h>
>  #include <linux/context_tracking.h>
>  #include <linux/mm.h>
>  
> @@ -1274,8 +1275,13 @@ static inline void tick_nohz_irq_enter(void)
>  	now = ktime_get();
>  	if (ts->idle_active)
>  		tick_nohz_stop_idle(ts, now);
> -	if (ts->tick_stopped)
> +	if (ts->tick_stopped) {
>  		tick_nohz_update_jiffies(now);
> +#ifdef CONFIG_SMP
> +		if (check_pending_deferrable_timers(smp_processor_id()))
> +			raise_softirq_irqoff(TIMER_SOFTIRQ);
> +#endif

... you failed to provide a stub function which avoids this #ifdef

> +	}
>  }
>  
>  #else
> diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> index 1bf9b49..5947c63 100644
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -221,6 +221,7 @@ static DECLARE_WORK(timer_update_work, timer_update_keys);
>  
>  #ifdef CONFIG_SMP
>  struct timer_base timer_base_deferrable;
> +static atomic_t deferrable_pending;
>  unsigned int sysctl_timer_migration = 1;
>  
>  DEFINE_STATIC_KEY_FALSE(timers_migration_enabled);
> @@ -1610,6 +1611,31 @@ static u64 cmp_next_hrtimer_event(u64 basem, u64 expires)
>  	return DIV_ROUND_UP_ULL(nextevt, TICK_NSEC) * TICK_NSEC;
>  }
>  
> +
> +#ifdef CONFIG_SMP
> +/*
> + * check_pending_deferrable_timers - Check for unbound deferrable timer expiry
> + * @cpu - Current CPU

Bogus doc format.

> + *
> + * The function checks whether any global deferrable pending timers
> + * are exipired or not. This function does not check cpu bounded
> + * diferrable pending timers expiry.

Editors have spell check.

> + *
> + * The function returns true when a cpu unbounded deferrable timer is expired.
> + */
> +bool check_pending_deferrable_timers(int cpu)
> +{
> +	if (cpu == tick_do_timer_cpu ||
> +		tick_do_timer_cpu == TICK_DO_TIMER_NONE) {

The second line conditional wants to be aligned with the first line
conditional.

> +		if (time_after_eq(jiffies, timer_base_deferrable.clk)
> +			&& !atomic_cmpxchg(&deferrable_pending, 0, 1)) {

Ditto. Aside of that this deferrable pending magic lacks any form of
explanation.

> +			return true;
> +		}
> +	}
> +	return false;
> +}
> +#endif
> +
>  /**
>   * get_next_timer_interrupt - return the time (clock mono) of the next timer
>   * @basej:	base time jiffies
> @@ -1801,7 +1827,8 @@ static __latent_entropy void run_timer_softirq(struct softirq_action *h)
>  	if (IS_ENABLED(CONFIG_NO_HZ_COMMON)) {
>  		__run_timers(this_cpu_ptr(&timer_bases[BASE_DEF]));
>  #ifdef CONFIG_SMP
> -		if (tick_do_timer_cpu == TICK_DO_TIMER_NONE ||
> +		if ((atomic_cmpxchg(&deferrable_pending, 1, 0) &&

How is that supposed to compile with NOHZ=n?

For every version of these patches the 0-day robot is complaining about
exactly the same problem:

    Your testing is solely done with one config, i.e. the config you are
    interested in.

Is it really so hard to compile test for a total of 4 combinations of
SMP and NOHZ?

Thanks,

        tglx