Re: [PATCH] tick/common: Align tick period during sched_timer setup.

[Mathias Krause]

On 15.06.23 11:18, Sebastian Andrzej Siewior wrote:
> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> 
> The tick period is aligned very early while the first clock_event_device
> is registered. The system runs in periodic mode and switches later to
> one-shot mode if possible.
> 
> The next wake-up event is programmed based on aligned value
> (tick_next_period) but the delta value, that is used to program the
> clock_event_device, is computed based on ktime_get().
> 
> With the subtracted offset, the devices fires in less than the exacted
> time frame. With a large enough offset the system programs the timer for
> the next wake-up and the remaining time left is too little to make any
> boot progress. The system hangs.
> 
> Move the alignment later to the setup of tick_sched timer. At this point
> the system switches to oneshot mode and a highres clocksource is
> available. It safe to update tick_next_period ktime_get() will now
> return accurate (not jiffies based) time.
> 
> [bigeasy: Patch description + testing].
> 
> Reported-by: Mathias Krause <minipli@xxxxxxxxxxxxxx>
> Reported-by: "Bhatnagar, Rishabh" <risbhat@xxxxxxxxxx>
> Fixes: e9523a0d81899 ("tick/common: Align tick period with the HZ tick.")

Cc: stable, maybe? This commit already ended up in quite a few "stable"
kernels (v6.3.2, v6.2.15, v6.1.28, v5.15.111, v5.10.180 and v5.4.243)
and it might be better to list them explicitly to avoid one of them
getting missed.

> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/5a56290d-806e-b9a5-f37c-f21958b5a8c0@xxxxxxxxxxxxxx
> Link: https://lore.kernel.org/12c6f9a3-d087-b824-0d05-0d18c9bc1bf3@xxxxxxxxxx
> ---
>  kernel/time/tick-common.c | 11 +----------
>  kernel/time/tick-sched.c  | 13 ++++++++++++-
>  2 files changed, 13 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
> index 65b8658da829e..b85f2f9c32426 100644
> --- a/kernel/time/tick-common.c
> +++ b/kernel/time/tick-common.c
> @@ -218,19 +218,10 @@ static void tick_setup_device(struct tick_device *td,
>  		 * this cpu:
>  		 */
>  		if (tick_do_timer_cpu == TICK_DO_TIMER_BOOT) {
> -			ktime_t next_p;
> -			u32 rem;
>  
>  			tick_do_timer_cpu = cpu;
>  
> -			next_p = ktime_get();
> -			div_u64_rem(next_p, TICK_NSEC, &rem);
> -			if (rem) {
> -				next_p -= rem;
> -				next_p += TICK_NSEC;
> -			}
> -
> -			tick_next_period = next_p;
> +			tick_next_period = ktime_get();
>  #ifdef CONFIG_NO_HZ_FULL
>  			/*
>  			 * The boot CPU may be nohz_full, in which case set
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 52254679ec489..42c0be3080bde 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -161,8 +161,19 @@ static ktime_t tick_init_jiffy_update(void)
>  	raw_spin_lock(&jiffies_lock);
>  	write_seqcount_begin(&jiffies_seq);
>  	/* Did we start the jiffies update yet ? */
> -	if (last_jiffies_update == 0)
> +	if (last_jiffies_update == 0) {
> +		u32 rem;
> +
> +		/*
> +		 * Ensure that the tick is aligned to a multiple of
> +		 * TICK_NSEC.
> +		 */
> +		div_u64_rem(tick_next_period, TICK_NSEC, &rem);
> +		if (rem)
> +			tick_next_period += TICK_NSEC - rem;
> +
>  		last_jiffies_update = tick_next_period;
> +	}
>  	period = last_jiffies_update;
>  	write_seqcount_end(&jiffies_seq);
>  	raw_spin_unlock(&jiffies_lock);

Hah, nice spot. So you implemented what I suggested and it, indeed,
works as expected, thereby:

Tested-by: Mathias Krause <minipli@xxxxxxxxxxxxxx>

Thanks,
Mathias