Re: [PATCH] bpf: Add support for absolute value BPF timers

[Tero Kristo <tero.kristo@linux.intel.com>]

Hi,

Please ignore this, I added a broken mailing list to CC.

Re-sending in a bit.

-Tero

On 21/02/2023 16:48, Tero Kristo wrote:
> Add a new flag BPF_F_TIMER_ABS that can be passed to bpf_timer_start()
> to start an absolute value timer instead of the default relative value.
> This makes the timer expire at an exact point in time, instead of a time
> with latencies and jitter induced by both the BPF and timer subsystems.
> This is useful e.g. in certain time sensitive profiling cases, where we
> need a timer to expire at an exact point in time.
>
> Signed-off-by: Tero Kristo <tero.kristo@linux.intel.com>
> ---
>   include/uapi/linux/bpf.h | 15 +++++++++++++++
>   kernel/bpf/helpers.c     | 11 +++++++++--
>   2 files changed, 24 insertions(+), 2 deletions(-)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 464ca3f01fe7..7f5b71847984 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -4951,6 +4951,12 @@ union bpf_attr {
>    *		different maps if key/value layout matches across maps.
>    *		Every bpf_timer_set_callback() can have different callback_fn.
>    *
> + *		*flags* can be one of:
> + *
> + *		**BPF_F_TIMER_ABS**
> + *			Start the timer in absolute expire value instead of the
> + *			default relative one.
> + *
>    *	Return
>    *		0 on success.
>    *		**-EINVAL** if *timer* was not initialized with bpf_timer_init() earlier
> @@ -7050,4 +7056,13 @@ struct bpf_core_relo {
>   	enum bpf_core_relo_kind kind;
>   };
>   
> +/*
> + * Flags to control bpf_timer_start() behaviour.
> + *     - BPF_F_TIMER_ABS: Timeout passed is absolute time, by default it is
> + *       relative to current time.
> + */
> +enum {
> +	BPF_F_TIMER_ABS = (1ULL << 0),
> +};
> +
>   #endif /* _UAPI__LINUX_BPF_H__ */
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index af30c6cbd65d..924849d89828 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -1253,10 +1253,11 @@ BPF_CALL_3(bpf_timer_start, struct bpf_timer_kern *, timer, u64, nsecs, u64, fla
>   {
>   	struct bpf_hrtimer *t;
>   	int ret = 0;
> +	enum hrtimer_mode mode;
>   
>   	if (in_nmi())
>   		return -EOPNOTSUPP;
> -	if (flags)
> +	if (flags > BPF_F_TIMER_ABS)
>   		return -EINVAL;
>   	__bpf_spin_lock_irqsave(&timer->lock);
>   	t = timer->timer;
> @@ -1264,7 +1265,13 @@ BPF_CALL_3(bpf_timer_start, struct bpf_timer_kern *, timer, u64, nsecs, u64, fla
>   		ret = -EINVAL;
>   		goto out;
>   	}
> -	hrtimer_start(&t->timer, ns_to_ktime(nsecs), HRTIMER_MODE_REL_SOFT);
> +
> +	if (flags & BPF_F_TIMER_ABS)
> +		mode = HRTIMER_MODE_ABS_SOFT;
> +	else
> +		mode = HRTIMER_MODE_REL_SOFT;
> +
> +	hrtimer_start(&t->timer, ns_to_ktime(nsecs), mode);
>   out:
>   	__bpf_spin_unlock_irqrestore(&timer->lock);
>   	return ret;