Re: [PATCH v2 1/3] rv: add per-edge dwell-time statistics primitive

[Gabriele Monaco <gmonaco@redhat.com>]

On Fri, 2026-09-11 at 14:34 +0200, Tobias Schaffner wrote:
> Add a small primitive that records, per automaton edge, how long the
> monitor dwelled before taking it with a count, a sum and a maximum.
> 
> The counters are per-CPU and lock-free, so a monitor's hot path can
> update them without disabling interrupts and without perturbing the latency
> being measured. Keep the state and its entry timestamp in one word so
> nested transitions cannot charge a dwell to the wrong edge.
> 
> Signed-off-by: Tobias Schaffner <tobias.schaffner@siemens.com>
> ---
>  include/rv/edge_stat.h | 69 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 69 insertions(+)
>  create mode 100644 include/rv/edge_stat.h
> 
> diff --git a/include/rv/edge_stat.h b/include/rv/edge_stat.h

...
> +#define RV_STATE_BITS		8
> +#define RV_STATE_MASK		GENMASK(RV_STATE_BITS - 1, 0)
> +#define RV_TS_MASK		GENMASK(BITS_PER_LONG - RV_STATE_BITS - 1, 0)
> +
> +#define da_state_of(w)		((unsigned int)((w) & RV_STATE_MASK))
> +#define da_ts_of(w)		((u64)(w) >> RV_STATE_BITS)
> +#define da_state_pack(s, ts)	(((((da_state_t)(ts)) & RV_TS_MASK) <<
> RV_STATE_BITS) | \
> +				 ((da_state_t)(s) & RV_STATE_MASK))

These are barely readable and error prone, can you use FIELD_GET()/FIELD_PREP()?

Thanks,
Gabriele

> +
> +static __always_inline
> +void rv_edge_stat_account(struct rv_edge_stat *s, u64 dwell_ns)
> +{
> +	s64 max;
> +	int i;
> +
> +	local64_inc(&s->count);
> +	local64_add(dwell_ns, &s->sum_ns);
> +
> +	/* Keep the largest dwell; bound retries if a nested update races us.
> */
> +	max = local64_read(&s->max_ns);
> +	for (i = 0; dwell_ns > (u64)max; i++) {
> +		if (i == MAX_DA_RETRY_RACING_EVENTS) {
> +			WARN_ONCE(1, "rv: edge-stat max update exceeded %d
> retries\n",
> +				  MAX_DA_RETRY_RACING_EVENTS);
> +			break;
> +		}
> +		if (local64_try_cmpxchg(&s->max_ns, &max, dwell_ns))
> +			break;
> +	}
> +}
> +
> +#endif /* CONFIG_RV_EDGE_STAT */
> +
> +#endif /* _RV_EDGE_STAT_H */