linux-kernel - Re: [RFC PATCH 08/17] rv: Add Hybrid Automata monitor type

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Message-ID: <aKRBg-KhyCqgFEg3@jlelli-thinkpadt14gen4.remote.csb>
Date: Tue, 19 Aug 2025 11:18:59 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: Gabriele Monaco <gmonaco@...hat.com>
Cc: linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	linux-trace-kernel@...r.kernel.org, Nam Cao <namcao@...utronix.de>,
	Tomas Glozar <tglozar@...hat.com>, Juri Lelli <jlelli@...hat.com>,
	Clark Williams <williams@...hat.com>,
	John Kacur <jkacur@...hat.com>
Subject: Re: [RFC PATCH 08/17] rv: Add Hybrid Automata monitor type

Hi!

On 14/08/25 17:08, Gabriele Monaco wrote:

...

> +/*
> + * ha_monitor_init_env - setup timer and reset all environment
> + *
> + * Called from a hook in the DA start functions, it supplies the da_mon
> + * corresponding to the current ha_mon.
> + * Not all hybrid automata require the timer, still set it for simplicity.
> + */
> +static inline void ha_monitor_init_env(struct da_monitor *da_mon)
> +{
> +	struct ha_monitor *ha_mon = to_ha_monitor(da_mon);
> +
> +	ha_monitor_reset_all_stored(ha_mon);
> +	if (unlikely(!ha_mon->timer.base))
> +		hrtimer_setup(&ha_mon->timer, ha_monitor_timer_callback,
> +			      CLOCK_MONOTONIC, HRTIMER_MODE_REL);
> +}

...

> +/*
> + * Helper functions to handle the monitor timer.
> + * Not all monitors require a timer, in such case the timer will be set up but
> + * never armed.
> + * Timers start since the last reset of the supplied env or from now if env is
> + * not an environment variable. If env was not initialised no timer starts.
> + * Timers can expire on any CPU unless the monitor is per-cpu,
> + * where we assume every event occurs on the local CPU.
> + */
> +static inline void ha_start_timer_ns(struct ha_monitor *ha_mon, enum envs env,
> +				     u64 expire)
> +{
> +	int mode = HRTIMER_MODE_REL;
> +	u64 passed = 0;
> +
> +	if (env >= 0 && env < ENV_MAX_STORED) {
> +		if (ha_monitor_env_invalid(ha_mon, env))
> +			return;
> +		passed = ha_get_env(ha_mon, env);
> +	}
> +	if (RV_MON_TYPE == RV_MON_PER_CPU)
> +		mode |= HRTIMER_MODE_PINNED;
> +	hrtimer_start(&ha_mon->timer, ns_to_ktime(expire - passed), mode);
> +}

Does this also need to be _HARD on RT for the monitor to work?

Also, my only concern with the usage of per-task timers is that
reprogramming add overhead, so I wonder if this gets noticeable when
running some kind of performance sensitive workload in production (as it
was reported for dl-server). Did you test such a case?

Thanks!
Juri