linux-kernel - Re: [PATCH v2 10/20] rv: Add Hybrid Automata monitor type

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Message-ID: <0c61c0bbbdc2002efb638dccda1f0a18c51d29df.camel@redhat.com>
Date: Fri, 17 Oct 2025 17:22:14 +0200
From: Gabriele Monaco <gmonaco@...hat.com>
To: Nam Cao <namcao@...utronix.de>, linux-kernel@...r.kernel.org, Steven
 Rostedt <rostedt@...dmis.org>, Masami Hiramatsu <mhiramat@...nel.org>, 
	linux-trace-kernel@...r.kernel.org
Cc: Tomas Glozar <tglozar@...hat.com>, Juri Lelli <jlelli@...hat.com>, Clark
 Williams <williams@...hat.com>, John Kacur <jkacur@...hat.com>
Subject: Re: [PATCH v2 10/20] rv: Add Hybrid Automata monitor type

On Fri, 2025-10-17 at 15:05 +0200, Nam Cao wrote:
> 
> Ok, now things start to make sense. Thanks for the explanation.
> 
> At least to me, using the same variable to store different time values
> is a bit confusing.
> 
> Is it possible that we always store the timestamp of the last clock reset?
> 
> The invariant bound value (N) is fixed for each state, so we can have
> the bound value in ha_verify_invariants() instead. For example, the
> Python script can generate something like
> 
> static inline bool ha_verify_invariants(struct ha_monitor *ha_mon,
>                                        enum states curr_state, enum events
> event,
>                                        enum states next_state, u64 time_ns)
> {
>        if (curr_state == enqueued_stall)
>                return ha_check_invariant_jiffy(ha_mon, threshold_jiffies,
> time_ns);
>        return true;
> }
> 
> Is that possible?

Alright, that /should/ be possible, provided the value used to set invariants is
constant or at least doesn't change until we leave the state.
This seems a fair assumption to make but doesn't stand for the throttle monitor,
in that case I read the remaining runtime from the dl entity, that one is
updated frequently, for instance when a task is throttled, it's negative, but
this doesn't mean the invariant should expect time to be negative.

Runtime is consumed only when a task is running, so here I use an invariant set
up on the /remaining/ runtime when reaching the running state, that's why also
switch_in resets the clock (runtime is not replenished, but the runtime_left
value doesn't need to be subtracted anything).

An alternative would be to have some sort of pause/resume operations on clocks,
and a task would just pause the clock when preempted, but those operations are
not backed up by theory and wouldn't really simplify the implementation (use 2
variables per clock or a single one and some hack to mark it as paused).

Again, there may be better ways, but I found this one the "simplest".

Does it makes sense or am I just crystallising to this implementation?

Thanks,
Gabriele

> 
> > Kinda, it would solve the problem for this specific subtraction, but racing
> > handlers could still lead to problems although the subtraction is "correct".
> > 
> > Since this is the only time the env storage needs to be an atomic_t and it's
> > fairly rare (only complicated models require calling this function at all,
> > others are happy with READ_ONCE/WRITE_ONCE) I didn't want to change the
> > storage
> > implementation for some perceived safety.
> > 
> > I wrote that comment exactly to motivate why we aren't using atomic_t, but I
> > should probably reword that. Does this make sense to you?
> 
> I think if we always store the timestamp since last reset, we can get
> rid of this function. Let's see how that discussion go..
> >