linux-kernel - Re: [PATCH v10 18/20] timers: Implement the hierarchical pull model

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Date: Sun, 28 Jan 2024 16:58:55 +0100
From: Anna-Maria Behnsen <anna-maria@...utronix.de>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
 John Stultz <jstultz@...gle.com>, Thomas Gleixner <tglx@...utronix.de>,
 Eric Dumazet <edumazet@...gle.com>, "Rafael J . Wysocki"
 <rafael.j.wysocki@...el.com>, Arjan van de Ven <arjan@...radead.org>,
 "Paul E . McKenney" <paulmck@...nel.org>, Rik van Riel <riel@...riel.com>,
 Steven Rostedt <rostedt@...dmis.org>, Sebastian Siewior
 <bigeasy@...utronix.de>, Giovanni Gherdovich <ggherdovich@...e.cz>, Lukasz
 Luba <lukasz.luba@....com>, "Gautham R . Shenoy" <gautham.shenoy@....com>,
 Srinivas Pandruvada <srinivas.pandruvada@...el.com>, K Prateek Nayak
 <kprateek.nayak@....com>
Subject: Re: [PATCH v10 18/20] timers: Implement the hierarchical pull model

Frederic Weisbecker <frederic@...nel.org> writes:

> On Mon, Jan 15, 2024 at 03:37:41PM +0100, Anna-Maria Behnsen wrote:
>> +/**
>> + * tmigr_quick_check() - Quick forecast of next tmigr event when CPU wants to
>> + *			 go idle
>> + *
>> + * Returns KTIME_MAX, when it is probable that nothing has to be done (not the
>> + * only one in the level 0 group; and if it is the only one in level 0 group,
>> + * but there are more than a single group active in top level)
>> + *
>> + * Returns first expiry of the top level group, when it is the only one in level
>> + * 0 and top level also only has a single active child.
>> + */
>> +u64 tmigr_quick_check(void)
>> +{
>> +	struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
>> +	struct tmigr_group *topgroup;
>> +	struct list_head lvllist;
>> +
>> +	if (tmigr_is_not_available(tmc))
>> +		return KTIME_MAX;
>
> Offline CPUs are supposed to handle their own global timers.
>
> So instead of returning KTIME_MAX here, shouldn't we pass instead
> tevt->global as a parameter and return that value?
>
> Otherwise the quick check will simply ignore the next global event of this CPU
> if it's before the next local event.

I thought about this as well. I skipped it to keep it as simple as
possible - as this is only a forecast and might change (CPUs will come
online/go offline/deletion of timers...). But I can integrate it to keep
it more precise.

>> +
>> +	if (WARN_ON_ONCE(tmc->idle))
>> +		return KTIME_MAX;
>
> Same here I guess...

Yes.

>> +
>> +	if (!tmigr_check_migrator_and_lonely(tmc->tmgroup, tmc->childmask))
>> +		return KTIME_MAX;
>
> This one makes sense.
>
>> +
>> +	for (int i = tmigr_hierarchy_levels; i > 0 ; i--) {
>> +		lvllist = tmigr_level_list[i - 1];
>> +		if (list_is_singular(&lvllist)) {
>> +			topgroup = list_first_entry(&lvllist, struct
>> tmigr_group, list);
>
> Is it safe against concurrent allocation failure in hotplug?

As you pointed out, it isn't!

> If the list is seen singular, then concurrently a CPU comes up and creates/add
> a new group. The current CPU actually fetches it instead of the current group
> because it's not singular anymore. But then some higher level group
> allocation fails and the newly added first entry is removed.
>
> list_is_singular() looks safe. But list_first_entry isn't. You can create
> list_first_entry_rcu:
>
> #define list_first_entry_rcu(ptr, type, member) \
> 	list_entry_rcu((ptr)->next, type, member)
>
> Protected inside rcu_read_lock() until the below READ_ONCE().
>
> And then use list_del_rcu/list_add_rcu/kfree_rcu on the update side.
>
> Isn't it possible to walk through group->parent instead?

Yes. Sure! This is possible and maybe also easier. I cannot remember why
I implemented it this way...

>> +
>> +			if (tmigr_check_lonely(topgroup))
>> +				return READ_ONCE(topgroup->next_expiry);

When I hand in tevt->global as a parameter, I'll need to compare the
first expiry of the toplevel group and the tevt->global value and return
the earlier expiry. Only a single child is active in top level, so it
might be that this CPU is the last active CPU in hierarchy.

I didn't check all the way to the top whether all groups are
'lonely'. So when the top level group has only a single active child, it
is also possible that the child of the top level group has two active
children... Then a return of KTIME_MAX would be also a more precise
forecast.

This quick check is there to keep the overhead minimal when checking
whether it might be possible to go idle. So I don't know, if we should
add this additional check per level (which is pretty simple when using
group->parent for walking the hierarchy). What do you think?

>> +		} else {
>> +			continue;
>> +		}
>> +	}
>> +
>> +	return KTIME_MAX;
>
> I'm less sure about that return value.

This is ok, because there is not only a single child active in top
level. This CPU is definitely not the last active CPU in hierarchy. So
it is likely that some other CPU will handle tevt->global of this CPU.

I'll add a comment :)

Thanks,

	Anna-Maria