linux-kernel - Re: [PATCH] sched/core: Cache timer busy housekeeping target

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Message-ID: <20190709013914.GA23714@lenoir>
Date:   Tue, 9 Jul 2019 03:39:16 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched/core: Cache timer busy housekeeping target

On Mon, Jul 01, 2019 at 08:24:37PM +0800, Wanpeng Li wrote:
> diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
> index 41dfff2..0d49bef 100644
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -195,8 +195,10 @@ struct hrtimer_cpu_base *get_target_base(struct hrtimer_cpu_base *base,
>  					 int pinned)
>  {
>  #if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
> -	if (static_branch_likely(&timers_migration_enabled) && !pinned)
> -		return &per_cpu(hrtimer_bases, get_nohz_timer_target());
> +	if (static_branch_likely(&timers_migration_enabled) && !pinned) {
> +		base->last_target_cpu = get_nohz_timer_target(base->last_target_cpu);
> +		return &per_cpu(hrtimer_bases, base->last_target_cpu);

I'm not sure this is exactly what we intend to cache here.

This doesn't return the last CPU for a given timer
(that would be timer->flags & TIMER_CPUMASK) but the last CPU that
was returned when "base" was passed.

First of all, it's always initialized to CPU 0, which is perhaps
not exactly what we want.

Also the result can be very stale and awkward. If for some reason we have:

        base(CPU 5)->last_target_cpu = 255

then later a timer is enqueued to CPU 5, the next time we re-enqueue that
timer will be to CPU 255, then the second re-enqueue will be to whatever
value we have in base(CPU 255)->last_target_cpu, etc...

For example imagine that:

	base(CPU 255)->last_target_cpu = 5

the timer will bounce between those two very distant CPU 5 and 255. So I think
you rather want "timer->flags & TIMER_CPUMASK". But note that those flags
can also be initialized to zero and therefore CPU 0, while we actually want
the CPU of the timer enqueuer for a first use. And I can't think of a
simple solution to solve that :-(  Perhaps keeping the enqueuer CPU as the
first choice (as we do upstream) is still the best thing we have.

Thanks.