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Date:   Tue, 20 Oct 2020 13:38:28 +0200
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Christian Eggers <ceggers@arri.de>, tglx@linutronix.de,
        linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: sched: system doesn't boot since  "sched: Add new
 migrate_disable() implementation"
Message-ID: <20201020113828.fv7ui4h3gkrcpqoc@linutronix.de>
References: <1654655.1jrfHnk7pZ@n95hx1g2>
 <20201019150935.koqbk57dmahhomdc@linutronix.de>
 <20201020113009.GR2628@hirez.programming.kicks-ass.net>
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On 2020-10-20 13:30:09 [+0200], Peter Zijlstra wrote:
> On Mon, Oct 19, 2020 at 05:09:35PM +0200, Sebastian Andrzej Siewior wrote:
> > On 2020-10-19 12:21:06 [+0200], Christian Eggers wrote:
> > > I have problems with the latest 5.9-rt releases on i.MX6ULL (!CONFIG_=
SMP):
> > >=20
> > =E2=80=A6
> > > Any hints?
> >=20
> > Thank you for the report. The reason is the migrate_disable()
> > implementation for !SMP.
>=20
> This should fix things I suppose. I'll fold it in.

It will. It will also break lazy-preemption. Each time a sleeping lock
is acquired there is also migrate_disable() and the migrate-disable
counter is !=3D 0 (even for UP). The result is that a wake up for a
SCHED_OTHER task with mg counter !=3D 0 will not lead to context switch
(same like preemption counter !=3D 0). The difference is that a wake up
for a RT task ignores this counter and perform a context switch anyway.

That way we have RT wake ups on time but avoid stumbling from one lock
to another.

> ---
> --- a/include/linux/preempt.h
> +++ b/include/linux/preempt.h
> @@ -378,7 +378,12 @@ static inline void preempt_notifier_init
>  extern void migrate_disable(void);
>  extern void migrate_enable(void);
> =20
> -#else /* !(CONFIG_SMP && CONFIG_PREEMPT_RT) */
> +#elif defined(CONFIG_PREEMPT_RT)
> +
> +static inline void migrate_disable(void) { }
> +static inline void migrate_enable(void { }
> +
> +#else /* !CONFIG_PREEMPT_RT */
> =20
>  /**
>   * migrate_disable - Prevent migration of the current task

Sebastian