linux-kernel - Re: [PATCH v12 10/11] sched: early boot clock

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Message-ID: <20180626112705.4dxrsd5acvobla5y@xakep.localdomain>
Date:   Tue, 26 Jun 2018 07:27:05 -0400
From:   Pavel Tatashin <pasha.tatashin@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        linux@...linux.org.uk, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, john.stultz@...aro.org,
        sboyd@...eaurora.org, x86@...nel.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, tglx@...utronix.de, hpa@...or.com,
        douly.fnst@...fujitsu.com, prarit@...hat.com, feng.tang@...el.com,
        pmladek@...e.com, gnomes@...rguk.ukuu.org.uk,
        linux-s390@...r.kernel.org
Subject: Re: [PATCH v12 10/11] sched: early boot clock

> 
> How's something like this? That moves sched_clock_init() to right before
> we enable IRQs for the first time (which is after we've started the
> whole timekeeping business).
> 
> The thing is, sched_clock_init_late() reall is far too late, we need to
> switch to unstable before we bring up SMP.

OK, sure.

> -	sched_clock_postinit();
> +	sched_clock_init();

Yes, we can move sched_clock_init(). But placing it after time_init() would
work on all arches with unstable clock except for x86.

See comment above time_init x86:
arch/x86/kernel/time.c

 99/*
100 * Initialize TSC and delay the periodic timer init to
101 * late x86_late_time_init() so ioremap works.
102 */
103void __init time_init(void)
104{
105	late_time_init = x86_late_time_init;
106}

Only After this:
> >     late_time_init()
> >         x86_late_time_init()
> >             x86_init.timers.timer_init()
> > 	        hpet_time_init() Only after this call we finally start
> > 		getting clock interrupts, and can get precise output from
> > 		sched_clock_local().

We start getting timer interrupts. Is it acceptable to move
sched_clock_init() after late_time_init()?

Thank you,
Pavel