linux-kernel - Re: [PATCH 03/15] sched/cputime: Correctly support generic vtime idle time

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Message-ID: <aW6johe2ZUR9l39Z@pavilion.home>
Date: Mon, 19 Jan 2026 22:35:30 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
	"Christophe Leroy (CS GROUP)" <chleroy@...nel.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Ben Segall <bsegall@...gle.com>, Boqun Feng <boqun.feng@...il.com>,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Heiko Carstens <hca@...ux.ibm.com>, Ingo Molnar <mingo@...hat.com>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Kieran Bingham <kbingham@...nel.org>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Mel Gorman <mgorman@...e.de>, Michael Ellerman <mpe@...erman.id.au>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Nicholas Piggin <npiggin@...il.com>,
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	Steven Rostedt <rostedt@...dmis.org>,
	Sven Schnelle <svens@...ux.ibm.com>,
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	Valentin Schneider <vschneid@...hat.com>,
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	Xin Zhao <jackzxcui1989@....com>, linux-pm@...r.kernel.org,
	linux-s390@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 03/15] sched/cputime: Correctly support generic vtime
 idle time

Le Mon, Jan 19, 2026 at 02:02:22PM +0100, Peter Zijlstra a écrit :
> On Fri, Jan 16, 2026 at 03:51:56PM +0100, Frederic Weisbecker wrote:
> 
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > index 8ddf74e705d3..f1d07a0276a5 100644
> > --- a/kernel/time/tick-sched.c
> > +++ b/kernel/time/tick-sched.c
> > @@ -780,7 +780,7 @@ static u64 get_cpu_sleep_time_us(struct tick_sched *ts, ktime_t *sleeptime,
> >  	ktime_t now, idle;
> >  	unsigned int seq;
> >  
> > -	if (!tick_nohz_active)
> > +	if (!tick_nohz_active || vtime_generic_enabled_cpu(cpu))
> >  		return -1;
> >  
> >  	now = ktime_get();
> 
> Is this not broken? IIUC this means that you can no longer use
> get_cpu_{idle,iowait}_time_us() the moment you have context tracking
> enabled.

It is supported again in patch 13/15. And it's not exactly breaking
bisection in the meantime because the sole user is cpufreq and cpufreq
shouldn't be relevant with nohz_full.

Ok a few subsystem rely on the resulting cpufreq API get_cpu_idle_time():

- the legacy drivers/macintosh/rack-meter.c
- drivers/scsi/lpfc/lpfc_init.c

But cpufreq provides a low-resolution version in the worst case for nohz_full
(again until 13/15).

Hmm, but you're right this is confusing. I think I should be able to fix that
in this patch.

Thanks.

-- 
Frederic Weisbecker
SUSE Labs