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Commit d375b4e0 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
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sched/clock: Fixup early initialization 



The code would assume sched_clock_stable() and switch to !stable
later, this switch brings a discontinuity in time.

The discontinuity on switching from stable to unstable was always
present, but previously we would set stable/unstable before
initializing TSC and usually stick to the one we start out with.

So the static_key bits brought an extra switch where there previously
wasn't one.

Things are further complicated by the fact that we cannot use
static_key as early as we usually call set_sched_clock_stable().

Fix things by tracking the stable state in a regular variable and only
set the static_key to the right state on sched_clock_init(), which is
ran right after late_time_init->tsc_init().

Before this we would not be using the TSC anyway.

Reported-and-Tested-by: default avatarSasha Levin <sasha.levin@oracle.com>
Reported-by: default avatar <dyoung@redhat.com>
Fixes: 35af99e6 ("sched/clock, x86: Use a static_key for sched_clock_stable")
Cc: jacob.jun.pan@linux.intel.com
Cc: Mike Galbraith <bitbucket@online.de>
Cc: hpa@zytor.com
Cc: paulmck@linux.vnet.ibm.com
Cc: John Stultz <john.stultz@linaro.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: lenb@kernel.org
Cc: rjw@rjwysocki.net
Cc: Eliezer Tamir <eliezer.tamir@linux.intel.com>
Cc: rui.zhang@intel.com
Signed-off-by: default avatarPeter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20140122115918.GG3694@twins.programming.kicks-ass.net


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 215393bc

kernel/sched/clock.c

+41 −12
Original line number Diff line number Diff line
@@ -77,35 +77,50 @@ __read_mostly int sched_clock_running; 


#ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK

static struct static_key __sched_clock_stable = STATIC_KEY_INIT;

static int __sched_clock_stable_early;



int sched_clock_stable(void)

{

	if (static_key_false(&__sched_clock_stable))

		return false;

	return true;

	return static_key_false(&__sched_clock_stable);

}



void set_sched_clock_stable(void)

static void __set_sched_clock_stable(void)

{

	if (!sched_clock_stable())

		static_key_slow_dec(&__sched_clock_stable);

		static_key_slow_inc(&__sched_clock_stable);

}



void set_sched_clock_stable(void)

{

	__sched_clock_stable_early = 1;



	smp_mb(); /* matches sched_clock_init() */



	if (!sched_clock_running)

		return;



	__set_sched_clock_stable();

}



static void __clear_sched_clock_stable(struct work_struct *work)

{

	/* XXX worry about clock continuity */

	if (sched_clock_stable())

		static_key_slow_inc(&__sched_clock_stable);

		static_key_slow_dec(&__sched_clock_stable);

}



static DECLARE_WORK(sched_clock_work, __clear_sched_clock_stable);



void clear_sched_clock_stable(void)

{

	if (keventd_up())

	__sched_clock_stable_early = 0;



	smp_mb(); /* matches sched_clock_init() */



	if (!sched_clock_running)

		return;



	schedule_work(&sched_clock_work);

	else

		__clear_sched_clock_stable(&sched_clock_work);

}



struct sched_clock_data {
@@ -140,6 +155,20 @@ void sched_clock_init(void) 
	}



	sched_clock_running = 1;



	/*

	 * Ensure that it is impossible to not do a static_key update.

	 *

	 * Either {set,clear}_sched_clock_stable() must see sched_clock_running

	 * and do the update, or we must see their __sched_clock_stable_early

	 * and do the update, or both.

	 */

	smp_mb(); /* matches {set,clear}_sched_clock_stable() */



	if (__sched_clock_stable_early)

		__set_sched_clock_stable();

	else

		__clear_sched_clock_stable(NULL);

}



/*
@@ -340,7 +369,7 @@ EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event); 
 */

u64 cpu_clock(int cpu)

{

	if (static_key_false(&__sched_clock_stable))

	if (!sched_clock_stable())

		return sched_clock_cpu(cpu);



	return sched_clock();
@@ -355,7 +384,7 @@ u64 cpu_clock(int cpu) 
 */

u64 local_clock(void)

{

	if (static_key_false(&__sched_clock_stable))

	if (!sched_clock_stable())

		return sched_clock_cpu(raw_smp_processor_id());



	return sched_clock();