linux-kernel - Re: [PATCH v6 1/4] sched/clock: interface to allow timestamps early in boot

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Date:   Wed, 27 Sep 2017 15:10:03 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>
Cc:     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
Subject: Re: [PATCH v6 1/4] sched/clock: interface to allow timestamps early
 in boot

On Wed, Sep 27, 2017 at 02:58:57PM +0200, Peter Zijlstra wrote:
> (we're violating "notsc" in any case and really should kill that
> option).

Something like so; in particular simple_udelay_calibrate() will issue
RDTSC _way_ early, so there is absolutely no point in then pretending we
can't use RDTSC for sched_clock.

---

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 796d96bb0821..1dd3849a42ca 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -37,13 +37,6 @@ EXPORT_SYMBOL(tsc_khz);
  */
 static int __read_mostly tsc_unstable;
 
-/* native_sched_clock() is called before tsc_init(), so
-   we must start with the TSC soft disabled to prevent
-   erroneous rdtsc usage on !boot_cpu_has(X86_FEATURE_TSC) processors */
-static int __read_mostly tsc_disabled = -1;
-
-static DEFINE_STATIC_KEY_FALSE(__use_tsc);
-
 int tsc_clocksource_reliable;
 
 static u32 art_to_tsc_numerator;
@@ -191,24 +184,7 @@ static void set_cyc2ns_scale(unsigned long khz, int cpu, unsigned long long tsc_
  */
 u64 native_sched_clock(void)
 {
-	if (static_branch_likely(&__use_tsc)) {
-		u64 tsc_now = rdtsc();
-
-		/* return the value in ns */
-		return cycles_2_ns(tsc_now);
-	}
-
-	/*
-	 * Fall back to jiffies if there's no TSC available:
-	 * ( But note that we still use it if the TSC is marked
-	 *   unstable. We do this because unlike Time Of Day,
-	 *   the scheduler clock tolerates small errors and it's
-	 *   very important for it to be as fast as the platform
-	 *   can achieve it. )
-	 */
-
-	/* No locking but a rare wrong value is not a big deal: */
-	return (jiffies_64 - INITIAL_JIFFIES) * (1000000000 / HZ);
+	return cycles_2_ns(rdtsc());
 }
 
 /*
@@ -244,27 +220,6 @@ int check_tsc_unstable(void)
 }
 EXPORT_SYMBOL_GPL(check_tsc_unstable);
 
-#ifdef CONFIG_X86_TSC
-int __init notsc_setup(char *str)
-{
-	pr_warn("Kernel compiled with CONFIG_X86_TSC, cannot disable TSC completely\n");
-	tsc_disabled = 1;
-	return 1;
-}
-#else
-/*
- * disable flag for tsc. Takes effect by clearing the TSC cpu flag
- * in cpu/common.c
- */
-int __init notsc_setup(char *str)
-{
-	setup_clear_cpu_cap(X86_FEATURE_TSC);
-	return 1;
-}
-#endif
-
-__setup("notsc", notsc_setup);
-
 static int no_sched_irq_time;
 
 static int __init tsc_setup(char *str)
@@ -1229,7 +1184,7 @@ static void tsc_refine_calibration_work(struct work_struct *work)
 
 static int __init init_tsc_clocksource(void)
 {
-	if (!boot_cpu_has(X86_FEATURE_TSC) || tsc_disabled > 0 || !tsc_khz)
+	if (!boot_cpu_has(X86_FEATURE_TSC) || !tsc_khz)
 		return 0;
 
 	if (tsc_clocksource_reliable)
@@ -1311,14 +1266,6 @@ void __init tsc_init(void)
 		set_cyc2ns_scale(tsc_khz, cpu, cyc);
 	}
 
-	if (tsc_disabled > 0)
-		return;
-
-	/* now allow native_sched_clock() to use rdtsc */
-
-	tsc_disabled = 0;
-	static_branch_enable(&__use_tsc);
-
 	if (!no_sched_irq_time)
 		enable_sched_clock_irqtime();
 
@@ -1348,7 +1295,7 @@ unsigned long calibrate_delay_is_known(void)
 	int sibling, cpu = smp_processor_id();
 	struct cpumask *mask = topology_core_cpumask(cpu);
 
-	if (!tsc_disabled && !cpu_has(&cpu_data(cpu), X86_FEATURE_CONSTANT_TSC))
+	if (!cpu_has(&cpu_data(cpu), X86_FEATURE_CONSTANT_TSC))
 		return 0;
 
 	if (!mask)