[PATCH v2 clocksource 5/5] clocksource: Do pairwise clock-desynchronization checking

paulmck
2021-01-12T00:46:54Z

From: "Paul E. McKenney" <paulmck@kernel.org>

Although smp_call_function() has the advantage of simplicity, using
it to check for cross-CPU clock desynchronization means that any CPU
being slow reduces the sensitivity of the checking across all CPUs.
And it is not uncommon for smp_call_function() latencies to be in the
hundreds of microseconds.

This commit therefore switches to smp_call_function_single(), so that
delays from a given CPU affect only those measurements involving that
particular CPU.

Cc: John Stultz <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mark Rutland <Mark.Rutland@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Reported-by: Chris Mason <clm@fb.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/time/clocksource.c | 41 +++++++++++++++++++++++++----------------
1 file changed, 25 insertions(+), 16 deletions(-)

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 67cf41c..3bae5fb 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -214,7 +214,7 @@ static void clocksource_watchdog_inject_delay(void)
}

static struct clocksource *clocksource_verify_work_cs;
-static DEFINE_PER_CPU(u64, csnow_mid);
+static u64 csnow_mid;
static cpumask_t cpus_ahead;
static cpumask_t cpus_behind;

@@ -228,7 +228,7 @@ static void clocksource_verify_one_cpu(void *csin)
sign = ((smp_processor_id() >> inject_delay_shift_percpu) & 0x1) * 2 - 1;
delta = sign * NSEC_PER_SEC;
}
- __this_cpu_write(csnow_mid, cs->read(cs) + delta);
+ csnow_mid = cs->read(cs) + delta;
}

static void clocksource_verify_percpu_wq(struct work_struct *unused)
@@ -236,9 +236,12 @@ static void clocksource_verify_percpu_wq(struct work_struct *unused)
int cpu;
struct clocksource *cs;
int64_t cs_nsec;
+ int64_t cs_nsec_max;
+ int64_t cs_nsec_min;
u64 csnow_begin;
u64 csnow_end;
- u64 delta;
+ s64 delta;
+ bool firsttime = 1;

cs = smp_load_acquire(&clocksource_verify_work_cs); // pairs with release
if (WARN_ON_ONCE(!cs))
@@ -247,19 +250,28 @@ static void clocksource_verify_percpu_wq(struct work_struct *unused)
cs->name, smp_processor_id());
cpumask_clear(&cpus_ahead);
cpumask_clear(&cpus_behind);
- csnow_begin = cs->read(cs);
- smp_call_function(clocksource_verify_one_cpu, cs, 1);
- csnow_end = cs->read(cs);
+ preempt_disable();
for_each_online_cpu(cpu) {
if (cpu == smp_processor_id())
continue;
- delta = (per_cpu(csnow_mid, cpu) - csnow_begin) & cs->mask;
- if ((s64)delta < 0)
+ csnow_begin = cs->read(cs);
+ smp_call_function_single(cpu, clocksource_verify_one_cpu, cs, 1);
+ csnow_end = cs->read(cs);
+ delta = (s64)((csnow_mid - csnow_begin) & cs->mask);
+ if (delta < 0)
cpumask_set_cpu(cpu, &cpus_behind);
- delta = (csnow_end - per_cpu(csnow_mid, cpu)) & cs->mask;
- if ((s64)delta < 0)
+ delta = (csnow_end - csnow_mid) & cs->mask;
+ if (delta < 0)
cpumask_set_cpu(cpu, &cpus_ahead);
+ delta = clocksource_delta(csnow_end, csnow_begin, cs->mask);
+ cs_nsec = clocksource_cyc2ns(delta, cs->mult, cs->shift);
+ if (firsttime || cs_nsec > cs_nsec_max)
+ cs_nsec_max = cs_nsec;
+ if (firsttime || cs_nsec < cs_nsec_min)
+ cs_nsec_min = cs_nsec;
+ firsttime = 0;
}
+ preempt_enable();
if (!cpumask_empty(&cpus_ahead))
pr_warn(" CPUs %*pbl ahead of CPU %d for clocksource %s.\n",
cpumask_pr_args(&cpus_ahead),
@@ -268,12 +280,9 @@ static void clocksource_verify_percpu_wq(struct work_struct *unused)
pr_warn(" CPUs %*pbl behind CPU %d for clocksource %s.\n",
cpumask_pr_args(&cpus_behind),
smp_processor_id(), cs->name);
- if (!cpumask_empty(&cpus_ahead) || !cpumask_empty(&cpus_behind)) {
- delta = clocksource_delta(csnow_end, csnow_begin, cs->mask);
- cs_nsec = clocksource_cyc2ns(delta, cs->mult, cs->shift);
- pr_warn(" CPU %d duration %lldns for clocksource %s.\n",
- smp_processor_id(), cs_nsec, cs->name);
- }
+ if (!firsttime && (!cpumask_empty(&cpus_ahead) || !cpumask_empty(&cpus_behind)))
+ pr_warn(" CPU %d check durations %lldns - %lldns for clocksource %s.\n",
+ smp_processor_id(), cs_nsec_min, cs_nsec_max, cs->name);
smp_store_release(&clocksource_verify_work_cs, NULL); // pairs with acquire.
}

--
2.9.5