From: "Steven Rostedt (VMware)" rostedt@goodmis.org
The preemptirq_delay_test module is used for the ftrace selftest code that tests the latency tracers. The problem is that it uses ktime for the delay loop, and then checks the tracer to see if the delay loop is caught, but the tracer uses trace_clock_local() which uses various different other clocks to measure the latency. As ktime uses the clock cycles, and the code then converts that to nanoseconds, it causes rounding errors, and the preemptirq latency tests are failing due to being off by 1 (it expects to see a delay of 500000 us, but the delay is only 499999 us). This is happening due to a rounding error in the ktime (which is totally legit). The purpose of the test is to see if it can catch the delay, not to test the accuracy between trace_clock_local() and ktime_get(). Best to use apples to apples, and have the delay loop use the same clock as the latency tracer does.
Cc: stable@vger.kernel.org Fixes: f96e8577da102 ("lib: Add module for testing preemptoff/irqsoff latency tracers") Acked-by: Joel Fernandes (Google) joel@joelfernandes.org Signed-off-by: Steven Rostedt (VMware) rostedt@goodmis.org --- kernel/trace/preemptirq_delay_test.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/kernel/trace/preemptirq_delay_test.c b/kernel/trace/preemptirq_delay_test.c index f704390db9fc..d8765c952fab 100644 --- a/kernel/trace/preemptirq_delay_test.c +++ b/kernel/trace/preemptirq_delay_test.c @@ -5,12 +5,12 @@ * Copyright (C) 2018 Joel Fernandes (Google) joel@joelfernandes.org */
+#include <linux/trace_clock.h> #include <linux/delay.h> #include <linux/interrupt.h> #include <linux/irq.h> #include <linux/kernel.h> #include <linux/kthread.h> -#include <linux/ktime.h> #include <linux/module.h> #include <linux/printk.h> #include <linux/string.h> @@ -25,13 +25,13 @@ MODULE_PARM_DESC(test_mode, "Mode of the test such as preempt or irq (default ir
static void busy_wait(ulong time) { - ktime_t start, end; - start = ktime_get(); + u64 start, end; + start = trace_clock_local(); do { - end = ktime_get(); + end = trace_clock_local(); if (kthread_should_stop()) break; - } while (ktime_to_ns(ktime_sub(end, start)) < (time * 1000)); + } while ((end - start) < (time * 1000)); }
static int preemptirq_delay_run(void *data)