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SubjectRe: [PATCH] trace: Default to using trace_global_clock if sched_clock is unstable
On Thu, 29 Mar 2018 23:25:57 +0100
Chris Wilson <chris@chris-wilson.co.uk> wrote:

> Across suspend, we may see a very large drift in timestamps if the sched
> clock is unstable, prompting the global trace's ringbuffer code to warn
> and suggest switching to the global clock. Preempt this request by
> detecting when the sched clock is unstable (determined during
> late_initcall) and automatically switching the default clock over to
> trace_global_clock.
>
> This should prevent requiring user interaction to resolve warnings such
> as:
>
> Delta way too big! 18446743856563626466 ts=18446744054496180323 write stamp = 197932553857
> If you just came from a suspend/resume,
> please switch to the trace global clock:
> echo global > /sys/kernel/debug/tracing/trace_clock

global clock has a much higher overhead than the local clock. I rather
not have it automatically switch even when there's no stable TSC. That
will be annoying to myself as I have boxes that this would switch on
and I prefer to keep the local clock.

One can also decide the clock with the kernel command line. Should we
update that message to also say:

Or set the global clock via the kernel command line with
"trace_clock=global"

?

-- Steve


>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
> ---
> kernel/trace/trace.c | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
>
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 13baf85b27d8..c5462513db90 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -41,6 +41,7 @@
> #include <linux/nmi.h>
> #include <linux/fs.h>
> #include <linux/trace.h>
> +#include <linux/sched/clock.h>
> #include <linux/sched/rt.h>
>
> #include "trace.h"
> @@ -8505,3 +8506,15 @@ __init static int clear_boot_tracer(void)
>
> fs_initcall(tracer_init_tracefs);
> late_initcall_sync(clear_boot_tracer);
> +
> +#ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
> +__init static int tracing_set_default_clock(void)
> +{
> + /* sched_clock_stable() is determined in late_initcall */
> + if (!trace_boot_clock && !sched_clock_stable())
> + tracing_set_clock(&global_trace, "global");
> +
> + return 0;
> +}
> +late_initcall_sync(tracing_set_default_clock);
> +#endif

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