linux-kernel - Re: [PATCH v3] trace: Default to using trace_global_clock if sched

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Date:   Wed, 04 Apr 2018 16:56:23 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Chris Wilson <chris@...is-wilson.co.uk>,
        linux-kernel@...r.kernel.org
CC:     intel-gfx@...ts.freedesktop.org
Subject: Re: [PATCH v3] trace: Default to using trace_global_clock if sched_clock is unstable

Can you add a patch on top. I already added your patch to my tree.

See branch ftrace/core
git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git

-- Steve


On April 4, 2018 4:51:02 PM EDT, Chris Wilson <chris@...is-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
>
>Signed-off-by: Chris Wilson <chris@...is-wilson.co.uk>
>Cc: Steven Rostedt (VMware) <rostedt@...dmis.org>
>---
>v2: Tell the user what's happening and what they can do to correct it.
>v3: Restore the correct logic to switch only if the default trace clock
>(no override) is found to be *not* stable.
>---
> kernel/trace/trace.c | 19 +++++++++++++++++++
> 1 file changed, 19 insertions(+)
>
>diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
>index 13baf85b27d8..a1810b053f65 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 +8506,21 @@ __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()) {
>+		printk(KERN_WARNING
>+		       "Unstable clock detected, switching default tracing clock to
>\"global\"\n"
>+		       "If you want to keep using the local clock, then add:\n"
>+		       "  \"trace_clock=local\"\n"
>+		       "on the kernel command line\n");
>+		tracing_set_clock(&global_trace, "global");
>+	}
>+
>+	return 0;
>+}
>+late_initcall_sync(tracing_set_default_clock);
>+#endif

-- 
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