Commit 58d4e21e authored Jul 18, 2014 by Tony Luck Committed by Steven Rostedt Jul 21, 2014

tracing: Fix wraparound problems in "uptime" trace clock

The "uptime" trace clock added in:

    commit 8aacf017
    tracing: Add "uptime" trace clock that uses jiffies

has wraparound problems when the system has been up more
than 1 hour 11 minutes and 34 seconds. It converts jiffies
to nanoseconds using:
        (u64)jiffies_to_usecs(jiffy) * 1000ULL
but since jiffies_to_usecs() only returns a 32-bit value, it
truncates at 2^32 microseconds.  An additional problem on 32-bit
systems is that the argument is "unsigned long", so fixing the
return value only helps until 2^32 jiffies (49.7 days on a HZ=1000
system).

Avoid these problems by using jiffies_64 as our basis, and
not converting to nanoseconds (we do convert to clock_t because
user facing API must not be dependent on internal kernel
HZ values).

Link: http://lkml.kernel.org/p/99d63c5bfe9b320a3b428d773825a37095bf6a51.1405708254.git.tony.luck@intel.com



Cc: stable@vger.kernel.org # 3.10+
Fixes: 8aacf017 "tracing: Add "uptime" trace clock that uses jiffies"
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

parent 9a3c4145

kernel/trace/trace.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -823,7 +823,7 @@ static struct {
		{ trace_clock_local, "local", 1 },
		{ trace_clock_global, "global", 1 },
		{ trace_clock_counter, "counter", 0 },
		{ trace_clock_jiffies, "uptime", 1 },
		{ trace_clock_jiffies, "uptime", 0 },
		{ trace_clock, "perf", 1 },
		ARCH_TRACE_CLOCKS
		};

kernel/trace/trace_clock.c

+5 −4

Original line number	Diff line number	Diff line
		@@ -59,13 +59,14 @@ u64 notrace trace_clock(void)

		/*
		* trace_jiffy_clock(): Simply use jiffies as a clock counter.
		* Note that this use of jiffies_64 is not completely safe on
		* 32-bit systems. But the window is tiny, and the effect if
		* we are affected is that we will have an obviously bogus
		* timestamp on a trace event - i.e. not life threatening.
		*/
		u64 notrace trace_clock_jiffies(void)
		{
		u64 jiffy = jiffies - INITIAL_JIFFIES;

		/* Return nsecs */
		return (u64)jiffies_to_usecs(jiffy) * 1000ULL;
		return jiffies_64_to_clock_t(jiffies_64 - INITIAL_JIFFIES);
		}

		/*