Messages in this thread | | | Date | Tue, 8 Apr 2014 14:15:43 -0400 | From | Steven Rostedt <> | Subject | Re: [RFC PATCH] time: Fix truncation in jiffies_to_usecs() |
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On Tue, 8 Apr 2014 19:49:51 +0200 Frederic Weisbecker <fweisbec@gmail.com> wrote:
> On Mon, Apr 07, 2014 at 10:34:51PM -0700, Tony Luck wrote: > > On Mon, Apr 7, 2014 at 3:25 PM, Tony Luck <tony.luck@intel.com> wrote: > > > > > c) If not this ... then what? Separate routine to convert large numbers > > > of jiffies to usec/nsecs? Should we make the existing one barf when > > > handed a number that overflows? > > > > Having thought about this a bit more - I'm leaning towards leaving > > jiffies_to_usecs() alone, but using it as a model for a from-scratch > > implementation of: > > u64 jiffies_to_nsecs(const unsigned long j) > > { > > } > > > > This is what the uptime tracer actually needs - and there is only > > one user of jiffies_to_nsecs() to worry about. > > I'm not sure I get what you're trying to do. We already have jiffies_to_nsecs(). > Anyway I'll just wait and check out the next patch :)
I believe the issue is the way it's implemented:
static inline u64 jiffies_to_nsecs(const unsigned long j) { return (u64)jiffies_to_usecs(j) * NSEC_PER_USEC; }
The problem is with jiffies_to_usecs(). Which we probably should change.
With HZ = 100, 1 second jiffies_to_usecs(100) = 1000,000. 1 minute jiffies_to_usec(6000) = 60,000,000. 1 hour jiffies_to_usecs(360000) = 3,600,000,000 1 hour 11 minutes 35 seconds - jiffies_to_usecs(429500) = 4,295,000,000
2^32 = 4294967296 < 4,295,000,000
Overflow!
That means after 1 hour, 11 minutes and 35 seconds, jiffies_to_usecs() will return a reset number. Time will go backwards. It doesn't matter what you typecast the return value of jiffies_to_usecs() to, the result is wrong.
Actually, I like Tony's first patch. I really think jiffies_to_usecs() should return a u64 number.
-- Steve
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