--- /dev/null
+From d78c9300c51d6ceed9f6d078d4e9366f259de28c Mon Sep 17 00:00:00 2001
+From: Andrew Hunter <ahh@google.com>
+Date: Thu, 4 Sep 2014 14:17:16 -0700
+Subject: jiffies: Fix timeval conversion to jiffies
+
+From: Andrew Hunter <ahh@google.com>
+
+commit d78c9300c51d6ceed9f6d078d4e9366f259de28c upstream.
+
+timeval_to_jiffies tried to round a timeval up to an integral number
+of jiffies, but the logic for doing so was incorrect: intervals
+corresponding to exactly N jiffies would become N+1. This manifested
+itself particularly repeatedly stopping/starting an itimer:
+
+setitimer(ITIMER_PROF, &val, NULL);
+setitimer(ITIMER_PROF, NULL, &val);
+
+would add a full tick to val, _even if it was exactly representable in
+terms of jiffies_ (say, the result of a previous rounding.) Doing
+this repeatedly would cause unbounded growth in val. So fix the math.
+
+Here's what was wrong with the conversion: we essentially computed
+(eliding seconds)
+
+jiffies = usec * (NSEC_PER_USEC/TICK_NSEC)
+
+by using scaling arithmetic, which took the best approximation of
+NSEC_PER_USEC/TICK_NSEC with denominator of 2^USEC_JIFFIE_SC =
+x/(2^USEC_JIFFIE_SC), and computed:
+
+jiffies = (usec * x) >> USEC_JIFFIE_SC
+
+and rounded this calculation up in the intermediate form (since we
+can't necessarily exactly represent TICK_NSEC in usec.) But the
+scaling arithmetic is a (very slight) *over*approximation of the true
+value; that is, instead of dividing by (1 usec/ 1 jiffie), we
+effectively divided by (1 usec/1 jiffie)-epsilon (rounding
+down). This would normally be fine, but we want to round timeouts up,
+and we did so by adding 2^USEC_JIFFIE_SC - 1 before the shift; this
+would be fine if our division was exact, but dividing this by the
+slightly smaller factor was equivalent to adding just _over_ 1 to the
+final result (instead of just _under_ 1, as desired.)
+
+In particular, with HZ=1000, we consistently computed that 10000 usec
+was 11 jiffies; the same was true for any exact multiple of
+TICK_NSEC.
+
+We could possibly still round in the intermediate form, adding
+something less than 2^USEC_JIFFIE_SC - 1, but easier still is to
+convert usec->nsec, round in nanoseconds, and then convert using
+time*spec*_to_jiffies. This adds one constant multiplication, and is
+not observably slower in microbenchmarks on recent x86 hardware.
+
+Tested: the following program:
+
+int main() {
+ struct itimerval zero = {{0, 0}, {0, 0}};
+ /* Initially set to 10 ms. */
+ struct itimerval initial = zero;
+ initial.it_interval.tv_usec = 10000;
+ setitimer(ITIMER_PROF, &initial, NULL);
+ /* Save and restore several times. */
+ for (size_t i = 0; i < 10; ++i) {
+ struct itimerval prev;
+ setitimer(ITIMER_PROF, &zero, &prev);
+ /* on old kernels, this goes up by TICK_USEC every iteration */
+ printf("previous value: %ld %ld %ld %ld\n",
+ prev.it_interval.tv_sec, prev.it_interval.tv_usec,
+ prev.it_value.tv_sec, prev.it_value.tv_usec);
+ setitimer(ITIMER_PROF, &prev, NULL);
+ }
+ return 0;
+}
+
+
+Cc: Thomas Gleixner <tglx@linutronix.de>
+Cc: Ingo Molnar <mingo@redhat.com>
+Cc: Paul Turner <pjt@google.com>
+Cc: Richard Cochran <richardcochran@gmail.com>
+Cc: Prarit Bhargava <prarit@redhat.com>
+Reviewed-by: Paul Turner <pjt@google.com>
+Reported-by: Aaron Jacobs <jacobsa@google.com>
+Signed-off-by: Andrew Hunter <ahh@google.com>
+[jstultz: Tweaked to apply to 3.17-rc]
+Signed-off-by: John Stultz <john.stultz@linaro.org>
+[bwh: Backported to 3.16: adjust filename]
+Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
+Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
+
+---
+ include/linux/jiffies.h | 12 ----------
+ kernel/time.c | 54 ++++++++++++++++++++++++++----------------------
+ 2 files changed, 30 insertions(+), 36 deletions(-)
+
+--- a/include/linux/jiffies.h
++++ b/include/linux/jiffies.h
+@@ -258,23 +258,11 @@ extern unsigned long preset_lpj;
+ #define SEC_JIFFIE_SC (32 - SHIFT_HZ)
+ #endif
+ #define NSEC_JIFFIE_SC (SEC_JIFFIE_SC + 29)
+-#define USEC_JIFFIE_SC (SEC_JIFFIE_SC + 19)
+ #define SEC_CONVERSION ((unsigned long)((((u64)NSEC_PER_SEC << SEC_JIFFIE_SC) +\
+ TICK_NSEC -1) / (u64)TICK_NSEC))
+
+ #define NSEC_CONVERSION ((unsigned long)((((u64)1 << NSEC_JIFFIE_SC) +\
+ TICK_NSEC -1) / (u64)TICK_NSEC))
+-#define USEC_CONVERSION \
+- ((unsigned long)((((u64)NSEC_PER_USEC << USEC_JIFFIE_SC) +\
+- TICK_NSEC -1) / (u64)TICK_NSEC))
+-/*
+- * USEC_ROUND is used in the timeval to jiffie conversion. See there
+- * for more details. It is the scaled resolution rounding value. Note
+- * that it is a 64-bit value. Since, when it is applied, we are already
+- * in jiffies (albit scaled), it is nothing but the bits we will shift
+- * off.
+- */
+-#define USEC_ROUND (u64)(((u64)1 << USEC_JIFFIE_SC) - 1)
+ /*
+ * The maximum jiffie value is (MAX_INT >> 1). Here we translate that
+ * into seconds. The 64-bit case will overflow if we are not careful,
+--- a/kernel/time.c
++++ b/kernel/time.c
+@@ -496,17 +496,20 @@ EXPORT_SYMBOL(usecs_to_jiffies);
+ * that a remainder subtract here would not do the right thing as the
+ * resolution values don't fall on second boundries. I.e. the line:
+ * nsec -= nsec % TICK_NSEC; is NOT a correct resolution rounding.
++ * Note that due to the small error in the multiplier here, this
++ * rounding is incorrect for sufficiently large values of tv_nsec, but
++ * well formed timespecs should have tv_nsec < NSEC_PER_SEC, so we're
++ * OK.
+ *
+ * Rather, we just shift the bits off the right.
+ *
+ * The >> (NSEC_JIFFIE_SC - SEC_JIFFIE_SC) converts the scaled nsec
+ * value to a scaled second value.
+ */
+-unsigned long
+-timespec_to_jiffies(const struct timespec *value)
++static unsigned long
++__timespec_to_jiffies(unsigned long sec, long nsec)
+ {
+- unsigned long sec = value->tv_sec;
+- long nsec = value->tv_nsec + TICK_NSEC - 1;
++ nsec = nsec + TICK_NSEC - 1;
+
+ if (sec >= MAX_SEC_IN_JIFFIES){
+ sec = MAX_SEC_IN_JIFFIES;
+@@ -517,6 +520,13 @@ timespec_to_jiffies(const struct timespe
+ (NSEC_JIFFIE_SC - SEC_JIFFIE_SC))) >> SEC_JIFFIE_SC;
+
+ }
++
++unsigned long
++timespec_to_jiffies(const struct timespec *value)
++{
++ return __timespec_to_jiffies(value->tv_sec, value->tv_nsec);
++}
++
+ EXPORT_SYMBOL(timespec_to_jiffies);
+
+ void
+@@ -533,31 +543,27 @@ jiffies_to_timespec(const unsigned long
+ }
+ EXPORT_SYMBOL(jiffies_to_timespec);
+
+-/* Same for "timeval"
++/*
++ * We could use a similar algorithm to timespec_to_jiffies (with a
++ * different multiplier for usec instead of nsec). But this has a
++ * problem with rounding: we can't exactly add TICK_NSEC - 1 to the
++ * usec value, since it's not necessarily integral.
++ *
++ * We could instead round in the intermediate scaled representation
++ * (i.e. in units of 1/2^(large scale) jiffies) but that's also
++ * perilous: the scaling introduces a small positive error, which
++ * combined with a division-rounding-upward (i.e. adding 2^(scale) - 1
++ * units to the intermediate before shifting) leads to accidental
++ * overflow and overestimates.
+ *
+- * Well, almost. The problem here is that the real system resolution is
+- * in nanoseconds and the value being converted is in micro seconds.
+- * Also for some machines (those that use HZ = 1024, in-particular),
+- * there is a LARGE error in the tick size in microseconds.
+-
+- * The solution we use is to do the rounding AFTER we convert the
+- * microsecond part. Thus the USEC_ROUND, the bits to be shifted off.
+- * Instruction wise, this should cost only an additional add with carry
+- * instruction above the way it was done above.
++ * At the cost of one additional multiplication by a constant, just
++ * use the timespec implementation.
+ */
+ unsigned long
+ timeval_to_jiffies(const struct timeval *value)
+ {
+- unsigned long sec = value->tv_sec;
+- long usec = value->tv_usec;
+-
+- if (sec >= MAX_SEC_IN_JIFFIES){
+- sec = MAX_SEC_IN_JIFFIES;
+- usec = 0;
+- }
+- return (((u64)sec * SEC_CONVERSION) +
+- (((u64)usec * USEC_CONVERSION + USEC_ROUND) >>
+- (USEC_JIFFIE_SC - SEC_JIFFIE_SC))) >> SEC_JIFFIE_SC;
++ return __timespec_to_jiffies(value->tv_sec,
++ value->tv_usec * NSEC_PER_USEC);
+ }
+ EXPORT_SYMBOL(timeval_to_jiffies);
+
projects / thirdparty / kernel / stable-queue.git / commitdiff
