jiffies: Fix timeval conversion to jiffies

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: stable@vger.kernel.org
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>
This commit is contained in:
Andrew Hunter 2014-09-04 14:17:16 -07:00 committed by John Stultz
parent 9bf2419fa7
commit d78c9300c5
2 changed files with 30 additions and 36 deletions

View file

@ -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,

View file

@ -559,17 +559,20 @@ 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;
@ -580,6 +583,13 @@ timespec_to_jiffies(const struct timespec *value)
(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
@ -596,31 +606,27 @@ jiffies_to_timespec(const unsigned long jiffies, struct timespec *value)
}
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.
*
* 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.
* 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.
*
* 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);