Messages in this thread | | | Date | Thu, 10 Jan 2013 13:26:22 +0100 | From | Stanislaw Gruszka <> | Subject | Re: [PATCH v2 repost] sched: cputime: avoid multiplication overflow (in common cases) |
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On Wed, Jan 09, 2013 at 07:33:03PM +0100, Frederic Weisbecker wrote: > On Mon, Jan 07, 2013 at 12:31:45PM +0100, Stanislaw Gruszka wrote: > > We scale stime, utime values based on rtime (sum_exec_runtime converted > > to jiffies). During scaling we multiple rtime * utime, what seems to be > > fine, since both values are converted to u64, but is not. > > > > Let assume HZ is 1000 - 1ms tick. Process consist of 64 threads, run > > for 1 day, threads utilize 100% cpu on user space. Machine has 64 cpus. > > > > Process rtime = utime will be 64 * 24 * 60 * 60 * 1000 jiffies, what is > > 0x149970000. Multiplication rtime * utime result is 0x1a855771100000000, > > which can not be covered in 64 bits. > > > > Result of overflow is stall of utime values visible in user space > > (prev_utime in kernel), even if application still consume lot of CPU > > time. > > > > Probably good fix for the problem, will be using 128 bit variable and > > proper mul128 and div_u128_u64 primitives. While mul128 is on it's > > way to kernel, there is no 128 bit division yet. I'm not sure, if we > > want to add it to kernel. Perhaps we could also change the way how > > stime and utime are calculated, but I don't know how, so I come with > > the below solution for the problem. > > > > To avoid overflow patch change value we scale to min(stime, utime). This > > is more like workaround, but will work for processes, which perform > > mostly on user space or mostly on kernel space. Unfortunately processes, > > which perform on kernel and user space equally, and additionally utilize > > lot of CPU time, still will hit this overflow pretty quickly. However > > such processes seems to be uncommon. > > > > Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com> > > I can easily imagine that overflow to happen with user time on intensive > CPU bound loads, or may be guests. > > But can we easily reach the same for system time? Even on intensive I/O bound > loads we shouldn't spend that much time in the kernel. Most of it probably goes > to idle. > > What do you think?
I think you are right :-)
> If that assumption is right in most cases, the following patch should solve the > issue:
I'm fine with this patch, it achives the same effect as my patch, but is simpler.
Thanks Stanislaw
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c > index 293b202..0650dd4 100644 > --- a/kernel/sched/cputime.c > +++ b/kernel/sched/cputime.c > @@ -509,11 +509,11 @@ EXPORT_SYMBOL_GPL(vtime_account); > # define nsecs_to_cputime(__nsecs) nsecs_to_jiffies(__nsecs) > #endif > > -static cputime_t scale_utime(cputime_t utime, cputime_t rtime, cputime_t total) > +static cputime_t scale_utime(cputime_t stime, cputime_t rtime, cputime_t total) > { > u64 temp = (__force u64) rtime; > > - temp *= (__force u64) utime; > + temp *= (__force u64) stime; > > if (sizeof(cputime_t) == 4) > temp = div_u64(temp, (__force u32) total); > @@ -531,10 +531,10 @@ static void cputime_adjust(struct task_cputime *curr, > struct cputime *prev, > cputime_t *ut, cputime_t *st) > { > - cputime_t rtime, utime, total; > + cputime_t rtime, stime, total; > > - utime = curr->utime; > - total = utime + curr->stime; > + stime = curr->stime; > + total = stime + curr->utime; > > /* > * Tick based cputime accounting depend on random scheduling > @@ -549,17 +549,17 @@ static void cputime_adjust(struct task_cputime *curr, > rtime = nsecs_to_cputime(curr->sum_exec_runtime); > > if (total) > - utime = scale_utime(utime, rtime, total); > + stime = scale_stime(stime, rtime, total); > else > - utime = rtime; > + stime = rtime; > > /* > * If the tick based count grows faster than the scheduler one, > * the result of the scaling may go backward. > * Let's enforce monotonicity. > */ > - prev->utime = max(prev->utime, utime); > - prev->stime = max(prev->stime, rtime - prev->utime); > + prev->stime = max(prev->stime, stime); > + prev->utime = max(prev->utime, rtime - prev->stime); > > *ut = prev->utime; > *st = prev->stime;
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