Messages in this thread | | | Date | Tue, 05 Oct 2004 17:33:22 +1000 | From | Peter Williams <> | Subject | Re: bug in sched.c:task_hot() |
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Chen, Kenneth W wrote: > Current implementation of task_hot() has a performance bug in it > that it will cause integer underflow. > > Variable "now" (typically passed in as rq->timestamp_last_tick) > and p->timestamp are all defined as unsigned long long. However, > If former is smaller than the latter, integer under flow occurs > which make the result of subtraction a huge positive number. Then > it is compared to sd->cache_hot_time and it will wrongly identify > a cache hot task as cache cold. > > This bug causes large amount of incorrect process migration across > cpus (at stunning 10,000 per second) and we lost cache affinity very > quickly and almost took double digit performance regression on a db > transaction processing workload. Patch to fix the bug. Diff'ed against > 2.6.9-rc3. > > Signed-off-by: Ken Chen <kenneth.w.chen@intel.com> > > > --- linux-2.6.9-rc3/kernel/sched.c.orig 2004-10-04 19:11:21.000000000 -0700 > +++ linux-2.6.9-rc3/kernel/sched.c 2004-10-04 19:19:27.000000000 -0700 > @@ -180,7 +180,8 @@ static unsigned int task_timeslice(task_ > else > return SCALE_PRIO(DEF_TIMESLICE, p->static_prio); > } > -#define task_hot(p, now, sd) ((now) - (p)->timestamp < (sd)->cache_hot_time) > +#define task_hot(p, now, sd) ((long long) ((now) - (p)->timestamp) \ > + < (long long) (sd)->cache_hot_time) > > enum idle_type > {
The interesting question is: How does now get to be less than timestamp? This probably means that timestamp_last_tick is not a good way of getting a value for "now". By the way, neither is sched_clock() when measuring small time differences as it is not monotonic (something that I had to allow for in my scheduling code). I applied no such safeguards to the timing used by the load balancing code as I assumed that it already worked.
Peter -- Peter Williams pwil3058@bigpond.net.au
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