RE: bug in sched.c:task_hot()

[Chen, Kenneth W]

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.
>

Peter Williams wrote on Tuesday, October 05, 2004 12:33 AM
> 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.

The reason now gets smaller than timestamp was because of random thing
that activate_task() do to p->timestamp.  Here are the sequence of events:

On timer tick, scheduler_tick() updates rq->timestamp_last_tick,
1 us later, process A wakes up, entering into run queue. activate_task()
updates p->timestamp.  At this time p->timestamp is larger than rq->
timestamp_last_tick.  Then another cpu goes into idle, tries load balancing,
It wrongly finds process A not cache hot (because of integer underflow),
moved it.  Then application runs on a cache cold CPU and suffer performance.

- Ken


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