Re: [PATCH] sched/fair: fix mul overflow on 32-bit systems

[Peter Zijlstra]

On Mon, Dec 14, 2015 at 06:42:24AM +0800, Yuyang Du wrote:
> > In most cases 'r' shouldn't exceed 1024 and util_sum not significantly
> > exceed 1024*47742, but in extreme cases like spawning lots of new tasks
> > it may potentially overflow 32 bit. Newly created tasks contribute
> > 1024*47742 each to the rq util_sum, which means that more than ~87 new
> > tasks on a single rq will get us in trouble I think.

> Both can workaround the issue with additional overhead. But I suspectthey
> will end up going in the wrong direction for util_avg. The question is a
> big util_sum (much bigger than 1024) may not be in a right range for it
> to be used in load balancing.

Right, it being >100% doesn't make any sense. We should look at ensuring
it saturates at 100%, or at least have it be bounded much tighter to
that, as currently its entirely unbounded, which is quite horrible.

> The problem is that it is not so good to initiate a new task's util_avg
> to 1024. At least, it makes much less sense than a new task's load_avg
> being initiated to its full weight. Because the top util_avg should be
> well bounded by 1024 - the CPU's full utilization.
> 
> So, maybe give the initial util_sum to an average of its cfs_rq, like:

>         cfs_rq->avg.util_sum / cfs_rq->load.weight * task->load.weight
> 
> And make sure that initial value's is bounded on various conditions.

That more or less results in an harmonic series, which is still very
much unbounded.

However, I think that makes sense, but would propose doing it
differently. That condition is generally a maximum (assuming proper
functioning of the weight based scheduling etc..) for any one task, so
on migrate we can hard clip to this value.

That still doesn't get rid of the harmonic series though, so we need
more. Now we can obviously also hard clip the sum on add, which I
suspect we'll need to do.

That laves us with a problem on remove though, at which point we can
clip to this max if needed, but that will add a fair amount of cost to
remove :/

Alternatively, and I still have to go look through the code, we should
clip when we've already calculated the weight based ratio anyway,
avoiding the cost of that extra division.

In any case, ideas, we'll have to play with I suppose.
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