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

[Yuyang Du <yuyang.du@intel.com>]

On Fri, Dec 11, 2015 at 11:18:56AM -0800, bsegall@google.com wrote:
> First, I believe in theory util_avg on a cpu should add up to 100% or
> 1024 or whatever. However, recently migrated-in tasks don't have their
> utilization cleared, so if they were quickly migrated again you could
> have up to the number of cpus or so times 100%, which could lead to
> overflow here. This just leads to more questions though:
> 
> The whole removed_util_avg thing doesn't seem to make a ton of sense -
> the code doesn't add util_avg for a migrating task onto
> cfs_rq->avg.util_avg

The code does add util_avg for a migrating task onto cfs_rq->avg.util_avg:

enqueue_entity_load_avg() calls attach_entity_load_avg()

> and doing so would regularly give >100% values (it
> does so on attach/detach where it's less likely to cause issues, but not
> migration). Removing it only makes sense if the task has accumulated all
> that utilization on this cpu, and even then mostly only makes sense if
> this is the only task on the cpu (and then it would make sense to add it
> on migrate-enqueue). The whole add-on-enqueue-migrate,
> remove-on-dequeue-migrate thing comes from /load/, where doing so is a
> more globally applicable approximation than it is for utilization,
> though it could still be useful as a fast-start/fast-stop approximation,
> if the add-on-enqueue part was added. It could also I guess be cleared
> on migrate-in, as basically the opposite assumption (or do something
> like add on enqueue, up to 100% and then set the se utilization to the
> amount actually added or something).
> 
> If the choice was to not do the add/remove thing, then se->avg.util_sum
> would be unused except for attach/detach, which currently do the
> add/remove thing. It's not unreasonable for them, except that currently
> nothing uses anything other than the root's utilization, so migration
> between cgroups wouldn't actually change the relevant util number
> (except it could because changing the cfs_rq util_sum doesn't actually
> do /anything/ unless it's the root, so you'd have to wait until the
> cgroup se actually changed in utilization).
> 
> 
> So uh yeah, my initial impression is "rip it out", but if being
> immediately-correct is important in the case of one task being most of
> the utilization, rather than when it is more evenly distributed, it
> would probably make more sense to instead put in the add-on-enqueue
> code.