Re: [PATCH v4 2/5] sched/fair: Fix attaching task sched avgs twice when switching to fair or changing task group

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

Hi Vincent,

On Mon, Jun 06, 2016 at 02:32:39PM +0200, Vincent Guittot wrote:
> > Unlike the switch to fair class case, if the task is on_rq, it will be
> > enqueued after we move task groups, so the simplest solution is to reset
> > the task's last_update_time when we do task_move_group(), but not to
> > attach sched avgs in task_move_group(), and then let enqueue_task() do
> > the sched avgs attachment.
> 
> According to the review of the previous version
> http://www.gossamer-threads.com/lists/linux/kernel/2450678#2450678,
> only the use case switch to fair with a task that has never been
> queued as a CFS task before, can have the issue and this will not
> happen for other use cases described above.
> So can you limit the description in the commit message to just this
> use case unless you have discovered new use cases and in this case
> please add the description.

I assure you that I will, :)
 
> Then, the problem with this use case, comes that last_update_time == 0
> has a special meaning ( task has migrated ) and we initialize
> last_update_time with this value.

In general, the meaning of last_update_time == 0 is: not yet attached to
any cfs queue.

> A much more simple solution would be
> to prevent last_update_time to be initialized with this special value.
> We can initialize the last_update_time of a sched_entity to 1 as an
> example which is easier than these changes.

Then at least, we must differentiate CONFIG_FAIR_GROUP_SCHED and
!CONFIG_FAIR_GROUP_SCHED. And I am not sure whether this is a simpler
solution, as I haven't sorted out the whole thing. IMO, this solution
seems a little too hacky and I'd prefer not if we have alternative.