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SubjectRe: [PATCH] Avoid moving tasks when a schedule can be made.
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On Thu, 2006-02-02 at 12:26 +1100, Peter Williams wrote:

> >
> > Actually, one of the tasks that was moved might need to resched right
> > away, since it preempts the current task that is doing the moving.
>
> Good point but I'd say that this was an instance when you didn't want to
> bail out of the load balance. E.g. during idle balancing the very first
> task moved would trigger it regardless of its priority. Also, if the
> task was of sufficiently high priority for it to warrant bailing out of
> the load balance why wasn't the current task (i.e. why didn't it preempt
> on its current CPU).

Because the task running on the current CPU is higher in priority. That
doesn't mean that the next one down shouldn't get scheduled on another
CPU if it is a higher priority than the currently running one. Of
course one needs to be careful not to cause too much cache blasting by
popping RT tasks all over CPUS.

>
> >
> >
> >>However, a newly woken task that preempts the current task isn't the
> >>only way that needs_resched() can become true just before load balancing
> >>is started. E.g. scheduler_tick() calls set_tsk_need_resched(p) when a
> >>task finishes a time slice and this patch would cause rebalance_tick()
> >>to be aborted after a lot of work has been done in this case.
> >
> >
> > No real work is lost. This is a loop that individually pulls tasks. So
> > the bail only stops the work of looking for more tasks to pull and we
> > don't lose the tasks that have already been pulled.
>
> I disagree. A bunch of work is done to determine which CPU to pull from
> and how many tasks to pull and then it will bail out before any of them
> are moved (and for no good reason).

Yeah, that was my mistake. There is work lost. So nuke that argument of
mine :)

>
> >
> >
> >>In summary, I think that the bail out is badly placed and needs some way
> >>of knowing if the reason need_resched() has become true is because of
> >>preemption of a newly woken task and not some other reason.
> >
> >
> > I need that bail in the loop, so it can stop if needed. Like I said, it
> > can be a task that is pulled to cause the bail. Also, having the run
> > queue locks and interrupts off for over a msec is really a bad idea.
>
> Clearly, the way to handle this is to impose some limit on the number of
> tasks to be moved or split large moves into a number of smaller moves
> (releasing and reacquiring locks in between). This could be done in the
> bits of code that set things up before move_tasks() is called.

I think that's something like what Ingo wants to do. Or something other
than my first brain dead patch.

>
> >
> >
> >>Peter
> >>PS I've added Nick Piggin to the CC list as he is interested in load
> >>balancing issues.
> >
> >
> > Thanks, and thanks for the comments too. I'm up for all suggestions and
> > ideas. I just feel it is important that we don't have unbounded
> > latencies of spin locks and interrupts off.
>
> Well, you seem to have succeeded in starting a discussion :-)

:)

-- Steve


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