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SubjectRe: [PATCH] Avoid moving tasks when a schedule can be made.
Steven Rostedt wrote:
> 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.

Yes, but I don't think that it warrants interrupting the load balancing.

> 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
>


--
Peter Williams pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
-- Ambrose Bierce
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