Re: [PATCH] Fix smpnice high priority task hopping problem

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Peter Williams wrote:
Siddha, Suresh B wrote:

Andrew, Please don't apply this patch. This breaks the existing HT
(and multi-core) scheduler optimizations.

Peter, on a DP system with HT, if we have only two runnable processes
and they end up running on the two threads of the same package, with your patch, migration thread will never move one of those processes to the idle package..


On a normal system, would either of them be moved anyway?


To fix my reported problem, we need to make sure that find_busiest_group()
doesn't find an imbalance..


I disagree. If this causes a problem with your "optimizations" then I think that you need to fix the "optimizations".

There's a rational argument (IMHO) that this patch should be applied even in the absence of the smpnice patches as it prevents active_load_balance() doing unnecessary work. If this isn't good for hypo threading then hypo threading is a special case and needs to handle it as such.

OK. The good news is that (my testing shows that) the "sched: fix smpnice abnormal nice anomalies" fixes the imbalance problem and the consequent CPU hopping.

BUT I still think that this patch (modified if necessary to handle any HT special cases) should be applied. On a normal system, it will (as I've already said) stop active_load_balance() from doing a lot of unnecessary work INCLUDING holding the run queue locks for TWO run queues for no good reason.

Peter
--
Peter Williams                                   [email protected]

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