Re: -mm seems significanty slower than mainline on kernbench

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Peter Williams wrote:
Martin Bligh wrote:



But I was thinking more about the code that (in the original) handled the case where the number of tasks to be moved was less than 1 but more than 0 (i.e. the cases where "imbalance" would have been reduced to zero when divided by SCHED_LOAD_SCALE). I think that I got that part wrong and you can end up with a bias load to be moved which is less than any of the bias_prio values for any queued tasks (in circumstances where the original code would have rounded up to 1 and caused a move). I think that the way to handle this problem is to replace 1 with "average bias prio" within that logic. This would guarantee at least one task with a bias_prio small enough to be moved.

I think that this analysis is a strong argument for my original patch being the cause of the problem so I'll go ahead and generate a fix. I'll try to have a patch available later this morning.




Attached is a patch that addresses this problem. Unlike the description above it does not use "average bias prio" as that solution would be very complicated. Instead it makes the assumption that NICE_TO_BIAS_PRIO(0) is a "good enough" for this purpose as this is highly likely to be the median bias prio and the median is probably better for this purpose than the average.

Signed-off-by: Peter Williams <[email protected]>



Doesn't fix the perf issue.


OK, thanks. I think there's a few more places where SCHED_LOAD_SCALE needs to be multiplied by NICE_TO_BIAS_PRIO(0). Basically, anywhere that it's added to, subtracted from or compared to a load. In those cases it's being used as a scaled version of 1 and we need a scaled

This would have been better said as "the load generated by 1 task" rather than just "a scaled version of 1". Numerically, they're the same but one is clearer than the other and makes it more obvious why we need NICE_TO_BIAS_PRIO(0) * SCHED_LOAD_SCALE and where we need it.

version of NICE_TO_BIAS_PRIO(0).  I'll have another patch later today.

I'm just testing this at the moment.

Peter
--
Peter Williams                                   [email protected]

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