Messages in this thread | | | Date | Tue, 30 Sep 2008 12:33:26 +0530 | From | "Amit K. Arora" <> | Subject | Re: [PATCH] sched: minor optimizations in wake_affine and select_task_rq_fair |
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On Mon, Sep 29, 2008 at 10:09:41AM -0600, Chris Friesen wrote: > Amit K. Arora wrote: >> sched: Minor optimizations in wake_affine and select_task_rq_fair >> >> This patch does following: >> o Reduces the number of arguments to wake_affine(). > > At what point is it cheaper to pass items as args rather than recalculating > them? If reducing the number of args is desirable, what about removing the > "this_cpu" and "prev_cpu" args and recalculating them in wake_affine()?
Thats a good question. Its kind of arguable and I wasn't sure if everyone will be happy if I removed more arguments from wake_affine() than what I did in my patch (because of the recalculations required).
wake_affine() currently has 11 arguments and I thought it may make sense in reducing it to a sane number. For that I chose arguments which I thought can be recalculated with minimum overhead (involves single struct dereference, a simple per cpu variable and/or a simple arithmetic). And one argument ("rq") which is being removed, isn't used at all in the function!
Regarding the two variables you have mentioned, I didn't remove them as args since I wasn't sure of "this_cpu" (which is nothing but smp_processor_id()) as it is arch dependent, and calculating "prev_cpu" involves two struct dereferences (((struct thread_info *)(task)->stack)->cpu). And the calculation for other arguments (like, this_sd, load and this_load) involves good amount of instructions.
If you disagree, what do you suggest we do here ?
Regards, Amit Arora
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