Re: [PATCH] sched: fix inv_weight calc

[Peter Zijlstra]

On Thu, 2008-05-01 at 10:54 -0600, Gregory Haskins wrote:
> (Peter and I have been discussing this on IRC, but thought we should
> take some new findings to a wider audience)....
> 
> >>> On Wed, Apr 30, 2008 at  2:45 PM, in message <1209581148.6433.47.camel@lappy>,
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: 
> > On Wed, 2008-04-30 at 13:15 -0400, Gregory Haskins wrote:
> >> We currently have a bug in sched-devel where the system will fail to
> >> balance tasks if CONFIG_FAIR_GROUP_SCHED=n.  To reproduce, simply launch
> >> a workload with multiple tasks and observe (either via top or
> >> /proc/sched_debug) that the tasks do not distribute much (if at all)
> >> around to all available cores. Instead, they tend to clump on one processor
> >> while the other cores are idle.
> >> 
> >> Bisecting, we found the culprit to be:
> >> 
> >> 	commit 1b9552e878a5db3388eba8660e8d8400020a07e9
> >> 	Author: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> >> 	Date:   Tue Apr 29 13:47:36 2008 +0200
> >> 	Subject: sched: higher granularity load on 64bit systems
> >> 
> >> Once we identified this patch as the problem, I studied what possible
> >> effect it could have with FAIR_GROUP_SCHED=n vs y.  Most of the code in
> >> 1b9552e8 would be compiled out if we disable group-scheduling, but there
> >> is one particular logic change in calc_delta_mine() that affects both modes
> >> that looked suspicious.  It changes the computation of the inverse-weight
> >> from:
> >> 
> >>     inv_weight = (WMULT_CONST-weight/2)/(weight+1)
> >> 
> >> to
> >> 
> >>     inv_weight = 1+(WMULT_CONST-weight/2)/(weight+1)
> >> 
> >> This patch restores the algorithm to its original logic, and seems to solve
> >> the regression for me.  I can't really wrap my head around the original
> >> intent of the "+1" change, or whether reverting the change will cause a
> >> ripple effect somewhere else.  All I can confirm is that the system will
> >> once again balance load with this logic reverted to its previous form.
> > 
> > I didn't intend that change to sneak into this patch - but it was
> > sort-of intentional. My rationale was, a normal rounding division does:
> > 
> >   (x + y/2) / y
> > 
> > Since our 'x' is at the upper end of our modulo space we can't add to it
> > for it would wrap and end up small. Therefore we do:
> > 
> >  (x - y/2) / y
> > 
> > Which would result in 1 less than expected, hence I added that 1 back.
> 
> Ah, yes.  That makes sense.
> 
> > 
> > Now I'm equally puzzled on its effect. Nor do I mind its removal, but I
> > would like to understand why it has such drastic effects.
> 
> Nevermind my patch, its bogus.  I was mistaken earlier in thinking it
> was better with the "+1" removed.  Subsequent testing has demonstrated
> that the issue is still present, even with my "fix" applied.  The root
> issue seems to be real, but I cant spy it in the code via visual
> inspection.  Reverting the patch outright does seem to restore proper
> balancer behavior.  (Note that the commit-id for Peter's patch has
> since changed...probably due to a recent rebase in sched-devel).
> Perhaps someone with a better understanding of the load calculation
> will see it.


Ingo, can you drop this patch for now? - it seems to cause grief to
several people. I'll keep it around in my tree in the hope of figuring
out why such a seemingly simple patch breaks so much.

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