Re: [PATCH] sched: fix inv_weight calc

From: Peter Zijlstra
Date: Wed Apr 30 2008 - 14:46:25 EST


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.

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.

> Thanks to my colleage, David Bahi, for doing all the legwork on the bisect.
> And thanks to Peter Zijlstra for guiding me on all things CFS as I stuggle
> to come up to speed on the non-RT portions of the scheduler.
>
> Signed-off-by: Gregory Haskins <ghaskins@xxxxxxxxxx>
> CC: David Bahi <dbahi@xxxxxxxxxx>
> CC: Ingo Molnar <mingo@xxxxxxx>
> CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> ---
>
> kernel/sched.c | 2 +-
> 1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 32ef6c8..8326e20 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -1562,7 +1562,7 @@ calc_delta_mine(unsigned long delta_exec, unsigned long weight,
> if (unlikely(!lw->inv_weight)) {
> unsigned long inv_wls = inv_WLS(lw->weight);
>
> - lw->inv_weight = 1 + (WMULT_CONST-inv_wls/2) / (inv_wls+1);
> + lw->inv_weight = (WMULT_CONST-inv_wls/2) / (inv_wls+1);
> }
>
> tmp = inv_WLS((u64)delta_exec * weight);
>

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