Re: [PATCH 0/4] sched: Various reweight_entity() fixes

[Peter Zijlstra]

On Thu, Feb 12, 2026 at 12:59:43PM +0100, Peter Zijlstra wrote:
> On Thu, Feb 12, 2026 at 01:13:30PM +0530, K Prateek Nayak wrote:

> >     Enqueue cfs_rq: depth(1) weight(2144337920) nr_queued(2045) sum_w_vruntime(18343811230990336)    sum_weight(2144337920) zero_vruntime(18337852274) sum_shift(0) avg_vruntime(18337852274)
> >     Dequeue cfs_rq: depth(1) weight(2144337920) nr_queued(2045) sum_w_vruntime(1303379968)           sum_weight(2143289344) zero_vruntime(18354971669) sum_shift(0) avg_vruntime(18354971669)

After staring at waaay too many traces of my own, which confirm what
you're seeing, but didn't want to make any sense either...

... I thinks I found it ...

Note that sum_weight, that has _just_ flipped bit 31 and thus turned
negative *if* it were a 32bit number. Every time, exactly at this point
things started to go sideways.

So I went looking for where this might be and I found the below.

My latest run has passed 0x8000000/(1024*1024) = 2048 tasks and is still
running. Numbers are still sane, fingers crossed.


---


diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 5b1a2abb727e..de155be02411 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -797,7 +797,11 @@ u64 avg_vruntime(struct cfs_rq *cfs_rq)
 		if (runtime < 0)
 			runtime -= (weight - 1);
 
+#ifndef CONFIG_64BIT
 		delta = div_s64(runtime, weight);
+#else
+		delta = runtime / weight;
+#endif
 	} else if (curr) {
 		/*
 		 * When there is but one element, it is the average.