Re: [PATCH] sched/fair: Add null pointer check to pick_next_entity()

Peter Zijlstra
2025-04-14T09:08:40Z

On Fri, Apr 11, 2025 at 10:51:34AM -0400, Rik van Riel wrote:
> On Wed, 9 Apr 2025 17:27:03 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
> > On Wed, Apr 09, 2025 at 10:29:43AM -0400, Rik van Riel wrote:
> > > Our trouble workload still makes the scheduler crash
> > > with this patch.
> > >
> > > I'll go put the debugging patch on our kernel.
> > >
> > > Should I try to get debugging data with this patch
> > > part of the mix, or with the debugging patch just
> > > on top of what's in 6.13 already?
> >
> > Whatever is more convenient I suppose.
> >
> > If you can dump the full tree that would be useful. Typically the
> > se::{vruntime,weight} and cfs_rq::{zero_vruntime,avg_vruntime,avg_load}
> > such that we can do full manual validation of the numbers.
>
> Here is a dump of the scheduler tree of the crashing CPU.
>
> Unfortunately the CPU crashed in pick_next_entity, and not in your
> debugging code. I'll add two more calls to avg_vruntime_validate(),
> one from avg_vruntime_update(), and one rfom __update_min_vruntime()
> when we skip the call to avg_vruntime_update(). The line numbers in
> the backtrace could be a clue.
>
> I have edited the cgroup names to make things more readable, but everything
> else is untouched.

Hmm, I didn't think you guys used the cgroup stuff.

Anyway, given cgroups, which group pick is the one that went boom? Also,
what is curr (for that cgroup).

curr lives outside of the tree, but is included in the eligibility
consideration (when still on_rq and all that).

> nr_running = 3
> min_vruntime = 107772371139014
> avg_vruntime = -1277161882867784752
> avg_load = 786
> tasks_timeline = [
> {
> cgroup /A
> weight = 10230 => 9

No vruntime, I'll assume !on_rq, but that makes avg_load above not match
:/ So something is off here.

> rq = {
> nr_running = 0
> min_vruntime = 458975898004
> avg_vruntime = 0
> avg_load = 0
> tasks_timeline = [
> ]
> }
> },
> {
> cgroup /B
> vruntime = 18445226958208703357
> weight = 319394 => 311
> rq = {
> nr_running = 2
> min_vruntime = 27468255210769
> avg_vruntime = 0
> avg_load = 93
> tasks_timeline = [
> {
> cgroup /B/a
> vruntime = 27468255210769
> weight = 51569 => 50
> rq = {
> nr_running = 1
> min_vruntime = 820449693961
> avg_vruntime = 0
> avg_load = 15
> tasks_timeline = [
> {
> task = 3653382 (fc0)
> vruntime = 820449693961
> weight = 15360 => 15
> },
> ]
> }
> },
> {
> cgroup /B/b
> vruntime = 27468255210769
> weight = 44057 => 43
> rq = {
> nr_running = 1
> min_vruntime = 563178567930
> avg_vruntime = 0
> avg_load = 15
> tasks_timeline = [
> {
> task = 3706454 (fc0)
> vruntime = 563178567930
> weight = 15360 => 15
> },
> ]
> }
> },
> ]
> }
> },
> {
> cgroup /C
> vruntime = 18445539757376619550
> weight = 477855 => 466
> rq = {
> nr_running = 0
> min_vruntime = 17163581720739
> avg_vruntime = 0
> avg_load = 0
> tasks_timeline = [
> ]
> }
> },
> ]

So given the above, I've created the below files, and that gives:

$ ./vruntime < root.txt
k: 0 w: 311 k*w: 0
k: 312799167916193 w: 466 k*w: 145764412248945938
v': 107772371139014 = v: 18445226958208703357 + d: 1624887871987273
V': -1116773464285165183 = V: 145764412248945938 - d: 1624887871987273 * W: 777
min_vruntime: 107772371139014
avg_vruntime: -1116773464285165183
avg_load: 777

> One thing that stands out to me is how the vruntime of each of the
> entities on the CPU's cfs_rq are really large negative numbers.
>
> vruntime = 18429030910682621789 equals 0xffc111f8d9ee675d
>
> I do not know how those se->vruntime numbers got to that point,
> but they are a suggestive cause of the overflow.
>
> I'll go comb through the se->vruntime updating code to see how those
> large numbers could end up as the vruntime for these sched entities.

As you can see from the output here, the large negative is the result
of min_vruntime being significantly ahead of the actual entities.

This can happen due to that monotonicity filter the thing has -- it
doesn't want to go backwards. Whereas the 0-lag point can move
backwards, seeing how it is the weighted average, and inserting a task
with positive lag will insert a task left of middle, moving the middle
left.

The zero_vruntime patch I gave earlier should avoid this particular
issue.

$ ./vruntime < B.txt
k: 0 w: 50 k*w: 0
k: 0 w: 43 k*w: 0
v': 27468255210769 = v: 27468255210769 + d: 0
V': 0 = V: 0 - d: 0 * W: 93
min_vruntime: 27468255210769
avg_vruntime: 0
avg_load: 93

C, B/a and B/b are not really interesting, they're single entries where
min_vruntime == vruntime and boring.

---8<---(root.txt)---8<---
entity 18445226958208703357 319394
entity 18445539757376619550 477855
group 107772371139014

---8<---(B.txt)---8<---
entity 27468255210769 51569
entity 27468255210769 44057
group 27468255210769

---8<---(vruntime.c)---8<---

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>

int main (int argc, char **argv)
{
unsigned long V, W;
unsigned long V0;
bool init = false;

for (;;) {
unsigned long vruntime, weight;
char type[32];

int r = scanf("%s\t%lu\t%lu\n", &type, &vruntime, &weight);
if (r == EOF)
break;

if (!strcmp(type, "entity")) {
if (!init) {
V = W = 0;
V0 = vruntime;
init = true;
}

unsigned long k = vruntime - V0;
unsigned long w = weight / 1024;

V += k * w;
W += w;

printf(" k: %ld w: %lu k*w: %ld\n", k, w, k*w);
}

if (!strcmp(type, "group")) {
unsigned long d = vruntime - V0;

printf(" v': %lu = v: %lu + d: %lu\n", V0 + d, V0, d);
printf(" V': %ld = V: %ld - d: %ld * W: %lu\n",
V - d * W, V, d, W);

V0 += d;
V -= d * W;
}
}

printf("min_vruntime: %lu\n", V0);
printf("avg_vruntime: %ld\n", V);
printf("avg_load: %lu\n", W);

return 0;
}