Re: [BUG] sched/fair: divide error in __calc_prop_weight() from the enqueue path (flat-hierarchy series)

Hello Jake,

Thank you for the report.

On 8/19/2026 4:43 AM, Jake S wrote:
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> 
> Hi,
> 
> I hit a divide-by-zero panic in __calc_prop_weight(), reached from
> enqueue_hierarchy() inside enqueue_task_fair(). This is the *enqueue*
> path, not the task_tick_fair() variant reported in May and addressed by
> the se->on_rq guard folded into 85570f10a4c6 -- enqueue_hierarchy() and
> dequeue_hierarchy() carry no equivalent check.
> 
> The code is from the tip sched/core flat-hierarchy rework; it is not in
> Linus' tree. I am running it via a distro kernel (CachyOS) that carries
> the series, on 7.2-rc7 and 7.2.0.
> 
> I have separated what I verified from what I am guessing. The last link
> in the causal chain is unexplained and I am asking about it rather than
> asserting it.
> 
> === The oops ===
> 
>   Oops: divide error: 0000 [#1] SMP NOPTI
>   CPU: 12 UID: 1000 PID: 312907 Comm: bash
>   Tainted: G     U   C OE       7.2.0-rc7-2-cachyos-rc #1 PREEMPT(full)
>   Hardware name: Dell Inc. XPS 16 DA16260/0RMV2Y, BIOS 1.5.1 04/01/2026
>   RIP: 0010:enqueue_task_fair.llvm.6536700009857788019+0x422/0x950
>   Code: 0f 84 74 01 00 00 83 bd 68 01 00 00 00 45 0f 4f f4 48 8b 4d 00
>         4c 89 e8 48 09 c8 48 c1 e8 20 0f 85 53 fd ff ff 44 89 e8 31 d2
>         <f7> f1 41 89 c5 e9 4f fd ff ff 0f 0b e9 1d fe ff ff 4c 89 e6
>   RAX: 0000000000000000 RBX: 0000000000000001 RCX: 0000000000000000
>   RDX: 0000000000000000 RSI: fffff46fbf98e680 RDI: fffff46fbf98ffc0
>   RBP: fffff46fbf98ffc0 R08: ffff8ee25f9b2a80 R09: 0000000000000000
>   R10: 0000000000000000 R11: 0000000000000110 R12: 0000000000000001
>   R13: 0000000000000000 R14: 0000000000000001 R15: fffff46fbf9901c0
>   Call Trace:
>    <TASK>
>    enqueue_task+0x8e/0x250
>    wake_up_new_task+0x148/0x2e0
>    kernel_clone+0x1c6/0x390
>    __x64_sys_clone+0xcc/0x100
>    do_syscall_64+0x147/0x3c0
>    asm_fred_entrypoint_user+0x41/0x41
>    </TASK>
> 
> Machine was idle, lid closed, 11.66 h into the boot. bash forked, the
> new task was enqueued, div trapped.

Was your laptop suspended at that point?

> 
> It is not survivable in practice. panic_on_oops was 0, so the kernel
> took the first #DE, printed the oops and continued for 476 ms. It then
> faulted at the same RIP with byte-identical registers and an identical
> RSP (ffffd46fff53bbb0):
> 
>   Kernel panic - not syncing: Fatal exception
>   Shutting down cpus with NMI
> 
> i.e. the oops-recovery path (kill task -> schedule()) re-entered the
> same enqueue with the rq lock already held mid-enqueue.
> 
> === Where it divides (confirmed) ===
> 
> kernel/sched/fair.c, __calc_prop_weight(), inlined into
> enqueue_hierarchy() -> enqueue_task_fair():
> 
>         weight *= se->load.weight;
>         if (parent_entity(se))
>                 weight /= cfs_rq->load.weight;  /* <-- #DE */
> 
> RCX = cfs_rq->load.weight = 0. R13 = 0 means se->load.weight was 0 as
> well, i.e. a group sched_entity carrying zero weight.
> 
> Not a miscompile: this is clang 22.1.8 + ThinLTO, hence the .llvm.<hash>
> suffix. The 32-bit "div %ecx" against 64-bit C operands is clang's
> BypassSlowDivision -- the preceding "or %rcx,%rax; shr $32,%rax; jne"
> is its guard. The 64-bit slow path is present in the same function.
> 
> === How the weight can reach zero (mechanism, partly inferred) ===
> 
> __calc_smp_shares() ends:
> 
>         return clamp_t(long, shares, MIN_SHARES, shares_max);
> 
> clamp() yields hi when hi < lo, so shares_max == 0 silently defeats the
> MIN_SHARES floor and returns 0 -- exactly the case the comment directly
> above it says must yield MIN_SHARES instead of 0. Note __clamp_once()
> already carries
> 
>         BUILD_BUG_ON_MSG(statically_true(ulo > uhi), ...)
> 
> so lo > hi is considered a bug upstream; it just cannot fire on a
> runtime-computed shares_max.
> 
> shares_max arrives from calc_concur_shares() as nr * tg_shares, where
> nr = min(tg_tasks(tg), tg_cpus(tg)). tg_cpus() returns
> cpuset_num_cpus(cgrp) unfloored, while its sibling tg_tasks() already
> floors at 1. That asymmetry is the hole.
> 
> concur is the live mode here:
> 
>   $ cat /sys/kernel/debug/sched/cgroup_mode
>   up smp (concur) max tasks
> 
> What I could NOT establish: that tg_cpus() actually returned 0, or what
> would produce an empty effective cpuset. update_cpumasks_hier()
> substitutes the parent's effective_cpus before storing; on this machine
> no cgroup has an empty cpuset.cpus.effective and every
> cpuset.cpus.partition reads "member". Twelve cgroups here have an empty
> cpuset.cpus and all report effective = 0-15. I suspected a power daemon
> that rewrites AllowedCPUs on the top-level systemd slices using an
> empty-then-set idiom, but I could not make that yield an empty effective
> mask, so I am not claiming it.
> 
> The missing floor looks like a hole regardless of what trips it, and I
> would rather ask than guess: is there a path where cpuset_num_cpus() can
> legitimately return 0, or should tg_cpus() simply floor at 1 the way
> tg_tasks() does?

Since you mentioned idle + lid closed, and
pm_sleep_disable_secondary_cpus() -> freeze_secondary_cpus() on the
suspend path, I'm wondering if that path can have any effect here but
afaict, sc->effective_cpus should be unaffected on that path and I'm
not sure if we can even get a fork() + wakeup before we thaw all the
process.

That said I do see a bunch of cpumask_empty(cs->effective_cpus) in
kernel/cgroup/cpuset.c so I'm not sure if that might be at play here.
I'll defer to folks who understand cpusets better.

> 
> === Proposed guard ===
> 
> Running locally on 7.2.0 for the past day. The WARN_ONCE in tg_cpus() is
> deliberately diagnostic -- it confirms or refutes the cpuset route the
> moment anyone reproduces this.
> 
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ __calc_prop_weight
> +       unsigned long div;
> +
>         weight *= se->load.weight;
> -       if (parent_entity(se))
> -               weight /= cfs_rq->load.weight;
> -       else
> +       if (parent_entity(se)) {
> +               div = cfs_rq->load.weight;
> +               if (unlikely(!div)) {
> +                       WARN_ONCE(1, "sched: cfs_rq->load.weight == 0 (se->load.weight=%lu)\n",
> +                                 se->load.weight);
> +                       return MIN_SHARES;
> +               }
> +               weight /= div;
> +       } else {
>                 weight /= NICE_0_LOAD;
> +       }
> 
>         return max(weight, MIN_SHARES);
> 
> @@ __calc_smp_shares
> -       return clamp_t(long, shares, MIN_SHARES, shares_max);
> +       /* clamp() yields hi when hi < lo, defeating the MIN_SHARES floor. */
> +       return clamp_t(long, shares, MIN_SHARES,
> +                      max_t(long, shares_max, MIN_SHARES));
> 
> @@ tg_cpus
> +       if (WARN_ONCE(nr < 1, "sched: tg_cpus() == 0, empty cpuset\n"))
> +               nr = 1;
>         return nr;
> 
> === Reproducer / caveats ===
> 
> Not reliably reproducible: one occurrence in ~11.7 h of idle uptime, and
> none since. I have no better trigger than "leave it running".
> 
> The kernel is tainted G U C OE -- out-of-tree camera drivers are loaded
> on this machine. I cannot categorically exclude memory corruption from
> those. Against that: no prior WARNs, no slab or list corruption, no DMAR
> faults, no EDAC events, and the two oopses 476 ms apart had byte-identical
> register state, which a wild write would not reproduce exactly. I mention
> it so nobody wastes time on a report I cannot fully vouch for.
> 
> Happy to test patches or run instrumented builds on the affected machine.

I think your current instrumentation from "Proposed guard" is good
enough to tell where the problem is if it reproduces. Are you running
with it on your setup currently?

> 
> Config: CONFIG_FAIR_GROUP_SCHED=y, CONFIG_SCHED_AUTOGROUP=y,
> CONFIG_SCHED_CLASS_EXT=y (sched_ext disabled, not in use),
> CONFIG_X86_NATIVE_CPU=y, no CONFIG_SCHED_BORE, no SCHED_ALT.
> Hardware: Intel Core Ultra X7 358H (Panther Lake), 16 CPUs.
> 
> Thanks,
> Jake

-- 
Thanks and Regards,
Prateek