Re: [patch V3 00/12] rseq: Implement time slice extension mechanism

[Mathieu Desnoyers]

On 2025-11-10 09:23, Mathieu Desnoyers wrote:
> On 2025-11-06 12:28, Prakash Sangappa wrote:
> [...]
> > Hit this watchdog panic.
> >
> > Using following tree. Assume this Is the latest.
> > https://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git/ rseq/ 
> > slice
> >
> > Appears to be spinning in mm_get_cid(). Must be the mm cid changes.
> > https://lore.kernel.org/all/20251029123717.886619142@xxxxxxxxxxxxx/
>
> When this happened during the development of the "complex" mm_cid
> scheme, this was typically caused by a stale "mm_cid" being kept around
> by a task even though it was not actually scheduled, thus causing
> over-reservation of concurrency IDs beyond the max_cids threshold. This
> ends up looping in:
>
> static inline unsigned int mm_get_cid(struct mm_struct *mm)
> {
>          unsigned int cid = __mm_get_cid(mm, READ_ONCE(mm- 
>  >mm_cid.max_cids));
>
>          while (cid == MM_CID_UNSET) {
>                  cpu_relax();
>                  cid = __mm_get_cid(mm, num_possible_cpus());
>          }
>          return cid;
> }
>
> Based on the stacktrace you provided, it seems to happen within
> sched_mm_cid_fork() within copy_process, so perhaps it's simply an
> initialization issue in fork, or an issue when cloning a new thread ?

One possible issue here: I note that kernel/sched/core.c:mm_init_cid()
misses the following initialization:

  mm->mm_cid.transit = 0;

Thanks,

Mathieu


> Thanks,
>
> Mathieu


--
Mathieu Desnoyers
EfficiOS Inc.
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