Re: [PATCH v5 17/29] sched/deadline: Leave a dying task out of a rebuild of the accounting

[Juri Lelli]

Hello,

On 08/11/26 13:53, Mathieu Desnoyers wrote:
> A rebuild of the root domains' accounting takes the pi_lock of each
> deadline task, not its rq lock. A dying task past its 0-lag time
> releases its bandwidth at its last schedule. It holds its rq lock there,
> not its pi_lock. Then it clears dl_bw. The rebuild can find the task
> between the two steps.
> 
> In this diagram, T is a deadline task that exits on CPU 0 past its 0-lag
> time. bw is its dl_bw. CPU 1 rebuilds the accounting:
> 
>   CPU 0 (T)                                  CPU 1 (rebuild)
> 
>   do_task_dead(): TASK_DEAD, under pi_lock
>   __schedule(): T's rq lock taken
>     task_non_contending(T)
>       dl_task_release_bw(T)
>         total_bw -= bw
>         dl_bw_gen recorded
>         record cleared
>                                              dl_rebuild_rd_accounting()
>                                                dl_clear_root_domains()
>                                                  dl_bw_gen++
>                                                  total_bw reset
>                                                dl_add_task_root_domain(T)
>                                                  T's pi_lock taken
>                                                  dl_bw != 0, dl_bw_gen old
>                                                  total_bw += bw
>       __dl_clear_params(T): dl_bw = 0
> 
> dl_bw_gen does not help, since the release came before the rebuild. The
> rebuild would count a bandwidth that nothing releases again. The root
> domain would then refuse deadline tasks it has room for, until the next
> rebuild.
> 
> Fix this with the record of the bandwidth, dl_bw_cpu. The release
> clears it under dl_departed_lock, which the walk holds. So leave out a
> dead task with no record. A dead task with a record has not reached its
> last schedule. The walk counts it as a live task. That schedule then
> releases the bandwidth, or puts the task on the departed list. A dead
> task on the departed list keeps its bandwidth until its timer, and the
> rebuild counts it as before.
> 
> The Sashiko AI review of v2 of this series found this window. Its review
> of v4 found a gap in the first version of this fix. That version also
> left out a task between TASK_DEAD and its last schedule, with its 0-lag
> time ahead.
> 
> Fixes: f9a25f776d78 ("cpusets: Rebuild root domain deadline accounting information")

It's kind of minor, but I'm not sure the above Fixes is right. This
patch alone can't be applied to fix the referenced commit, it needs the
(at least 2?) preceeding patches.

> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> ---

Other than that,

Reviewed-by: Juri Lelli <juri.lelli@xxxxxxxxxx>

Thanks,
Juri