On Fri, Dec 20, 2024 at 10:42:34PM +0900, Koichiro Den wrote:
> Even after mm/vmstat:online teardown, shepherd may still queue work for
> the dying cpu until the cpu is removed from online mask. While it's
> quite rare, this means that after unbind_workers() unbinds a per-cpu
> kworker, it potentially runs vmstat_update for the dying CPU on an
> irrelevant cpu before entering STARTING section.
> When CONFIG_DEBUG_PREEMPT=y, it results in the following error with the
> backtrace.
>
> BUG: using smp_processor_id() in preemptible [00000000] code: \
> kworker/7:3/1702
> caller is refresh_cpu_vm_stats+0x235/0x5f0
> CPU: 0 UID: 0 PID: 1702 Comm: kworker/7:3 Tainted: G
> Tainted: [N]=TEST
> Workqueue: mm_percpu_wq vmstat_update
> Call Trace:
> <TASK>
> dump_stack_lvl+0x8d/0xb0
> check_preemption_disabled+0xce/0xe0
> refresh_cpu_vm_stats+0x235/0x5f0
> vmstat_update+0x17/0xa0
> process_one_work+0x869/0x1aa0
> worker_thread+0x5e5/0x1100
> kthread+0x29e/0x380
> ret_from_fork+0x2d/0x70
> ret_from_fork_asm+0x1a/0x30
> </TASK>
>
> So, disable vmstat_work reliably on vmstat_cpu_down_prep().
>
> Signed-off-by: Koichiro Den <koichiro.den@xxxxxxxxxxxxx>
> ---
> mm/vmstat.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 4d016314a56c..44e1d87dcf01 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -2154,7 +2154,7 @@ static int vmstat_cpu_online(unsigned int cpu)
>
> static int vmstat_cpu_down_prep(unsigned int cpu)
> {
> - cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu));
> + disable_delayed_work_sync(&per_cpu(vmstat_work, cpu));
> return 0;
> }
>
> --
Andrew,
I just noticed my silly mistake - I needed to enable the work in the
opposite direction. It looks like you've already queued this (v1)
to mm-hotfixes-unstable. Could you please replace it with v2?
(https://lore.kernel.org/all/20241221033321.4154409-1-koichiro.den@xxxxxxxxxxxxx/)
Sorry to bother you. Let me know if submitting a separate follow-up patch
would be more appropriate.
Thank you.
-Koichiro Den
