Re: [PATCH] writeback: report a Tasks-RCU quiescent state per cgwb drain pass

[Jan Kara]

On Wed 09-09-26 18:01:07, Josef Bacik wrote:
> cleanup_offline_cgwbs_workfn() drains a dying cgwb by calling
> cleanup_offline_cgwb() until it returns false, with a cond_resched()
> between passes.  On a CONFIG_PREEMPTION kernel that cond_resched() does
> nothing: _cond_resched() is a plain "return 0", and under
> PREEMPT_DYNAMIC the full and lazy modes disable it.  Since commit
> 7dadeaa6e851 ("sched: Further restrict the preemption modes") those are
> the only two models on the architectures with PREEMPT_LAZY support,
> arm64 and x86 among them, so the drain loop never reports a Tasks-RCU
> quiescent state.
> 
> A worker draining a cgwb with millions of attached inodes runs for
> minutes.  On a 6.18 arm64 host in lazy mode the cgwb worker drained one
> dying cgroup's writeback domain for over 11 minutes.  A BPF program
> unlink (bpf_trampoline_unlink_prog -> bpf_trampoline_update ->
> unregister_ftrace_direct -> ftrace_shutdown -> synchronize_rcu_tasks())
> waited on that grace period while holding the trampoline mutex, 42
> tasks queued behind it in D state, and the hung task detector fired at
> 614 s and panicked the host.  Any BPF or ftrace detach during a long
> drain inherits the drain's length.
> 
> Fix this by calling cond_resched_tasks_rcu_qs() so we do not stall out
> anybody who calls sycnrhonize_rcu_tasks().  We put this in a do { } while
> loop because if we have many small cgroups cleanup_offline_cgwb() will
> return false and we will never call cond_resched_tasks_rcu_qs(), creating
> the same problem.
> 
> Fixes: c22d70a162d3 ("writeback, cgroup: release dying cgwbs by switching attached inodes")
> Cc: stable@xxxxxxxxxxxxxxx
> Link: https://lore.kernel.org/bpf/9d444098-7c03-4163-af12-bd0a79a51443@paulmck-laptop/
> Assisted-by: LLM
> Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>

Makes sense to me. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

And I fully agree about the fragility. It looks as an oversight that
cond_resched() doesn't imply RCU quiescent period after the recent
preemption overhaul.

								Honza

> ---
>  mm/backing-dev.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index cecbcf9060a6..18e999053bae 100644
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -910,8 +910,9 @@ static void cleanup_offline_cgwbs_workfn(struct work_struct *work)
>  			continue;
>  
>  		spin_unlock_irq(&cgwb_lock);
> -		while (cleanup_offline_cgwb(wb))
> -			cond_resched();
> +		do {
> +			cond_resched_tasks_rcu_qs();
> +		} while (cleanup_offline_cgwb(wb));
>  		spin_lock_irq(&cgwb_lock);
>  
>  		wb_put(wb);
> 
> ---
> base-commit: 893e11787f78e43b534e252249ac3fff4d1333f8
> change-id: 20260909-cgwb-tasks-rcu-qs-42341609f3e1
> 
> Best regards,
> --  
> Josef Bacik <josef@xxxxxxxxxxxxxx>
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR