Re: [PATCH RFC 0/2] rcu: Add debugfs interface for pending callback monitoring

[Joel Fernandes <joelagnelf@nvidia.com>]

On 5/7/2026 1:37 PM, Gustavo Luiz Duarte wrote:
> There is currently no easy way to monitor how many RCU callbacks are
> pending system-wide. The existing trace points provide per-event data
> but require active tracing, which makes them awkward for fleet-wide
> monitoring. Knowing the depth and stage of pending callbacks helps
> admins reason about RCU health, gives an indirect signal of memory
> held back by RCU, and is useful when tuning RCU parameters.
> 
> This series adds a debugfs file at:
> 
>   /sys/kernel/debug/rcu/pending_cbs
> 
> that reports per-CPU pending callback counts with a "total" row.
> 
> Patch 1 introduces the file with per-CPU columns for each segcblist
> segment (done, wait, next_ready, next) plus a "lazy" column.
> 
> Patch 2 extends the file with a "kfree_rcu" column reporting objects
> queued in the batched kfree_rcu()/kvfree_rcu() path
> (CONFIG_KVFREE_RCU_BATCHED), which has its own per-CPU queues outside
> the main segmented callback list.
> 
> Signed-off-by: Gustavo Luiz Duarte <gustavold@gmail.com>

You actually don't need debugfs for this. You can just use bpftrace and
instrument trace_rcu_ (with other RCU tracing Kconfig options enabled?). I had
something like that working sometime ago.

Generally RCU doesn't add userspace interfaces randomly like that. I remember
Paul ripped similar things out some time ago.