bpf: arena: reschedule while freeing the arena pages

[kernel-patches-daemon-bpf]

Pull request for series with
subject: bpf: arena: reschedule while freeing the arena pages
version: 1
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=1151075

[kernel-patches-daemon-bpf]

Upstream branch: 5e289c5
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=1151075
version: 1

[kernel-patches-review-bot]

commit f31c4f03899a5d4ce4ad5dd663a0d92c0c4d694e
Author: Breno Leitao <leitao@debian.org>

bpf: arena: reschedule while freeing the arena pages

This patch addresses RCU stall and softlockup issues on non-preemptible
kernels by adding cond_resched() calls during arena page freeing.

> bpf: arena: reschedule while freeing the arena pages
>
> On non-preemptible kernels that is long enough to trip both the RCU
> stall detector and the softlockup watchdog. Observed on production arm64
> hosts at Meta:
>
>   rcu: INFO: rcu_sched self-detected stall on CPU
>   rcu:     37-....: (20999 ticks this GP) idle=3834/1/0x4000000000000000
>   Workqueue: events_unbound bpf_map_free_deferred
>   Call trace:
>    __apply_to_page_range+0x404/0x518
>    apply_to_existing_page_range+0x18/0x28
>    arena_map_free+0x54/0x98
>    bpf_map_free_deferred+0x70/0xf8
>    process_scheduled_works+0x17c/0x400
>    worker_thread+0x198/0x498
>   [...]
>   watchdog: BUG: soft lockup - CPU#37 stuck for 44s! [kworker/u288:15]
>   Kernel panic - not syncing: softlockup: hung tasks
>
> arena_map_free() sweeps the whole 4GiB arena address range in a single
> apply_to_existing_page_range() call and drops every populated page from
> the walk callback.
>
> Give up the CPU after releasing each page, using cond_resched().

This isn't a bug, but the opening sentence "On non-preemptible kernels
that is long enough ..." uses "that" without an antecedent.  The phrase
that explains what takes long enough (the 4GiB arena sweep) appears two
paragraphs later, after the stall trace.

Would moving the arena_map_free() description ahead of the stall trace
make the flow clearer?

---

AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

In-Reply-To-Subject: bpf: arena: reschedule while freeing the arena pages
CI run summary: https://github.com/kernel-patches/bpf/actions/runs/32755827346

[kernel-patches-daemon-bpf]

Forwarding comment 5399007475 via email
In-Reply-To: 20260824-b4-bpf-arena-free-resched-v1-1-5c447477a64b@debian.org
Patch: https://patchwork.kernel.org/project/netdevbpf/patch/20260824-b4-bpf-arena-free-resched-v1-1-5c447477a64b@debian.org/

[kernel-patches-daemon-bpf]

Upstream branch: d83fba2
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=1151075
version: 1

[kernel-patches-daemon-bpf]

Upstream branch: ce36e38
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=1151075
version: 1

[kernel-patches-daemon-bpf]

Upstream branch: 05ea1b6
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=1151075
version: 1

[kernel-patches-daemon-bpf]

Upstream branch: 1555de3
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=1151075
version: 1