| From: |
| Sebastian Andrzej Siewior <bigeasy-AT-linutronix.de> |
| To: |
| linux-kernel-AT-vger.kernel.org, netdev-AT-vger.kernel.org |
| Subject: |
| [PATCH v2 net-next 00/15] locking: Introduce nested-BH locking. |
| Date: |
| Fri, 03 May 2024 20:25:04 +0200 |
| Message-ID: |
| <20240503182957.1042122-1-bigeasy@linutronix.de> |
| Cc: |
| "David S. Miller" <davem-AT-davemloft.net>, Boqun Feng <boqun.feng-AT-gmail.com>, Daniel Borkmann <daniel-AT-iogearbox.net>, Eric Dumazet <edumazet-AT-google.com>, Frederic Weisbecker <frederic-AT-kernel.org>, Ingo Molnar <mingo-AT-redhat.com>, Jakub Kicinski <kuba-AT-kernel.org>, Paolo Abeni <pabeni-AT-redhat.com>, Peter Zijlstra <peterz-AT-infradead.org>, Thomas Gleixner <tglx-AT-linutronix.de>, Waiman Long <longman-AT-redhat.com>, Will Deacon <will-AT-kernel.org> |
| Archive-link: |
| Article |
Disabling bottoms halves acts as per-CPU BKL. On PREEMPT_RT code within
local_bh_disable() section remains preemtible. As a result high prior
tasks (or threaded interrupts) will be blocked by lower-prio task (or
threaded interrupts) which are long running which includes softirq
sections.
The proposed way out is to introduce explicit per-CPU locks for
resources which are protected by local_bh_disable() and use those only
on PREEMPT_RT so there is no additional overhead for !PREEMPT_RT builds.
The series introduces the infrastructure and converts large parts of
networking which is largest stake holder here. Once this done the
per-CPU lock from local_bh_disable() on PREEMPT_RT can be lifted.
v1…v2 https://lore.kernel.org/all/20231215171020.687342-1-bigea...
- Jakub complained about touching networking drivers to make the
additional locking work. Alexei complained about the additional
locking within the XDP/eBFP case.
This led to a change in how the per-CPU variables are accessed for the
XDP/eBPF case. On PREEMPT_RT the variables are now stored on stack and
the task pointer to the structure is saved in the task_struct while
keeping every for !RT unchanged. This was proposed as a RFC in
v1: https://lore.kernel.org/all/20240213145923.2552753-1-bige...
and then updated
v2: https://lore.kernel.org/all/20240229183109.646865-1-bigea...
- Renamed the container struct from xdp_storage to bpf_net_context.
Suggested by Toke Høiland-Jørgensen.
- Use the container struct also on !PREEMPT_RT builds. Store the
pointer to the on-stack struct in a per-CPU variable. Suggested by
Toke Høiland-Jørgensen.
This reduces the initial queue from 24 to 15 patches.
- There were complains about the scoped_guard() which shifts the whole
block and makes it harder to review because the whole gets removed and
added again. The usage has been replaced with local_lock_nested_bh()+
its unlock counterpart.
Sebastian
