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[34.228.114.98]) by smtp.gmail.com with ESMTPSA id af79cd13be357-939ecc38105sm175654485a.4.2026.09.11.07.09.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 11 Sep 2026 07:09:42 -0700 (PDT) From: Josef Bacik Subject: [PATCH RFC v2 00/15] rcu-tasks: let preemption outside trampolines be a quiescent state Date: Fri, 11 Sep 2026 14:08:38 +0000 Message-Id: <20260911-b4-rcu-tasks-preempt-qs-v2-0-eaaa61ed2da4@toxicpanda.com> Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit X-B4-Tracking: v=1; b=H4sIAGgLpGoC/4WOzQ6CMBCEX8Xs2ZWWFAyeTEx8AK+GQ7sUqT+A3 UowhHcX8GricWZnvp0B2HpnGXarAbztHLumnkS8XgFVur5YdMWkIRZxKjIp0Cj09MKg+cbYems fbcAnoxKyLFWi0oS2MLWnU+n6hXyG0/EA+dfkl7laCjNzjhnNFo3XNVWzdW8jo6LF/PVlblSOQ +Pfy+JOLvi/4zqJAgul0qxURFLIfWh6R62uC72h5gH5OI4f1jKPpQwBAAA= X-Change-ID: 20260910-b4-rcu-tasks-preempt-qs-401ff45465c7 To: "Paul E. McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Boqun Feng , Thomas Gleixner , Peter Zijlstra , Steven Rostedt , Masami Hiramatsu , Mark Rutland , Jiri Olsa , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , x86@kernel.org, Catalin Marinas , Will Deacon , Puranjay Mohan , Xu Kuohai Cc: Andy Lutomirski , Josh Triplett , Uladzislau Rezki , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Juergen Gross , Luis Chamberlain , Ihor Solodrai , linux-kernel@vger.kernel.org, rcu@vger.kernel.org, linux-trace-kernel@vger.kernel.org, bpf@vger.kernel.org, linux-arm-kernel@lists.infradead.org, xen-devel@lists.xenproject.org, Josef Bacik X-Mailer: b4 0.15.2 X-Developer-Signature: v=1; a=openssh-sha256; t=1789135736; l=9888; i=josef@toxicpanda.com; h=from:subject:message-id; bh=ciq/+qqHqSRSh7FRYtq3aWelW6Wh5G33c0OaJbqUUSk=; b=U1NIU0lHAAAAAQAAADMAAAALc3NoLWVkMjU1MTkAAAAgUBr36M/n0nWN0DNbnxwzIiCZez6MG JiruuNaSCI/zXsAAAAGcGF0YXR0AAAAAAAAAAZzaGE1MTIAAABTAAAAC3NzaC1lZDI1NTE5AAAA QAo5VaRxmnCLTA74sHK3/IXlJk4maBdHJ/o8BcYQaWxLTsL0V/6t0pY/lmq70yYasPVK7mviH+F JmwKt5LbXdQw= X-Developer-Key: i=josef@toxicpanda.com; a=openssh; fpr=SHA256:C8kOX2QUJCMqnCX+KEeoqRAjLo9L+ELOSH2NSAJHqGA v1: https://lore.kernel.org/all/20260910-b4-rcu-tasks-preempt-qs-v1-0-d4469f4cc101@toxicpanda.com/ v1->v2: - Only walk the kprobe hash while the optimizer is actually waiting (Sashiko). - Re-check the kprobe jump window at every QS decision instead of once at preemption time (AI review). - Updated Documentation/RCU for the new rule (AI review). - Added 14/15 and 15/15 to address Paul's comments. - Added a comment in trace_recursion.h per Steve. - No change for the arm64 ftrace_static_tramp_end report, the Kconfig dependency already covers it (Sashiko). - Re-ran the x86-64 QEMU tests, still 0.2-0.3s and clean. --- Original email --- Tasks RCU only treats a voluntary context switch, usermode or idle as a quiescent state, because a preempted task may be sitting in a trampoline that is about to be freed. That was a fine trade when PREEMPT_NONE servers compiled Tasks RCU away and PREEMPT desktops rarely ran long-lived in-kernel loops. PREEMPT_LAZY changes both halves at once: Tasks RCU is now real on server configs, and cond_resched() is a no-op, so a CPU-bound kthread or kworker only ever loses the CPU by being preempted, which is exactly the event Tasks RCU refuses to count. The way this showed up for us was a cgroup writeback worker draining a very large cgwb for around eleven minutes on an arm64 box. Nothing wrong with that on its own, but a BPF program detach on another CPU went bpf_trampoline_update() -> ftrace_shutdown() -> synchronize_rcu_tasks() while holding trampoline_mutex, forty-odd tasks piled up behind the mutex, and the hung task detector panicked the machine. The kprobe jump optimizer is worse in principle: it does synchronize_rcu_tasks() under kprobe_mutex, text_mutex and cpus_read_lock(), so one long-running kthread can stall static key updates and CPU hotplug for its whole run. The current answer is to find each such loop and add cond_resched_tasks_rcu_qs() to it, which is the kind of annotation PREEMPT_LAZY was supposed to let us stop writing. This series tries the other direction: have the trampolines say when a task is inside them, so that a preemption anywhere else can be a quiescent state. - task_struct grows an int, rcu_tramp_nesting. Every trampoline whose lifetime Tasks RCU guards increments it before calling out and decrements it before returning: ftrace_caller and its dynamic copies, the BPF trampoline (which drops it again around the call to the original function, since im->pcref covers that), the x86 optprobe template, and out-of-line register_ftrace_direct() trampolines. Only current writes it and nested users are balanced, so it is a plain non-atomic inc/dec, one load of current plus one RMW per entry/exit. - The inc/dec are inside the trampoline, so there is a window of a few instructions on each side where the count is zero but the task is in (or on its way into) trampoline text. Nothing there can be preempted synchronously, only from an interrupt, so the irq-exit preemption path looks at regs->ip and holds the count across preempt_schedule_irq() when the IP is somewhere the counter cannot cover: outside core and module text (all the dynamically allocated trampolines and slots), in the static ftrace stubs or the x86 return thunks that still hold a direct-call target, in a module that hosts its own direct trampoline, or inside the bytes after a kprobe that the jump optimizer may be about to rewrite (the one synchronize_rcu_tasks() user that is not about trampolines at all). - With those in place, rcu_tasks_classic_qs() also clears the holdout flag on a preemption when the count is zero, on architectures that opt in. x86-64 and arm64 do so here. Everyone else keeps the voluntary-only rule and is untouched apart from the (unused) field. A running holdout already gets poked via rcu_request_urgent_qs_task(), which makes the next tick set NEED_RESCHED, so with this the resulting preemption retires it and a Tasks RCU grace period is bounded by roughly a tick plus the longest preempt-off section rather than by the longest stretch without a voluntary schedule(). Patches 1-12 are scaffolding and change no behaviour on their own; patch 13 flips the rule and selects the option for the two architectures. Testing so far is QEMU only: x86-64, PREEMPT_LAZY with PREEMPT_RCU=n, PROVE_RCU and lockdep, with and without PREEMPT_DYNAMIC. A kthread spinning in-kernel for 30s with the function tracer, an ftrace kprobe, an optimized kprobe and fentry/fexit programs attached: synchronize_rcu_tasks() goes from 29.7s to 0.1-0.3s, tearing down a DYNAMIC ftrace_ops (tracefs instance function -> nop) from 27s to 0.2-0.8s, and the ftrace-direct sample modules load, fire and unload in about 2.5s each while the spinner runs, with no warnings and the new return-to-user assertion quiet. arm64 is build-tested only at this point; real hardware numbers for both are the obvious next step and I did not want to sit on the idea waiting for them. Things I would particularly like opinions on: - Whether hooking rcu_tasks_classic_qs() is the right place, or whether Paul would rather see this expressed differently inside Tasks RCU. - return_to_handler and the rethook/kretprobe trampolines are not instrumented. Their C callees take the ftrace recursion lock before touching any ops and the trampolines themselves are static text, so I believe they do not need it, but I would like Steven and Masami to confirm. - The register_ftrace_direct() contract change: out-of-line direct trampolines now have to maintain the count themselves (the samples are converted). I do not know of out-of-tree users beyond BPF, but this is the one place an existing user could be silently weakened. - Whether arm64 folks are comfortable with the ldr/add/str in ftrace_caller and the BPF trampoline, and with treating all of ftrace_caller as trampoline text for the IP check. - If this holds up, cond_resched_tasks_rcu_qs() and rcu_softirq_qs_periodic() become unnecessary on the opted-in architectures; I have not touched them here. Based on v7.3-rc2+ (893e11787f78). --- Josef Bacik (15): rcu-tasks: Add per-task trampoline nesting count entry: Pass pt_regs to irqentry_exit_cond_resched() rcu-tasks: Hold trampoline nesting across irq-exit preemption in trampoline text kprobes: Let Tasks RCU recognise tasks preempted in an optprobe jump window ftrace: Mark modules hosting direct-call trampolines for Tasks RCU x86/ftrace: Maintain Tasks RCU trampoline nesting in ftrace_caller x86/kprobes: Maintain Tasks RCU trampoline nesting in the optprobe template bpf, x86: Maintain Tasks RCU trampoline nesting in the BPF trampoline arm64: ftrace: Maintain Tasks RCU trampoline nesting in ftrace_caller bpf, arm64: Maintain Tasks RCU trampoline nesting in the BPF trampoline samples: ftrace: Maintain Tasks RCU trampoline nesting in direct-call trampolines rcutorture: Bracket Tasks RCU readers with trampoline nesting rcu-tasks: Treat preemption outside trampolines as a quiescent state rcu-tasks: Retire switched-out tasks with no trampoline nesting at scan time rcu-tasks: Kick running holdouts through the scheduler .../RCU/Design/Requirements/Requirements.rst | 28 +++- Documentation/RCU/checklist.rst | 8 +- arch/arm64/Kconfig | 1 + arch/arm64/kernel/asm-offsets.c | 3 + arch/arm64/kernel/entry-ftrace.S | 35 +++++ arch/arm64/kernel/ftrace.c | 16 +++ arch/arm64/net/bpf_jit_comp.c | 46 +++++++ arch/x86/Kconfig | 1 + arch/x86/kernel/asm-offsets.c | 3 + arch/x86/kernel/ftrace.c | 37 ++++++ arch/x86/kernel/ftrace_64.S | 43 +++++++ arch/x86/kernel/kprobes/opt.c | 20 +++ arch/x86/kernel/vmlinux.lds.S | 4 + arch/x86/net/bpf_jit_comp.c | 43 +++++++ arch/x86/xen/enlighten_pv.c | 2 +- include/linux/irq-entry-common.h | 14 +- include/linux/kprobes.h | 8 +- include/linux/module.h | 7 + include/linux/rcupdate.h | 86 ++++++++++++- include/linux/sched.h | 2 + include/linux/trace_recursion.h | 11 ++ kernel/entry/common.c | 38 +++++- kernel/fork.c | 2 + kernel/kprobes.c | 46 +++++++ kernel/rcu/Kconfig | 17 ++- kernel/rcu/rcutorture.c | 6 + kernel/rcu/tasks.h | 141 ++++++++++++++++++++- kernel/rcu/update.c | 2 + kernel/trace/ftrace.c | 39 ++++++ samples/ftrace/ftrace-direct-modify.c | 9 ++ samples/ftrace/ftrace-direct-multi-modify.c | 9 ++ samples/ftrace/ftrace-direct-multi.c | 5 + samples/ftrace/ftrace-direct-too.c | 5 + samples/ftrace/ftrace-direct.c | 5 + samples/ftrace/ftrace-direct.h | 64 ++++++++++ 35 files changed, 776 insertions(+), 30 deletions(-) --- base-commit: 893e11787f78e43b534e252249ac3fff4d1333f8 change-id: 20260910-b4-rcu-tasks-preempt-qs-401ff45465c7 Best regards, -- Josef Bacik