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[2a00:1028:83b8:1e7a:3010:3bd6:8521:caf1]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-47d7f6f0e15sm148940005e9.10.2026.01.08.05.21.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 08 Jan 2026 05:21:34 -0800 (PST) From: Petr Tesarik To: Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Sebastian Andrzej Siewior , Clark Williams Cc: Frederic Weisbecker , linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, Petr Tesarik Subject: [PATCH v2 1/1] ring-buffer: Use a housekeeping CPU to wake up waiters Date: Thu, 8 Jan 2026 14:21:32 +0100 Message-ID: <20260108132132.2473515-1-ptesarik@suse.com> X-Mailer: git-send-email 2.52.0 Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Avoid running the wakeup irq_work on an isolated CPU. Since the wakeup can run on any CPU, let's pick a housekeeping CPU to do the job. This change reduces additional noise when tracing isolated CPUs. For example, the following ipi_send_cpu stack trace was captured with nohz_full=2 on the isolated CPU: -0 [002] d.h4. 1255.379293: ipi_send_cpu: cpu=2 callsite=irq_work_queue+0x2d/0x50 callback=rb_wake_up_waiters+0x0/0x80 -0 [002] d.h4. 1255.379329: => trace_event_raw_event_ipi_send_cpu => __irq_work_queue_local => irq_work_queue => ring_buffer_unlock_commit => trace_buffer_unlock_commit_regs => trace_event_buffer_commit => trace_event_raw_event_x86_irq_vector => __sysvec_apic_timer_interrupt => sysvec_apic_timer_interrupt => asm_sysvec_apic_timer_interrupt => pv_native_safe_halt => default_idle => default_idle_call => do_idle => cpu_startup_entry => start_secondary => common_startup_64 The IRQ work interrupt alone adds considerable noise, but the impact can get even worse with PREEMPT_RT, because the IRQ work interrupt is then handled by a separate kernel thread. This requires a task switch and makes tracing useless for analyzing latency on an isolated CPU. After applying the patch, the trace is similar, but ipi_send_cpu always targets a non-isolated CPU. Unfortunately, irq_work_queue_on() is not NMI-safe. When running in NMI context, fall back to queuing the irq work on the local CPU. Signed-off-by: Petr Tesarik --- kernel/trace/ring_buffer.c | 24 +++++++++++++++++++++--- 1 file changed, 21 insertions(+), 3 deletions(-) diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 41c9f5d079beb..5cf4c6baa2771 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -4,6 +4,7 @@ * * Copyright (C) 2008 Steven Rostedt */ +#include #include #include #include @@ -4011,19 +4012,36 @@ static void rb_commit(struct ring_buffer_per_cpu *cpu_buffer) rb_end_commit(cpu_buffer); } +static bool +rb_irq_work_queue(struct rb_irq_work *irq_work) +{ + int cpu; + + /* irq_work_queue_on() is not NMI-safe */ + if (unlikely(in_nmi())) + return irq_work_queue(&irq_work->work); + + /* + * If CPU isolation is not active, cpu is always the current + * CPU, and the following is equivallent to irq_work_queue(). + */ + cpu = housekeeping_any_cpu(HK_TYPE_KERNEL_NOISE); + return irq_work_queue_on(&irq_work->work, cpu); +} + static __always_inline void rb_wakeups(struct trace_buffer *buffer, struct ring_buffer_per_cpu *cpu_buffer) { if (buffer->irq_work.waiters_pending) { buffer->irq_work.waiters_pending = false; /* irq_work_queue() supplies it's own memory barriers */ - irq_work_queue(&buffer->irq_work.work); + rb_irq_work_queue(&buffer->irq_work); } if (cpu_buffer->irq_work.waiters_pending) { cpu_buffer->irq_work.waiters_pending = false; /* irq_work_queue() supplies it's own memory barriers */ - irq_work_queue(&cpu_buffer->irq_work.work); + rb_irq_work_queue(&cpu_buffer->irq_work); } if (cpu_buffer->last_pages_touch == local_read(&cpu_buffer->pages_touched)) @@ -4043,7 +4061,7 @@ rb_wakeups(struct trace_buffer *buffer, struct ring_buffer_per_cpu *cpu_buffer) cpu_buffer->irq_work.wakeup_full = true; cpu_buffer->irq_work.full_waiters_pending = false; /* irq_work_queue() supplies it's own memory barriers */ - irq_work_queue(&cpu_buffer->irq_work.work); + rb_irq_work_queue(&cpu_buffer->irq_work); } #ifdef CONFIG_RING_BUFFER_RECORD_RECURSION -- 2.52.0