[PATCH] ring-buffer: Use a housekeeping CPU to wake up waiters

Petr Tesarik
2026-01-06T09:10:52Z

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:

<idle>-0 [002] d.h4. 1255.379293: ipi_send_cpu: cpu=2 callsite=irq_work_queue+0x2d/0x50 callback=rb_wake_up_waiters+0x0/0x80
<idle>-0 [002] d.h4. 1255.379329: <stack trace>
=> 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.

Signed-off-by: Petr Tesarik <ptesarik@suse.com>
---
kernel/trace/ring_buffer.c | 19 ++++++++++++++++---
1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 41c9f5d079beb..ed9160599091d 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -4,6 +4,7 @@
*
* Copyright (C) 2008 Steven Rostedt <srostedt@redhat.com>
*/
+#include <linux/sched/isolation.h>
#include <linux/trace_recursion.h>
#include <linux/trace_events.h>
#include <linux/ring_buffer.h>
@@ -4011,19 +4012,31 @@ static void rb_commit(struct ring_buffer_per_cpu *cpu_buffer)
rb_end_commit(cpu_buffer);
}

+static inline bool
+rb_irq_work_queue(struct rb_irq_work *irq_work)
+{
+ int cpu = housekeeping_any_cpu(HK_TYPE_KERNEL_NOISE);
+
+ /*
+ * If CPU isolation is not active, cpu is always the current
+ * CPU, and the following is equivallent to irq_work_queue().
+ */
+ 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 +4056,7 @@ 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