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

Steven Rostedt
2026-01-06T22:03:49Z

On Tue, 6 Jan 2026 10:10:39 +0100
Petr Tesarik <ptesarik@suse.com> wrote:

> 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

I take it that even with this patch you would still get the above events.
The only difference would be the "cpu=" in the event info will not be the
same as the CPU it executed on, right?

>
> 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>

LGTM,

I'll queue it up for the next merge window.

-- Steve