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

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Message-ID: <20260106170405.425f469e@gandalf.local.home>
Date: Tue, 6 Jan 2026 17:04:05 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Petr Tesarik <ptesarik@...e.com>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Sebastian Andrzej Siewior
 <bigeasy@...utronix.de>, Clark Williams <clrkwllms@...nel.org>,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 linux-rt-devel@...ts.linux.dev
Subject: Re: [PATCH] ring-buffer: Use a housekeeping CPU to wake up waiters

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

LGTM,

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

-- Steve