On Tue, 6 Jan 2026 17:04:05 -0500 Steven Rostedt <[email protected]> wrote:
> On Tue, 6 Jan 2026 10:10:39 +0100 > Petr Tesarik <[email protected]> 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? Yes, this is trace of a similar event after applying the patch: <idle>-0 [002] d.h4. 313.334367: ipi_send_cpu: cpu=1 callsite=irq_work_queue_on+0x55/0x90 callback=generic_smp_call_function_single_interrupt+0x0/0x20 <idle>-0 [002] d.h4. 313.334390: <stack trace> => trace_event_raw_event_ipi_send_cpu => __smp_call_single_queue => irq_work_queue_on => 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 callback function in the trace event is different. That's because send_call_function_single_ipi() always uses this value. Maybe it can be improved, and I can look into it, but that's clearly a very separate issue. > > 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 <[email protected]> > > LGTM, > > I'll queue it up for the next merge window. Thank you! Petr T
