linux-kernel - Re: Kernel 6.14.11 dl_server_timer(...) causing IPI/Function Call Interrupts on isolcpu/nohz

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Message-ID: <CAKJHwtPyF_2xvdQb0b3b=6qsgZXrn5CQNA7pk5PKWJV63_TO1Q@mail.gmail.com>
Date: Thu, 31 Jul 2025 12:50:02 -0500
From: David Haufe <dhaufe@...plextrading.com>
To: Juri Lelli <juri.lelli@...hat.com>
Cc: linux-kernel@...r.kernel.org
Subject: Re: Kernel 6.14.11 dl_server_timer(...) causing IPI/Function Call
 Interrupts on isolcpu/nohz_full cores, performance regression

Kernel 6.16 shows the issue. /kernel/sched/fair.c calls
dl_server_start() and there is no assessment prior to that point or
later of the isolcpu/nohz_full+single-process condition of the core.
Same function_graph trace generated. Code is the same at
tip+sched/core.

On Thu, Jul 31, 2025 at 2:02 AM Juri Lelli <juri.lelli@...hat.com> wrote:
>
> Hello,
>
> Thanks for the report.
>
> On 30/07/25 11:51, David Haufe wrote:
> > [1.] Kernel 6.14.11 dl_server_timer(...) causing IPI/Function Call
> > Interrupts on isolcpu/nohz_full cores, performance regression
> > [2.] The code for dl_server_timer is causing new IPI/Function Call
> > Interrupts to fire on isolcpu/nohz_full cores which previously had no
> > interrupts. When there is a single, SCHED_OTHER process running on an
> > isolcpu/nohz_full core, dl_server_timer executes on a housekeeping
> > core. This ultimately invokes add_nr_running() and
> > sched_update_tick_dependency() and finally tick_nohz_dep_set_cpu().
> > Setting the single process running on an isolcpu/nohz_full core to
> > FIFO (rt priority) prevents this new interrupt, as it is not seen as a
> > fair schedule process anymore. Having to use rt priority is
> > unnecessary and a regression to prior kernels. Kernel function_graph
> > trace below showing core 0 (housekeeping) sending the IPI to core 19
> > (nohz_full, isolcpu, rcu_nocb_poll) which is running a single
> > SCHED_OTHER process. I believe this has been observed by others.
> > https://community.clearlinux.org/t/sysjitter-worse-in-kernel-6-12-than-6-6/10206
>
> Would you be able to check if the following branch, containing multiple
> fixes for dl-server, is still affected by the regression?
>
> Thanks,
> Juri
>

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