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

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Message-ID: <aIsUwT1Ai0zcMRpT@jlelli-thinkpadt14gen4.remote.csb>
Date: Thu, 31 Jul 2025 09:01:21 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: David Haufe <dhaufe@...plextrading.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

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