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

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Message-ID: <CAKJHwtPnBGMGfWKwn3L6OiaXYDWn8KFhzQOQZQfPnXGF-wRT_g@mail.gmail.com>
Date: Mon, 24 Nov 2025 14:30:28 -0600
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

Hi Juri,
Working with 6.17.7 (code appears the same in .8 and 6.18rc), we are
still having isolated/nohz cores interrupted by deadline
functionality. The dl_task_timer is firing once a second with a single
SCHED_OTHER process spinning on the core. I am no longer seeing the
IPI interrupts, but the hrtime activity is still causing a performance
regression compared to kernels prior to the deadline merge.

Thanks,
Dave

On Mon, Aug 4, 2025 at 11:59 AM Juri Lelli <juri.lelli@...hat.com> wrote:
>
> On 04/08/25 10:44, David Haufe wrote:
> > My apologies, I see what you mean now. add_nr_running() is not being
> > invoked if it is the dl_server. We are still trying to get this branch
> > to boot to verify ourselves. We will be on the lookout for this to be
> > merged for release.
>
> No worries, guess I wasn't clear the first time. :)
>
> I added a very much experimental commit on
>
> https://github.com/jlelli/linux/tree/upstream/fix-dlserver-1
>
> that seems to be able to remove the one per second dl_server_timer and
> start it back as needed. But, I just played briefly with it, so I am not
> fully convinced is what we want. Anyway, if you could test with it as
> well it would be a useful data point. In principle you could try porting
> the following commits to your current tree and check if they do improve
> things (in reverse order starting from the bottom from the branch above):
>
> f237e524f3c7 ("sched/deadline: Make dl-server nohz full aware")
> 219a63335b67 ("sched/deadline: Don't count nr_running twice for dl_server proxy tasks")
> 7620177e8108 ("sched/deadline: Fix RT task potential starvation when expiry time passed")
> cccb45d7c429 ("sched/deadline: Less agressive dl_server handling")
>
> Thanks!
> Juri
>

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