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

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Message-ID: <aJDmzxq1mB_DxIFV@jlelli-thinkpadt14gen4.remote.csb>
Date: Mon, 4 Aug 2025 17:58:55 +0100
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

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