Re: [PATCH 0/5] sched/deadline: Fix GRUB accounting

[Marcel Ziswiler]

Hi Juri

On Fri, 2025-06-27 at 13:51 +0200, Juri Lelli wrote:
> Hi All,
> 
> This patch series addresses a significant regression observed in
> `SCHED_DEADLINE` performance, specifically when `SCHED_FLAG_RECLAIM`
> (Greedy Reclamation of Unused Bandwidth - GRUB) is enabled alongside
> overrunning jobs. This issue was reported by Marcel [1].
> 
> Marcel's team extensive real-time scheduler (`SCHED_DEADLINE`) tests on
> mainline Linux kernels (amd64-based Intel NUCs and aarch64-based RADXA
> ROCK5Bs) typically show zero deadline misses for 5ms granularity tasks.
> However, with reclaim mode enabled and the same two overrunning jobs in
> the mix, they observed a dramatic increase in deadline misses: 43
> million on NUC and 600 thousand on ROCK55B. This highlights a critical
> accounting issue within `SCHED_DEADLINE` when reclaim is active.
> 
> This series fixes the issue by doing the following.
> 
> - 1/5: sched/deadline: Initialize dl_servers after SMP
>   Currently, `dl-servers` are initialized too early during boot, before
>   all CPUs are online. This results in an incorrect calculation of
>   per-runqueue `DEADLINE` variables, such as `extra_bw`, which rely on a
>   stable CPU count. This patch moves the `dl-server` initialization to a
>   later stage, after SMP initialization, ensuring all CPUs are online and
>   correct `extra_bw` values can be computed from the start.
> 
> - 2/5: sched/deadline: Reset extra_bw to max_bw when clearing root domains
>   The `dl_clear_root_domain()` function was found to not properly account
>   for the fact that per-runqueue `extra_bw` variables retained stale
>   values computed before root domain changes. This led to broken
>   accounting. This patch fixes the issue by resetting `extra_bw` to
>   `max_bw` before restoring `dl-server` contributions, ensuring a clean
>   state.
> 
> - 3/5: sched/deadline: Fix accounting after global limits change
>   Changes to global `SCHED_DEADLINE` limits (handled by
>   `sched_rt_handler()` logic) were found to leave stale or incorrect
>   values in various accounting-related variables, including `extra_bw`.
>   This patch properly cleans up per-runqueue variables before implementing
>   the global limit change and then rebuilds the scheduling domains. This
>   ensures that the accounting is correctly restored and maintained after
>   such global limit adjustments.
> 
> - 4/5 and 5/5 are simple drgn scripts I put together to help debugging
>   this issue. I have the impression that they might be useful to have
>   around for the future.
> 
> Please review and test.

Over the weekend I run 312 mio. test runs on NUC and 231 mio. on ROCK55B without any single deadline misses.
Therefore,

for the whole series:

Tested-by: Marcel Ziswiler <marcel.ziswiler@xxxxxxxxxxxxxxx> # nuc & rock5b

Thanks!

> The set is also availabe at
> 
> git@xxxxxxxxxx:jlelli/linux.git upstream/fix-grub-tip
> 
> 1 - https://lore.kernel.org/lkml/ce8469c4fb2f3e2ada74add22cce4bfe61fd5bab.camel@xxxxxxxxxxxxxxx/
> 
> Thanks,
> Juri
> 
> Juri Lelli (5):
>   sched/deadline: Initialize dl_servers after SMP
>   sched/deadline: Reset extra_bw to max_bw when clearing root domains
>   sched/deadline: Fix accounting after global limits change
>   tools/sched: Add root_domains_dump.py which dumps root domains info
>   tools/sched: Add dl_bw_dump.py for printing bandwidth accounting info
> 
>  MAINTAINERS                      |  1 +
>  kernel/sched/core.c              |  2 +
>  kernel/sched/deadline.c          | 61 +++++++++++++++++++---------
>  kernel/sched/rt.c                |  6 +++
>  kernel/sched/sched.h             |  1 +
>  tools/sched/dl_bw_dump.py        | 57 ++++++++++++++++++++++++++
>  tools/sched/root_domains_dump.py | 68 ++++++++++++++++++++++++++++++++
>  7 files changed, 177 insertions(+), 19 deletions(-)
>  create mode 100755 tools/sched/dl_bw_dump.py
>  create mode 100755 tools/sched/root_domains_dump.py

Cheers

Marcel