Re: [REGRESSION] [PATCH v3 7/7] sched/eevdf: Move to a single runqueue

[Chen Yu]

On Wed, Sep 23, 2026 at 07:51:35PM +0530, Aishwarya Rambhadran wrote:
> Hi Peter,
> 
> We have observed some schbench p99 request latency regressions while
> comparing v7.2 against v7.3 cycle on an AWS Graviton3 (m7g.metal)
> using Fastpath (Linux kernel performance benchmarking tool).
> 
> The schbench suite as a whole does not show a significant regression,
> and the results appear to be a mix of improvements and regressions
> depending on the thread configuration. However, a few configurations
> show fairly large p99 request latency regressions. For example,
> comparing v7.2 against v7.3-rc4:
> 
> message-thread:16, worker-thread:16
> v7.2 = 662869 us
> v7.3-rc4 = 969557 us
> change% = -31.63%
> 
> message-thread:64, worker-thread:4
> v7.2 = 730453 us
> v7.3-rc4 = 942251 us
> change% = -22.48%
> 
> message-thread:32, worker-thread:4
> v7.2 = 74251 us
> v7.3-rc4 = 56725 us
> change% = +30.90%
> 
> 
> So this does not appear to be a general latency degradation; some
> configs improve while others regress.
> 
> I also ran a Fastpath automated git bisection for the m 16, t 16 p99
> request latency regression between v7.2 and v7.3-rc1, which identified
> this commit as the first bad commit. The original regression was ~30.9%,
> & the bisection reproducibility check showed ~27.8% regression with 20
> repeats across 2 boot sessions.
> 
> This commit looks relevant since it changes EEVDF runqueue organization
> and is intended, in part, to address latency issues with hierarchical fair/
> cgroup scheduling. I also noticed that there have been fixes on top of this
> change, including 68e37487810a ("sched/fair: Fix flat hierarchy"), which
> fixes vruntime handling and has a Fixes: 85570f10a4c6 tag.
> 
> Given that the overall schbench result is mostly neutral while some p99
> configurations move considerably in both directions, I wanted to check
> whether this could be an expected trade-off from the single-runqueue
> change, or whether the p99 regressions are worth investigating further.
> 
> I can run additional tests or provide the full results/bisection data if
> useful.
> 
>

Would disabling WA_WEIGHT help? I happened to find that this workaround works
for me to restore netperf performance (though it is a throughput score rather
than a latency one), it seems that task_h_load() becomes much
larger(also on sched/task_h_load):
echo NO_WA_WEIGHT > /sys/kernel/debug/sched/features
https://lore.kernel.org/lkml/aoxah90s0bQ4tcUW@three-body/ 

thanks,
Chenyu