Re: [PATCH 0/4] sched: Various reweight_entity() fixes

[Shubhang Kaushik <shubhang@os.amperecomputing.com>]

Hello Peter,

On Fri, 30 Jan 2026, Peter Zijlstra wrote:

> Two issues related to reweight_entity() were raised; poking at all that got me
> these patches.
>
> They're in queue.git/sched/core and I spend most of yesterday staring at traces
> trying to find anything wrong. So far, so good.
>
> Please test.
>
>

Analysis of commit bdba3187771c ("sched/fair: Use full weight to 
__calc_delta()") confirms it resolves a scaling bottleneck present in the 
v7.0-rc baseline (cee73b1e840c).

On Ampere Altra (80 cores) non-SMT SMP with CONFIG_NOHZ_FULL and 
CONFIG_SCHED_CLUSTER enabled, the baseline suffered from vruntime 
distortion that caused significant scheduling jitter, resulting in 
measurable performance cliffs under high parallel load.

Llama AIO for Inference Scaling : The baseline shows a performance cliffs 
at high thread counts, due to the scheduler miscalculating time slices 
for tail threads, which forced the entire synchronous inference batch to 
wait. The patch corrects the delta calculation for predictable scaling.

n_threads	Baseline (cee73b1e840c)	Patched (bdba3187771c)	Diff
51		8.46 t/s		12.12 t/s		+43.3%
52		17.75 t/s		22.26 t/s		+25.4%
64		12.40 t/s		14.66 t/s		+18.2%

Hackbench scaling in the baseline was non-linear indicating excessive 
scheduling overhead as process counts increased. By using the full weight 
for delta calculations, the patch reduces unnecessary migrations and 
improves message throughput.

Task Count	Baseline(sec)	Patched(sec)	Throughput
16 Processes	51.25s		44.09s		+16.2%
32 Processes	92.42s		78.09s		+18.3%

Tested-by: Shubhang Kaushik <shubhang@os.amperecomputing.com>

Regards,
Shubhang Kaushik