Re: [PATCH v3 0/7] sched: Flatten the pick

[Shubhang Kaushik]

Hello Peter,

I applied the `sched/flat` patchset from your tree on top of the
`tip/sched/core` base commit (9ebe5c3c29f62)/(7.1-rc2)

The evaluation was performed on an 80-core, Ampere Altra
system running Fedora Linux 41.

Benchmark Runs:-

1. Hackbench (Execution time in seconds: lower is better)
    The data reveals a clear architectural pivot point at 4 tasks:
    - Low Concurrency (< 4 tasks): Regresses by +1.8% to +4.0%.
      Removing cgroup isolation boundaries expands the idle CPU search
      adding slight overhead to the wake-up path.
      * 1 Thread:  (+1.8%)
      * 2 Threads: (+4.0%)
      * 2 Procs:   (+3.3%)
    - Tipping Point (4 tasks): Performance is completely flat.
      * 4 Threads: (+0.03%)
      * 4 Procs:   (+0.1%)
    - High Concurrency (>= 8 tasks): Improves by -0.7% to -2.3%.
      Collapsing the tree structure down to a flat layout removes
      multi-layer load tracking updates (update_load_avg), saving cycles
      under load.
      * 8 Threads:  (-0.7%)
      * 16 Threads: (-1.8%)
      * 8 Procs:    (-1.2%)
      * 16 Procs:   (-2.3%)
      * 32 Procs:   (-1.6%)

2. Schbench (Wakeup Tail Latency)
    - 16 Threads (128kb footprint): 99.9th percentile tail latency drops
      significantly by -12.21% (us). Operating on a unified runqueue layer
      prevents induced group-level throttling.
    - 32 Threads (128kb footprint): 99.9th percentile tail latency
      regresses by +5.50% (us). Eliminating nested queues increases lock
      contention during heavy simultaneous wakeups.

3. Sysbench
    - Sysbench RAM: Throughput increases by +1.55% (MiB/sec). Fewer tree
      traversals reduce cache-line bouncing, freeing up cycles.

The patchset trades minor low-load performance for better scaling and
tighter tail latencies under distributed load. However, the majority of
these deltas remain small and sit near the measurement noise floor (<=
4%).

Regards,
Shubhang Kaushik

On Fri, 5 Jun 2026, Peter Zijlstra wrote:

> Hi!
>
> New version, same story [1]. TL;DR:
>
> - Adds new cgroup_mode knob and implements new policies to address the
>   hierarchy level weight mismatch.
>
> - Builds upon that base to create a flat / single runqueue scheduler where the
>   cgroup hierarchy is expressed through dynamic weight management.
>
> I'm hoping to be able to merge these patches early in the next cycle (after
> 7.2-rc1).
>
> Random benchmark:
>
> Game vs 'for ((i=0; i<8; i++)) do nice ./spin.sh; done':
>
>  Lutris / GE-Proton10-34 / Steam Runtime 3 (sniper)
>  Intel Core i7-2600K
>  AMD Radeon RX 580
>
>  Shadows Awakening (GOG)
>
> 	  default slice(*)
>
>  FPS min   4.0   29.0
>      avg  47.5   59.2
>      max  83.7   83.7
>
>  FT  min   9.3   10.2
>      avg  34.0   17.0
>      max 121.2   30.0
>
>  FPS (Frames Per Second)
>  FT  (FrameTime)
>
>  [*] Command prefix: 'chrt -o --sched-runtime 100000 0'
>
>
> Changes since v2:
>
> - merged debug and prep patches
> - fixed update_entity_lag() on dequeue (Vincent)
> - fixed throttle vs tick (Prateek)
> - fixed wakeup_preempt_fair()
> - rebased on tip/sched/core
> - rewritten cgroup_mode changelogs
> - reworked cgroup_mode concur
> - added cgroup_mode tasks
> - changed default cgroup_mode
>
>
> [1] - https://lore.kernel.org/r/20260511113104.563854162@xxxxxxxxxxxxx
>
> Can also be had:
>
>  git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git sched/flat
>
> include/linux/cpuset.h |    6
> include/linux/sched.h  |    1
> kernel/cgroup/cpuset.c |   15
> kernel/sched/core.c    |    5
> kernel/sched/debug.c   |   89 ++++
> kernel/sched/fair.c    |  943 ++++++++++++++++++++++++-------------------------
> kernel/sched/pelt.c    |    6
> kernel/sched/sched.h   |   30 -
> 8 files changed, 607 insertions(+), 488 deletions(-)