linux-kernel - [RFC PATCH 0/3] sched/fair: Optimize cfs_rq and sched_entity allocation for better data locality

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Message-ID: <20250604195846.193159-1-zecheng@google.com>
Date: Wed,  4 Jun 2025 19:58:40 +0000
From: Zecheng Li <zecheng@...gle.com>
To: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, 
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Valentin Schneider <vschneid@...hat.com>, Xu Liu <xliuprof@...gle.com>, 
	Blake Jones <blakejones@...gle.com>, Josh Don <joshdon@...gle.com>, linux-kernel@...r.kernel.org, 
	Zecheng Li <zecheng@...gle.com>
Subject: [RFC PATCH 0/3] sched/fair: Optimize cfs_rq and sched_entity
 allocation for better data locality

Accessing cfs_rq and sched_entity instances incurs many cache misses.
This series of patches aims to reduce these cache misses. A struct
cfs_rq instance is per CPU and per task_group. Each task_group instance
(and the root runqueue) holds cfs_rq instances per CPU. Additionally,
there are corresponding struct sched_entity instances for each cfs_rq
instance (except the root). Currently, both cfs_rq and sched_entity
instances are allocated in NUMA-local memory using kzalloc_node, and
tg->cfs_rq and tg->se are arrays of pointers.

Original memory layout:

	tg->cfs_rq = kcalloc(nr_cpu_ids, sizeof(cfs_rq), GFP_KERNEL);
	tg->se = kcalloc(nr_cpu_ids, sizeof(se), GFP_KERNEL);

	+----+       +-----------------+
	| tg | ----> | cfs_rq pointers |
	+----+       +-----------------+
	                |     |     |
	                v     v     v
	            cfs_rq cfs_rq cfs_rq

	+----+       +--------------------+
	| tg | ----> | sched_entity ptrs  |
	+----+       +--------------------+
	                |     |     |
	                v     v     v
	                se    se    se

Layout after Optimization:

	+--------+      | CPU 0  |	| CPU 1  |	| CPU 2  |
	|   tg   |      | percpu |	| percpu |	| percpu |
	|        |         ...             ...             ...
	| percpu |  ->  | cfs_rq |	| cfs_rq |	| cfs_rq |
	| offset |      |   se   |	|   se   |	|   se   |
	+--------+      +--------+	+--------+	+--------+

The optimization includes two parts:

1) Embed sched_entity to cfs_rq.

- This benefits loading the sched_entity for the parent runqueue.
Currently it incurs pointer chasing, i.e., cfs_rq->tg->se[cpu]. After
embedding, the sched_entity fields can be loaded with simple offset
computations. As a tradeoff, the root task_group instance needs to
allocate memory for its sched_entity, which is not needed before
optimization. In the worst case, this will increase #CPU *
sizeof(sched_entity) RAM usage, which is small.

2) Allocate cfs_rq using percpu allocator.

- Accesses to cfs_rq instances in hot paths are mostly iterating through
multiple task_groups for the same CPU. Therefore, the new percpu layout
can reuse the base pointer, and they are more likely to reside in the
CPU cache than the per-task_group pointer arrays.

- This optimization also reduces the memory needed for the array of
pointers.

To measure the impact of the patch series, we construct a tree structure
hierarchy of cgroups, with “width” and “depth” parameters controlling
the number of children per node and the depth of the tree. Each leaf
cgroup runs a schbench workload and gets an 80% quota of the total CPU
quota divided by number of leaf cgroups (in other words, the target CPU
load is set to 80%) to exercise the throttling functions. Bandwidth
control period is set to 10ms. We run the benchmark on Intel and AMD
machines; each machine has hundreds of threads.

Tests were conducted on Kernel 6.15.

| Kernel LLC Misses | depth 3 width 10    | depth 5 width 4     |
+-------------------+---------------------+---------------------+
| AMD-orig          | [2218.98, 2241.89]M | [2599.80, 2645.16]M |
| AMD-opt           | [1957.62, 1981.55]M | [2380.47, 2431.86]M |
| Change            | -11.69%             | -8.248%             |
| Intel-orig        | [1580.53, 1604.90]M | [2125.37, 2208.68]M |
| Intel-opt         | [1066.94, 1100.19]M | [1543.77, 1570.83]M |
| Change            | -31.96%             | -28.13%             |

There's also a 25% improvement on kernel IPC on the AMD system. On
Intel, the improvement is 3% despite a greater LLC miss reduction.

Other workloads without CPU share limits, while also running in a cgroup
hierarchy with O(1000) instances, show no obvious regression:

sysbench, hackbench - lower is better; ebizzy - higher is better.

workload  | base                  | opt                   | metric
----------+-----------------------+-----------------------+------------
sysbench  | 63.55, [63.04, 64.05] | 64.36, [62.97, 65.75] | avg latency
hackbench | 36.95, [35.45, 38.45] | 37.12, [35.81, 38.44] | time
ebizzy    | 610.7, [569.8, 651.6] | 613.5, [592.1, 635.0] | record/s

Zecheng Li (3):
  sched/fair: Embed sched_entity into cfs_rq
  sched/fair: Remove task_group->se pointer
  sched/fair: Allocate cfs_rq structs per-cpu

 kernel/sched/core.c  | 40 ++++++++-------------
 kernel/sched/debug.c |  2 +-
 kernel/sched/fair.c  | 83 ++++++++++++++++----------------------------
 kernel/sched/sched.h | 40 ++++++++++++++++-----
 4 files changed, 76 insertions(+), 89 deletions(-)


base-commit: 0ff41df1cb268fc69e703a08a57ee14ae967d0ca
-- 
2.50.0