Re: [PATCH v2] sched: Initialize the vruntime of a new task when it is first enqueued

From: Mukesh Kumar Chaurasiya
Date: Mon Jul 01 2024 - 12:02:25 EST



On Thu, 27 Jun 2024 21:33:59 +0800, Zhang Qiao wrote:
> When creating a new task, we initialize vruntime of the newly task at
> sched_cgroup_fork(). However, the timing of executing this action is too
> early and may not be accurate.
>
> Because it uses current CPU to init the vruntime, but the new task
> actually runs on the cpu which be assigned at wake_up_new_task().
>
> [...]

I tried this on a 104 cpu ppc64le Power9 system

The experiment was to stress the cpus with 416 threads(4 * nproc)
and then run small commands like date, time, ls etc

With bpf i collected the time it took for the tasks from waking up
to switching on to the cpu.

I collected 3600 samples for each

The results are as follows:

With patch

@: count 3601, average 4609830, total 16599997845

@c: 3709
@latency:
[4K, 8K) 1 | |
[8K, 16K) 4 | |
[16K, 32K) 2270 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[32K, 64K) 45 |@ |
[64K, 128K) 3 | |
[128K, 256K) 141 |@@@ |
[256K, 512K) 24 | |
[512K, 1M) 7 | |
[1M, 2M) 8 | |
[2M, 4M) 38 | |
[4M, 8M) 226 |@@@@@ |
[8M, 16M) 321 |@@@@@@@ |
[16M, 32M) 501 |@@@@@@@@@@@ |
[32M, 64M) 12 | |

# Without Patch

@: count 3600, average 12786599, total 46031757085

@c: 3608
@latency:
[4K, 8K) 1 | |
[8K, 16K) 6 | |
[16K, 32K) 297 |@@@@@@@@@@@ |
[32K, 64K) 25 | |
[64K, 128K) 4 | |
[128K, 256K) 149 |@@@@@ |
[256K, 512K) 21 | |
[512K, 1M) 5 | |
[1M, 2M) 14 | |
[2M, 4M) 45 |@ |
[4M, 8M) 857 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ |
[8M, 16M) 812 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ |
[16M, 32M) 1325 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[32M, 64M) 31 |@ |

The patch leads to a significant decrease in latency of a newly woken task.

Tested-by: Mukesh Kumar Chaurasiya <mchauras@xxxxxxxxxxxxx>
Reviewed-by: Mukesh Kumar Chaurasiya <mchauras@xxxxxxxxxxxxx>