Re: BUG Report: Fork benchmark drop by 30% on aarch64

[Dietmar Eggemann]

Hi Hagar,

On 05/02/2025 16:10, Hagar Hemdan wrote:
> Hi,
> 
> There is about a 30% drop in fork benchmark [1] on aarch64 and a 10%
> drop on x86_64 using kernel v6.13.1.
> 
> Git bisect pointed to commit eff6c8ce8d4d ("sched/core: Reduce cost
> of sched_move_task when config autogroup") which merged starting
> v6.4-rc1.
> 
> The regression only happens when number of CPUs is equal to number
> of threads [2] that fork test is creating which means it's only visible
> under CPU contention.
> 
> I used m6g.xlarge AWS EC2 Instance with 4 vCPUs and 16 GiB RAM for ARM64
> and m6a.xlarge with also 4 vCPUs and 16 GiB RAM for x86_64.
> 
> I noticed this regression exists only when autogroup config is enabled.

So '# CONFIG_SCHED_AUTOGROUP is not set' in .config so we have: 

static inline void sched_autogroup_exit_task(struct task_struct *p) { }

I.e. doing a 'echo 0 > /proc/sys/kernel/sched_autogroup_enabled' still
shows this issue?

> 
> Run the fork test with these combinations and autogroup is enabled:
> 
> Arch      | commit eff6c8ce8d4d | Fork Result (lps)  |  %Cpu(s)
> ----------+---------------------+--------------------+------------------
> aarch64   | without             | 28677.0            |  3.2 us, 96.7 sy
> aarch64   | with                | 19860.7 (30% drop) |  2.7 us, 79.4 sy
> x86_64    | without             | 27776.2            |  3.1 us, 96.9 sy
> x86_64    | with                | 25020.6 (10% drop) |  4.1 us, 93.2 sy
> ----------+---------------------+--------------------+------------------

Can you rerun with:

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3e5a6bf587f9..62cc50c79a78 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -9057,7 +9057,7 @@ void sched_move_task(struct task_struct *tsk)
         * group changes.
         */
        group = sched_get_task_group(tsk);
-       if (group == tsk->sched_task_group)
+       if ((group == tsk->sched_task_group) && !(tsk->flags & PF_EXITING))
                return;

> 
> It seems that the commit is capping the amount of CPU resources that can
> be utilized leaving around 18% idle in case of aarch64 and 3% idle in
> x86_64 case which is likely the main reason behind the reported fork
> regression.
> 
> When autogroup is disabled:
> 
> Arch      | commit eff6c8ce8d4d | Fork Result (lps)  |  %Cpu(s)
> ----------+---------------------+--------------------+------------------
> aarch64   | without             | 19877.8            |  2.2 us, 80.1 sy  
> aarch64   | with                | 20086.3 (~same)    |  1.9 us, 80.2 sy
> x86_64    | without             | 24974.2            |  4.9 us, 92.5 sy 
> x86_64    | with                | 24921.5 (~same)    |  4.9 us, 92.4 sy
> ----------+---------------------+--------------------+------------------
> 
> So when autogroup disabled, I still see the amount of idle CPU resources 
> 18%, 3% on aarch64 and x86_64 regardless of commit.
> 
> Is this performance drop an expected of this commit when autogroup is
> enabled?
> 
> Thanks,
> Hagar
> 
> [1] https://github.com/kdlucas/byte-unixbench/blob/master/UnixBench
> [2] Used command: ./Run -c 4 spawn
>