Re: [RESEND PATCH] sched/fair: Skip sched_balance_running cmpxchg when balance is not due

[Shrikanth Hegde]

On 10/3/25 4:30 AM, Tim Chen wrote:
> Repost comments:
>
> There have been past discussions about avoiding serialization in load
> balancing, but no objections were raised to this patch itself during
> its last posting:
> https://lore.kernel.org/lkml/20250416035823.1846307-1-tim.c.chen@xxxxxxxxxxxxxxx/
>
> Vincent and Chen Yu have already provided their Reviewed-by tags.
>
> We recently encountered this issue again on a 2-socket, 240-core
> Clearwater Forest server running SPECjbb. In this case, 14% of CPU
> cycles were wasted on unnecessary acquisitions of
> sched_balance_running. This reinforces the need for the change, and we
> hope it can be merged.
>
> Tim
>
> ---
>
> During load balancing, balancing at the LLC level and above must be
> serialized. The scheduler currently checks the atomic
> `sched_balance_running` flag before verifying whether a balance is
> actually due. This causes high contention, as multiple CPUs may attempt
> to acquire the flag concurrently.
>
> On a 2-socket Granite Rapids system with sub-NUMA clustering enabled
> and running OLTP workloads, 7.6% of CPU cycles were spent on cmpxchg
> operations for `sched_balance_running`. In most cases, the attempt
> aborts immediately after acquisition because the load balance time is
> not yet due.
>
> Fix this by checking whether a balance is due *before* trying to
> acquire `sched_balance_running`. This avoids many wasted acquisitions
> and reduces the cmpxchg overhead in `sched_balance_domain()` from 7.6%
> to 0.05%. As a result, OLTP throughput improves by 11%.
>
> Reviewed-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> ---

Hi Tim.

Fine by me. unnecessary atomic operations do hurt on large systems.
The further optimization that i pointed out can come in later i guess.
That would help only further. this should be good to begin with.

With that.
Reviewed-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>