Re: [PATCH] sched: Skip useless sched_balance_running acquisition if load balance is not due

[Chen, Yu C]

On 4/16/2025 11:58 AM, Tim Chen wrote:
> At load balance time, balance of last level cache domains and
> above needs to be serialized. The scheduler checks the atomic var
> sched_balance_running first and then see if time is due for a load
> balance. This is an expensive operation as multiple CPUs can attempt
> sched_balance_running acquisition at the same time.
>
> On a 2 socket Granite Rapid systems enabling sub-numa cluster and
> running OLTP workloads, 7.6% of cpu cycles are spent on cmpxchg of
> sched_balance_running.  Most of the time, a balance attempt is aborted
> immediately after acquiring sched_balance_running as load balance time
> is not due.
>
> Instead, check balance due time first before acquiring
> sched_balance_running. This skips many useless acquisitions
> of sched_balance_running and knocks the 7.6% CPU overhead on
> sched_balance_domain() down to 0.05%.  Throughput of the OLTP workload
> improved by 11%.
>
> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Reported-by: Mohini Narkhede <mohini.narkhede@xxxxxxxxx>
> Tested-by: Mohini Narkhede <mohini.narkhede@xxxxxxxxx>
> ---

This change is straightforward(simple enough) to mitigate the
costly atomic operation, it looks good from my understanding, so:

Reviewed-by: Chen Yu yu.c.chen@xxxxxxxxx


thanks,
Chenyu