Re: [PATCH v4 4/4] sunrpc: eliminate a modulus operation from the enqueueing codepath

[NeilBrown]

On Thu, 02 Jul 2026, Jeff Layton wrote:
> Currently we do this to determine the pool to enqueue on:
> 
>     pidx = m->to_pool[cpu_to_node(raw_smp_processor_id())] % serv->sv_nrpools;
> 
> ...but a modulus is rather expensive. Replace this instead with an
> explicit check for running off the end of the array.
> 
> This situation should never occur, but if it does, just fall back to
> pool 0.
> 
> This trades a ~20-30 cycle operation that isn't pipelined and
> monopolizes the divider for a ~1 cycle well-predicted branch.

I would rather discard ->sv_nrpools as described in previous reply, then
this modulus would disappear as it isn't needed.

Thanks,
NeilBrown


> 
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> ---
>  net/sunrpc/svc.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
> index ae93a6f51087..ed841ea09079 100644
> --- a/net/sunrpc/svc.c
> +++ b/net/sunrpc/svc.c
> @@ -252,7 +252,9 @@ struct svc_pool *svc_pool_for_cpu(struct svc_serv *serv)
>  	if (serv->sv_nrpools <= 1)
>  		return serv->sv_pools;
>  
> -	pidx = m->to_pool[cpu_to_node(raw_smp_processor_id())] % serv->sv_nrpools;
> +	pidx = m->to_pool[cpu_to_node(raw_smp_processor_id())];
> +	if (pidx >= serv->sv_nrpools)
> +		pidx = 0;
>  
>  	/*
>  	 * Threads are spread evenly across the pools, but when there are
> 
> -- 
> 2.54.0
> 
>