Re: [PATCH v2 4/4] sched/rt: Split cpupri_vec->cpumask to per NUMA node to reduce contention

[Chen, Yu C]

On 3/24/2026 8:00 PM, Peter Zijlstra wrote:
> On Mon, Mar 23, 2026 at 11:45:01AM -0700, Tim Chen wrote:
> > On Fri, 2026-03-20 at 13:40 +0100, Peter Zijlstra wrote:
> > > On Mon, Jul 21, 2025 at 02:10:26PM +0800, Pan Deng wrote:
> > >
> > > > This change splits `cpupri_vec->cpumask` into per-NUMA-node data to
> > > > mitigate false sharing.
> > >
> > > So I really do think we need something here. We're running into the
> > > whole cpumask contention thing on a semi regular basis.

[ ... ]

> +
> +unsigned int sbm_find_next_bit(struct sbm *sbm, int start)
> +{
> +	struct sbm_leaf *leaf = (void *)sbm;
> +	struct sbm_root *root = (void *)sbm;
> +	int nr = start >> arch_sbm_shift;
> +	int bit = start & arch_sbm_mask;
> +	unsigned long tmp, mask = (~0UL) << bit;
> +	if (sbm->type == st_root) {
> +		for (; nr < arch_sbm_leafs; nr++, mask = ~0UL) {
> +			leaf = root->leafs[nr];
> +			tmp = leaf->bitmap & mask;
> +			if (!tmp)
> +				continue;

I suppose this should be
	if (tmp)
		break;
otherwise
[   40.071616] watchdog: BUG: soft lockup - CPU#0 stuck for 30s! 
[swapper/0:0]
[   40.071616] Modules linked in:
[   40.071616] CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 
7.0.0-rc5-sbm-+ #16 PREEMPT(full)
[   40.071616] RIP: 0010:sbm_find_next_bit+0x2a/0xa0

> +		}
> +	} else {
> +		tmp = leaf->bitmap & mask;
> +	}
> +	if (!tmp)
> +		return -1;
> +	return (nr << arch_sbm_shift) | __ffs(tmp);
> +}

update of the test:
With above change, I did a simple hackbench test on
a system with multiple LLCs within 1 node, so the benefit
is significant(+12%~+30%) when system is under-loaded, while
some regression when overloaded(-10%)(need to figure out)

thanks,
Chenyu