Re: [Patch v4 13/16] sched/cache: Fix cache aware scheduling enabling for multi LLCs system

[Chen, Yu C]

On 5/18/2026 11:06 PM, Peter Zijlstra wrote:
> On Wed, May 13, 2026 at 01:39:24PM -0700, Tim Chen wrote:
> > From: Chen Yu <yu.c.chen@xxxxxxxxx>
> >
> > If there are multiple LLCs in the system, cache aware scheduling
> > should be enabled. However, there is a corner case where, if there
> > is a single NUMA node and a single LLC per node, cache aware
> > scheduling will be turned on in the current implementation -
> > because at this moment, the parent domain has not yet been
> > degenerated, and it is possible that the current domain has the
> > same cpu span as its parent. There is no need to turn cache aware
> > scheduling on in this scenario.
> >
> > Fix it by iterating the parent domains to find a domain that is
> > a superset of the current sd_llc, so that later, after the duplicated
> > parent domains have been degenerated, cache aware scheduling will
> > take effect.
> >
> > For example, the expected behavior would be:
> > 2 sockets, 1 LLC per socket: MC span=0-3, PKG span=0-7, has_multi_llcs=true
> > 1 socket, 2 LLCs per socket: MC span=0-3, PKG span=0-7, has_multi_llcs=true
> > 2 sockets, 2 LLCs per socket: MC span=0-3, PKG span=0-7, has_multi_llcs=true
> > 1 socket, 1 LLC per socket: MC span=0-3, PKG span=0-3, has_multi_llcs=false
> >
> > This bug was reported by sashiko.
> >
> > Fixes: d59f4fd1d303 ("sched/cache: Enable cache aware scheduling for
> > multi LLCs NUMA node")
>
> FWIW, this and the next patch have this Fixes: line mangled -- I fixed
> it. But please figure out what did that so it doesn't happen again.

Thanks for fixing it. We will pay attention to this in the next version.

thanks,
Chenyu