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

["Chen, Yu C" <yu.c.chen@intel.com>]

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@intel.com>
>>
>> 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