Re: [PATCH 06/19] sched/fair: Assign preferred LLC ID to processes

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

On 10/15/2025 7:15 PM, Peter Zijlstra wrote:
> On Tue, Oct 14, 2025 at 01:16:16PM +0800, Chen, Yu C wrote:
> 
>> The question becomes: how can we figure out the threads that share
>> data? Can the kernel detect this, or get the hint from user space?
> 
> This needs the PMU, then you can steer using cache-miss ratios. But then
> people will hate us for using counters.
> 
>> Yes, the numa_group in NUMA load balancing indicates
>> that several tasks manipulate the same page, which could be an
>> indicator. Besides, if task A frequently wakes up task B, does it
>> mean A and B have the potential to share data? Furthermore, if
>> task A wakes up B via a pipe, it might also indicate that A has
>> something to share with B. I just wonder if we can introduce a
>> structure to gather this information together.
> 
> The wakeup or pipe relation might be small relative to the working set.
> Consider a sharded in memory database, where the query comes in through
> the pipe/socket/wakeup. This query is small, but then it needs to go
> trawl through its memory to find the answer.
> 
> Something we *could* look at -- later -- is an interface to create
> thread groups, such that userspace that is clever enough can communicate
> this. But then there is the ago old question, will there be sufficient
> users to justify the maintenance of said interface.

OK, that could be an enhancement in the future if we encounter more
use cases.

thanks,
Chenyu