Re: [RFC patch v3 07/20] sched: Add helper function to decide whether to allow cache aware scheduling

[Libo Chen <libo.chen@oracle.com>]

On 7/8/25 01:29, Chen, Yu C wrote:
> On 7/8/2025 8:41 AM, Libo Chen wrote:
>> Hi Tim and Chenyu,
>>
>>
>> On 6/18/25 11:27, Tim Chen wrote:
>>> Cache-aware scheduling is designed to aggregate threads into their
>>> preferred LLC, either via the task wake up path or the load balancing
>>> path. One side effect is that when the preferred LLC is saturated,
>>> more threads will continue to be stacked on it, degrading the workload's
>>> latency. A strategy is needed to prevent this aggregation from going too
>>> far such that the preferred LLC is too overloaded.
>>>
>>> Introduce helper function _get_migrate_hint() to implement the LLC
>>> migration policy:
>>>
>>> 1) A task is aggregated to its preferred LLC if both source/dest LLC
>>>     are not too busy (<50% utilization, tunable), or the preferred
>>>     LLC will not be too out of balanced from the non preferred LLC
>>>     (>20% utilization, tunable, close to imbalance_pct of the LLC
>>>     domain).
>>> 2) Allow a task to be moved from the preferred LLC to the
>>>     non-preferred one if the non-preferred LLC will not be too out
>>>     of balanced from the preferred prompting an aggregation task
>>>     migration later.  We are still experimenting with the aggregation
>>>     and migration policy. Some other possibilities are policy based
>>>     on LLC's load or average number of tasks running.  Those could
>>>     be tried out by tweaking _get_migrate_hint().
>>>
>>> The function _get_migrate_hint() returns migration suggestions for the upper-le
>>> +__read_mostly unsigned int sysctl_llc_aggr_cap       = 50;
>>> +__read_mostly unsigned int sysctl_llc_aggr_imb       = 20;
>>> +
>>
>>
>> I think this patch has a great potential.
>>
>> Since _get_migrate_hint() is tied to an individual task anyway, why not add a
>> per-task llc_aggr_imb which defaults to the sysctl one? Tasks have different
>> preferences for llc stacking, they can all be running in the same system at the
>> same time. This way you can offer a greater deal of optimization without much
>> burden to others.
> 
> Yes, this doable. It can be evaluated after the global generic strategy
> has been verified to work, like NUMA balancing :)
> 

I will run some real-world workloads and get back to you (may take some time)

>>
>> Also with sysctl_llc_aggr_imb, do we really need SCHED_CACHE_WAKE? 
> 
> Do you mean the SCHED_CACHE_WAKE or SCHED_CACHE_LB?
> 

Ah I was thinking sysctl_llc_aggr_imb alone can help reduce overstacking on
target LLC from a few hyperactive wakees (may consider to ratelimit those
wakees as a solution), but just realize this can affect lb as well and doesn't
really reduce overheads from frequent wakeups (no good idea on top of my head
but we should find a better solution than sched_feat to address the overhead issue).



>> Does setting sysctl_llc_aggr_imb to 0 basically say no preference for either LLC, no?
>>
> 
> My understanding is that, if sysctl_llc_aggr_imb is 0, the task aggregation
> might still consider other aspects, like if that target LLC's utilization has
> exceeded 50% or not.
> 

which can be controlled by sysctl_llc_aggr_cap, right? Okay so if both LLCs have
<$(sysctl_llc_aggr_cap)% utilization, should sysctl_llc_aggr_cap be the only
determining factor here barring NUMA balancing?

Libo

> thanks,
> Chenyu> Thanks,
>> Libo
>>
>>> +static enum llc_mig_hint _get_migrate_hint(int src_cpu, int dst_cpu,
>>> +                       unsigned long tsk_util,
>>> +                       bool to_pref)
>>> +{
>>> +    unsigned long src_util, dst_util, src_cap, dst_cap;
>>> +
>>> +    if (cpus_share_cache(src_cpu, dst_cpu))
>>> +        return mig_allow;
>>> +
>>> +    if (!get_llc_stats(src_cpu, &src_util, &src_cap) ||
>>> +        !get_llc_stats(dst_cpu, &dst_util, &dst_cap))
>>> +        return mig_allow;
>>> +
>>> +    if (!fits_llc_capacity(dst_util, dst_cap) &&
>>> +        !fits_llc_capacity(src_util, src_cap))
>>> +        return mig_ignore;
>>> +
>>> +    src_util = src_util < tsk_util ? 0 : src_util - tsk_util;
>>> +    dst_util = dst_util + tsk_util;
>>> +    if (to_pref) {
>>> +        /*
>>> +         * sysctl_llc_aggr_imb is the imbalance allowed between
>>> +         * preferred LLC and non-preferred LLC.
>>> +         * Don't migrate if we will get preferred LLC too
>>> +         * heavily loaded and if the dest is much busier
>>> +         * than the src, in which case migration will
>>> +         * increase the imbalance too much.
>>> +         */
>>> +        if (!fits_llc_capacity(dst_util, dst_cap) &&
>>> +            util_greater(dst_util, src_util))
>>> +            return mig_forbid;
>>> +    } else {
>>> +        /*
>>> +         * Don't migrate if we will leave preferred LLC
>>> +         * too idle, or if this migration leads to the
>>> +         * non-preferred LLC falls within sysctl_aggr_imb percent
>>> +         * of preferred LLC, leading to migration again
>>> +         * back to preferred LLC.
>>> +         */
>>> +        if (fits_llc_capacity(src_util, src_cap) ||
>>> +            !util_greater(src_util, dst_util))
>>> +            return mig_forbid;
>>> +    }
>>> +    return mig_allow;
>>> +}
>>
>>