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

[Tim Chen <tim.c.chen@linux.intel.com>]

On Mon, 2025-07-07 at 17:41 -0700, 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.
> 

Thanks for taking a look.

> 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? 
> 

_get_migrate_hint() could also be called from llc_balance(). At that time
we make a determination of whether we should do llc_balance() without knowing
which exact task we're going to move, but still observe the migration policy
that shouldn't cause too much imbalance.  So it may not be strictly tied to a task
in the current implementation.

> 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.

You're thinking of something like a prctl knob that will bias aggregation for
some process?  Wonder if Peter has some opinion on this.

> 
> Also with sysctl_llc_aggr_imb, do we really need SCHED_CACHE_WAKE?
> 

Actually we think that we can do without SCHED_CACHE_WAKE feature and rely only
on load balance SCHED_CACHE_LB.  But still keeping 

>  Does setting
> sysctl_llc_aggr_imb to 0 basically say no preference for either LLC, no?

Aggregation will tend to make utilization on the preferred LLC to be more
than the non-preferred one.  Parameter "sysctl_llc_aggr_imb" is the imbalance
allowed.  If we set this to 0, as long as the preferred LLC is not utilized
more than the source LLC, we could still aggregate towards the preferred LLC
and a preference could still be there.  

Tim

> 
> 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;
> > +}
> 
>