Re: [PATCH] sched/cache: Check reduced capacity before alb_break_llc()

[Chen Yu]

On Thu, Oct 29, 2026 at 10:13:10AM +0300, Kayra Cizmeci wrote:
> On Thu, Oct 29, 2026 at 10:36:50AM +0800, Zhan Xusheng wrote:
> 
> Hi Zhan,
> 
> > From: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>
> > 
> > need_active_balance() moves the only CFS task off a CPU whose capacity
> > is reduced, for example by IRQ load, to an idle CPU with more.
> > alb_break_llc() runs first and refuses to move a lone task off its
> > preferred LLC, so if no other CPU in that LLC is idle the task stays on
> > the reduced CPU.  Like asym packing, which was moved ahead of
> > alb_break_llc() for the same reason, this is a performance case that
> > cache locality should not override.
> > 
> > With two LLCs of two CPUs, the other CPU of the task's LLC busy and 60%
> > of the task's CPU spent in a hardirq, a CPU-bound two-thread process
> > stayed there for 20 seconds at 38% of its normal throughput.  With this
> > change it moves to the idle LLC within a second, as it does with cache
> > aware scheduling disabled.
> 
> Sweet.
> 
> > 
> > Fixes: 714059f79ff0 ("sched/cache: Handle moving single tasks to/from their preferred LLC")
> > Signed-off-by: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>
> > ---
> >  kernel/sched/fair.c | 12 ++++++------
> >  1 file changed, 6 insertions(+), 6 deletions(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index cfc982205b98..e313c6d212c6 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -13585,12 +13585,6 @@ static int need_active_balance(struct lb_env *env)
> >  	if (asym_active_balance(env))
> >  		return 1;
> >  
> > -	if (alb_break_llc(env))
> > -		return 0;
> > -
> > -	if (imbalanced_active_balance(env))
> > -		return 1;
> > -
> >  	/*
> >  	 * The dst_cpu is idle and the src_cpu CPU has only 1 CFS task.
> >  	 * It's worth migrating the task if the src_cpu's capacity is reduced
> > @@ -13604,6 +13598,12 @@ static int need_active_balance(struct lb_env *env)
> >  			return 1;
> >  	}
> >  
> > +	if (alb_break_llc(env))
> > +		return 0;
> > +
> > +	if (imbalanced_active_balance(env))
> > +		return 1;
> > +
> >  	if (env->migration_type == migrate_misfit ||
> >  	    env->migration_type == migrate_llc_task)
> >  		return 1;
> > 
> > base-commit: 65ba8f34ec886ec72b2c07ddc8cf3d815c32869d
> > -- 
> > 2.43.0
> > 
> 
> Well yeah. alb_break_llc() just returns true in the nr_running <= 1
> case. FWIW, on my tip tree this just creates a conflict. On me
> alb_break_llc() is first, then asym_active_balance() and then
> imbalanced_active_balance(). I think in that case moving all three
> functions would be better? (below this other block that's for this case)
>

The position of asym has been adjusted in
https://lore.kernel.org/lkml/179284198232.4109677.752653731564503349.tip-bot2@tip-bot2/

thanks,
Chenyu