Re: [PATCH 08/10] sched_ext: idle: introduce SCX_PICK_IDLE_NODE

[Andrea Righi <arighi@nvidia.com>]

On Mon, Dec 23, 2024 at 07:53:21PM -0800, Yury Norov wrote:
> On Mon, Dec 23, 2024 at 06:48:48PM -0800, Yury Norov wrote:
> > On Fri, Dec 20, 2024 at 04:11:40PM +0100, Andrea Righi wrote:
> > > Introduce a flag to restrict the selection of an idle CPU to a specific
> > > NUMA node.
> > > 
> > > Signed-off-by: Andrea Righi <arighi@nvidia.com>
> > > ---
> > >  kernel/sched/ext.c      |  1 +
> > >  kernel/sched/ext_idle.c | 11 +++++++++--
> > >  2 files changed, 10 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> > > index 143938e935f1..da5c15bd3c56 100644
> > > --- a/kernel/sched/ext.c
> > > +++ b/kernel/sched/ext.c
> > > @@ -773,6 +773,7 @@ enum scx_deq_flags {
> > >  
> > >  enum scx_pick_idle_cpu_flags {
> > >  	SCX_PICK_IDLE_CORE	= 1LLU << 0,	/* pick a CPU whose SMT siblings are also idle */
> > > +	SCX_PICK_IDLE_NODE	= 1LLU << 1,	/* pick a CPU in the same target NUMA node */
> > 
> > SCX_FORCE_NODE or SCX_FIX_NODE?
> > 
> > >  };
> > >  
> > >  enum scx_kick_flags {
> > > diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
> > > index 444f2a15f1d4..013deaa08f12 100644
> > > --- a/kernel/sched/ext_idle.c
> > > +++ b/kernel/sched/ext_idle.c
> > > @@ -199,6 +199,12 @@ static s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, int node, u64 f
> 
> This function begins with:
> 
>  static s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, int node, u64 flags)
>  {
>       nodemask_t hop_nodes = NODE_MASK_NONE;
>       s32 cpu = -EBUSY;
>  
>       if (!static_branch_maybe(CONFIG_NUMA, &scx_builtin_idle_per_node))
>               return pick_idle_cpu_from_node(cpus_allowed, NUMA_FLAT_NODE, flags);
> 
>       ...
>  
> So if I disable scx_builtin_idle_per_node and then call:
> 
>         scx_pick_idle_cpu(some_cpus, numa_node_id(), SCX_PICK_IDLE_NODE)
> 
> I may get a CPU from any non-local node, right? I think we need to honor user's
> request:  
> 
>       if (!static_branch_maybe(CONFIG_NUMA, &scx_builtin_idle_per_node))
>               return pick_idle_cpu_from_node(cpus_allowed,
>                      flags & SCX_PICK_IDLE_NODE ? node :  NUMA_FLAT_NODE, flags);
> 
> That way the code will be coherent: if you enable idle cpumasks, you
> will be able to follow all the NUMA hierarchy. If you disable them, at
> least you honor user's request to return a CPU from a given node, if
> he's very explicit about his intention.
> 
> You can be even nicer:
> 
>       if (!static_branch_maybe(CONFIG_NUMA, &scx_builtin_idle_per_node)) {
>                 node = pick_idle_cpu_from_node(cpus, node, flags);
>                 if (node == MAX_NUM_NODES && flags & SCX_PICK_IDLE_NODE == 0)
>                         node = pick_idle_cpu_from_node(cpus, NUMA_FLAT_NODE, flags);
> 
>                 return node;
>       }
> 

Sorry, I'm not following, if scx_builtin_idle_per_node is disabled, we’re
only tracking idle CPUs in a single NUMA_FLAT_NODE (which is node 0). All
the other cpumasks are just empty, and we would always return -EBUSY if we
honor the user request.

Maybe we should just return an error if scx_builtin_idle_per_node is
disabled and the user is requesting an idle CPU in a specific node?

-Andrea