Re: [PATCH 10/15] sched_ext: Add per-CPU data to DSQs

[Andrea Righi <arighi@nvidia.com>]

On Fri, Mar 06, 2026 at 11:54:15PM +0100, Andrea Righi wrote:
> Hi Tejun,
> 
> On Fri, Mar 06, 2026 at 09:06:18AM -1000, Tejun Heo wrote:
> > Add per-CPU data structure to dispatch queues. Each DSQ now has a percpu
> > scx_dsq_pcpu which contains a back-pointer to the DSQ. This will be used by
> > future changes to implement per-CPU reenqueue tracking for user DSQs.
> > 
> > init_dsq() now allocates the percpu data and can fail, so it returns an
> > error code. All callers are updated to handle failures. exit_dsq() is added
> > to free the percpu data and is called from all DSQ cleanup paths.
> > 
> > In scx_bpf_create_dsq(), init_dsq() is called before rcu_read_lock() since
> > alloc_percpu() requires GFP_KERNEL context, and dsq->sched is set
> > afterwards.
> > 
> > Signed-off-by: Tejun Heo <tj@kernel.org>
> > ---
> ...
> > @@ -5849,8 +5884,11 @@ static struct scx_sched *scx_alloc_and_add_sched(struct sched_ext_ops *ops,
> >  		goto err_free_pnode;
> >  	}
> >  
> > -	for_each_possible_cpu(cpu)
> > -		init_dsq(bypass_dsq(sch, cpu), SCX_DSQ_BYPASS, sch);
> > +	for_each_possible_cpu(cpu) {
> > +		ret = init_dsq(bypass_dsq(sch, cpu), SCX_DSQ_BYPASS, sch);
> > +		if (ret)
> > +			goto err_free_pcpu;
> 
> If we fail here...
> 
> > +	}
> >  
> >  	for_each_possible_cpu(cpu) {
> >  		struct scx_sched_pcpu *pcpu = per_cpu_ptr(sch->pcpu, cpu);
> > @@ -5932,6 +5970,10 @@ static struct scx_sched *scx_alloc_and_add_sched(struct sched_ext_ops *ops,
> >  err_stop_helper:
> >  	kthread_destroy_worker(sch->helper);
> >  err_free_pcpu:
> > +	for_each_possible_cpu(cpu) {
> > +		if (bypass_dsq(sch, cpu))
> > +			exit_dsq(bypass_dsq(sch, cpu));
> > +	}
> 
> ...we are still calling exit_dsq() for all the CPUs. We should probably
> call it only on the initialized ones, or we may touch uninitialized mem.

Moreover, bypass_dsq() is always non-NULL so we can remove the redundant
check if bypass_dsq().

Thanks,
-Andrea