Re: [PATCH v9 06/11] sched/core: Push current task from non preferred CPU

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Hi Yury,

On 7/25/26 3:34 AM, Yury Norov wrote:
> On Fri, Jul 24, 2026 at 07:37:27PM +0530, Shrikanth Hegde wrote:
>> Actively push out task running on a non-preferred CPU. Since the task is
>> running on the CPU, need to stop the cpu and push the task out.
>> However, if the task is pinned only to non-preferred CPUs, it will continue
>> running there. This will help in maintaining the userspace affinities
>> unlike CPU hotplug or isolated cpusets.
>>
>> Though code is similar to  __balance_push_cpu_stop and quite close to
>> push_cpu_stop, it is being kept separate as it provides a cleaner
>> implementation with CONFIG_PREFERRED_CPU.
>>
>> Add push_task_work_done flag to protect work buffer.
>> Works only with FAIR class.
>>
>> For now, only current running task is pushed out. This keeps the code
>> simpler. In future optimization maybe done to move all the queued
>> task on the rq.
>>
>> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
>> ---
>>   kernel/sched/core.c  | 78 ++++++++++++++++++++++++++++++++++++++++++++
>>   kernel/sched/sched.h |  8 +++++
>>   2 files changed, 86 insertions(+)
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 9e8eec4451b6..704043531b24 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -5774,6 +5774,9 @@ void sched_tick(void)
>>   	unsigned long hw_pressure;
>>   	u64 resched_latency;
>>   
>> +	if (!cpu_preferred(cpu))
>> +		sched_push_current_non_preferred_cpu(rq);
>> +
>>   	if (housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE))
>>   		arch_scale_freq_tick();
>>   
>> @@ -11292,3 +11295,78 @@ void sched_change_end(struct sched_change_ctx *ctx)
>>   		p->sched_class->prio_changed(rq, p, ctx->prio);
>>   	}
>>   }
>> +
>> +#ifdef CONFIG_PREFERRED_CPU
>> +static DEFINE_PER_CPU(struct cpu_stop_work, npc_push_task_work);
>> +
>> +static int sched_non_preferred_cpu_push_stop(void *arg)
>> +{
>> +	struct task_struct *p = arg;
>> +	struct rq *rq = this_rq();
>> +	struct rq_flags rf;
>> +	int cpu;
>> +
>> +	if (cpu_preferred(rq->cpu)) {
>> +		scoped_guard(rq_lock, rq)
>> +			rq->push_task_work_done = false;
>> +		put_task_struct(p);
>> +		return 0;
>> +	}
>> +
>> +	raw_spin_lock_irq(&p->pi_lock);
>> +
>> +	/* This could take rq lock. So call it before rq lock is taken */
>> +	cpu = select_fallback_rq(rq->cpu, p);
>> +	rq_lock(rq, &rf);
>> +	rq->push_task_work_done = false;
>> +	update_rq_clock(rq);
>> +
>> +	context_unsafe_alias(rq);
>> +
>> +	if (task_rq(p) == rq && task_on_rq_queued(p) &&
>> +	    !is_migration_disabled(p))
>> +		rq = __migrate_task(rq, &rf, p, cpu);
>> +
>> +	rq_unlock(rq, &rf);
>> +	raw_spin_unlock_irq(&p->pi_lock);
>> +	put_task_struct(p);
>> +
>> +	return 0;
> 
> You always return 0, and don't test the return value. Just make it
> void, or return (and handle) some error, please.
> 

Currently all the function callbacks of stop_one_cpu_nowait return 0.
This can't be changed to void today since signature mandates int.

typedef int (*cpu_stop_fn_t)(void *arg);

Also even if return value is of some error, it doesn't make any difference.
This is because in cpu_stopper_thread, return value is propagated only if it
wait for work to complete semantic. stop_one_cpu_nowait sets done=NULL.

bool stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
                         struct cpu_stop_work *work_buf)
{
         *work_buf = (struct cpu_stop_work){ .fn = fn, .arg = arg, .caller = _RET_IP_, };
         return cpu_stop_queue_work(cpu, work_buf);
}

cpu_stopper_thread:
                 ret = fn(arg);
                 if (done) {
                         if (ret)
                                 done->ret = ret;
                         cpu_stop_signal_done(done);
                 }

If we really need to return void then we need a new function signature for nowait
variant. Adding separate function signature just for nowait isn't probably worth.
What do you think?