Re: [PATCH] scheduler: conditional statement cleanup

[Pierce Griffiths]

Peter,
Is there anything in this patch that you'd consider salvageable, or
would it be better to just throw the whole thing out? In either case, I
appreciate your honesty regarding this patch's (lack of) quality, and
apologize for what is most likely a waste of your time.

On Sat, Sep 22, 2018 at 12:26:40PM +0200, Peter Zijlstra wrote:
> On Fri, Sep 21, 2018 at 03:22:03PM -0500, PierceGriffiths wrote:
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 625bc9897f62..443a1f235cfd 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -617,12 +617,8 @@ bool sched_can_stop_tick(struct rq *rq)
> >  	 * If there are more than one RR tasks, we need the tick to effect the
> >  	 * actual RR behaviour.
> >  	 */
> > -	if (rq->rt.rr_nr_running) {
> > -		if (rq->rt.rr_nr_running == 1)
> > -			return true;
> > -		else
> > -			return false;
> > -	}
> > +	if (rq->rt.rr_nr_running)
> > +		return rq->rt.rr_nr_running == 1;
> >  
> >  	/*
> >  	 * If there's no RR tasks, but FIFO tasks, we can skip the tick, no
> 
> That one is OK I suppose.
> 
> > diff --git a/kernel/sched/cpufreq.c b/kernel/sched/cpufreq.c
> > index 5e54cbcae673..a8fd4bd68954 100644
> > --- a/kernel/sched/cpufreq.c
> > +++ b/kernel/sched/cpufreq.c
> > @@ -34,10 +34,7 @@ void cpufreq_add_update_util_hook(int cpu, struct update_util_data *data,
> >  			void (*func)(struct update_util_data *data, u64 time,
> >  				     unsigned int flags))
> >  {
> > -	if (WARN_ON(!data || !func))
> > -		return;
> > -
> > -	if (WARN_ON(per_cpu(cpufreq_update_util_data, cpu)))
> > +	if (WARN_ON(!data || !func || per_cpu(cpufreq_update_util_data, cpu)))
> >  		return;
> >  
> >  	data->func = func;
> 
> But I'm not a fan of this one. It mixes a different class of function
> and the WARN condition gets too complicated. Its easier to have separate
> warns.
> 
> > diff --git a/kernel/sched/cpupri.c b/kernel/sched/cpupri.c
> > index daaadf939ccb..152c133e8247 100644
> > --- a/kernel/sched/cpupri.c
> > +++ b/kernel/sched/cpupri.c
> > @@ -29,20 +29,16 @@
> >  #include "sched.h"
> >  
> >  /* Convert between a 140 based task->prio, and our 102 based cpupri */
> > -static int convert_prio(int prio)
> > +static int convert_prio(const int prio)
> >  {
> > -	int cpupri;
> > -
> >  	if (prio == CPUPRI_INVALID)
> > -		cpupri = CPUPRI_INVALID;
> > +		return CPUPRI_INVALID;
> >  	else if (prio == MAX_PRIO)
> > -		cpupri = CPUPRI_IDLE;
> > +		return CPUPRI_IDLE;
> >  	else if (prio >= MAX_RT_PRIO)
> > -		cpupri = CPUPRI_NORMAL;
> > +		return CPUPRI_NORMAL;
> >  	else
> > -		cpupri = MAX_RT_PRIO - prio + 1;
> > -
> > -	return cpupri;
> > +		return MAX_RT_PRIO - prio + 1;
> 
> The code looks even better if you leave out the last else.
> 
> >  }
> >  
> >  /**
> > @@ -95,10 +91,8 @@ int cpupri_find(struct cpupri *cp, struct task_struct *p,
> >  		smp_rmb();
> >  
> >  		/* Need to do the rmb for every iteration */
> > -		if (skip)
> > -			continue;
> > -
> > -		if (cpumask_any_and(&p->cpus_allowed, vec->mask) >= nr_cpu_ids)
> > +		if (skip || cpumask_any_and(&p->cpus_allowed, vec->mask)
> > +				>= nr_cpu_ids)
> >  			continue;
> >  
> >  		if (lowest_mask) {
> 
> That just makes the code ugly for no reason.
> 
> > @@ -222,7 +216,7 @@ int cpupri_init(struct cpupri *cp)
> >  	return 0;
> >  
> >  cleanup:
> > -	for (i--; i >= 0; i--)
> > +	while (--i >= 0)
> >  		free_cpumask_var(cp->pri_to_cpu[i].mask);
> >  	return -ENOMEM;
> >  }
> > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> > index 2e2955a8cf8f..acf1b94669ad 100644
> > --- a/kernel/sched/rt.c
> > +++ b/kernel/sched/rt.c
> > @@ -142,10 +142,12 @@ void free_rt_sched_group(struct task_group *tg)
> >  		destroy_rt_bandwidth(&tg->rt_bandwidth);
> >  
> >  	for_each_possible_cpu(i) {
> > -		if (tg->rt_rq)
> > -			kfree(tg->rt_rq[i]);
> > -		if (tg->rt_se)
> > -			kfree(tg->rt_se[i]);
> > +		/* Don't need to check if tg->rt_rq[i]
> > +		 * or tg->rt_se[i] are NULL, since kfree(NULL)
> > +		 * simply performs no operation
> > +		 */
> 
> That's an invalid comment style.
> 
> > +		kfree(tg->rt_rq[i]);
> > +		kfree(tg->rt_se[i]);
> >  	}
> >  
> >  	kfree(tg->rt_rq);
> > @@ -1015,10 +1017,7 @@ enqueue_top_rt_rq(struct rt_rq *rt_rq)
> >  
> >  	BUG_ON(&rq->rt != rt_rq);
> >  
> > -	if (rt_rq->rt_queued)
> > -		return;
> > -
> > -	if (rt_rq_throttled(rt_rq))
> > +	if (rt_rq->rt_queued || rt_rq_throttled(rt_rq))
> >  		return;
> >  
> >  	if (rt_rq->rt_nr_running) {
> 
> The compiler can do this transformation and the old code was simpler.
> 
> > @@ -1211,10 +1210,7 @@ void dec_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
> >   */
> >  static inline bool move_entity(unsigned int flags)
> >  {
> > -	if ((flags & (DEQUEUE_SAVE | DEQUEUE_MOVE)) == DEQUEUE_SAVE)
> > -		return false;
> > -
> > -	return true;
> > +	return !((flags & (DEQUEUE_SAVE | DEQUEUE_MOVE)) == DEQUEUE_SAVE)
> >  }
> 
> Again, I find the new code harder to read.
> 
> >  
> > @@ -2518,12 +2513,10 @@ static int tg_set_rt_bandwidth(struct task_group *tg,
> >  	/*
> >  	 * Disallowing the root group RT runtime is BAD, it would disallow the
> >  	 * kernel creating (and or operating) RT threads.
> > +	 *
> > +	 * No period doesn't make any sense.
> >  	 */
> > -	if (tg == &root_task_group && rt_runtime == 0)
> > -		return -EINVAL;
> > -
> > -	/* No period doesn't make any sense. */
> > -	if (rt_period == 0)
> > +	if ((tg == &root_task_group && !rt_runtime) || !rt_period)
> >  		return -EINVAL;
> >  
> >  	mutex_lock(&rt_constraints_mutex);
> 
> Again, far harder to read.
> 
> In short, while all the transformations are 'correct' the end result is
> horrible. Please don't do this.