Messages in this thread Patch in this message | | | Date | Wed, 4 Mar 2020 11:27:39 -0500 | From | Steven Rostedt <> | Subject | Re: [PATCH v3 1/6] sched/rt: cpupri_find: Implement fallback mechanism for !fit case |
| |
On Mon, 2 Mar 2020 13:27:16 +0000 Qais Yousef <qais.yousef@arm.com> wrote:
> /** > * cpupri_find - find the best (lowest-pri) CPU in the system > * @cp: The cpupri context > @@ -62,80 +115,72 @@ int cpupri_find(struct cpupri *cp, struct task_struct *p, > struct cpumask *lowest_mask, > bool (*fitness_fn)(struct task_struct *p, int cpu)) > { > - int idx = 0; > int task_pri = convert_prio(p->prio); > + int best_unfit_idx = -1; > + int idx = 0, cpu;
Nit, but if you moved idx, might as well remove the unnecessary initialization of it as well ;-)
> > BUG_ON(task_pri >= CPUPRI_NR_PRIORITIES); > > for (idx = 0; idx < task_pri; idx++) {
It's initialized here.
> - struct cpupri_vec *vec = &cp->pri_to_cpu[idx]; > - int skip = 0; > > - if (!atomic_read(&(vec)->count)) > - skip = 1; > - /* > - * When looking at the vector, we need to read the counter, > - * do a memory barrier, then read the mask. > - * > - * Note: This is still all racey, but we can deal with it. > - * Ideally, we only want to look at masks that are set. > - * > - * If a mask is not set, then the only thing wrong is that we > - * did a little more work than necessary. > - * > - * If we read a zero count but the mask is set, because of the > - * memory barriers, that can only happen when the highest prio > - * task for a run queue has left the run queue, in which case, > - * it will be followed by a pull. If the task we are processing > - * fails to find a proper place to go, that pull request will > - * pull this task if the run queue is running at a lower > - * priority. > - */ > - smp_rmb(); > - > - /* Need to do the rmb for every iteration */ > - if (skip) > - continue; > - > - if (cpumask_any_and(p->cpus_ptr, vec->mask) >= nr_cpu_ids) > + if (!__cpupri_find(cp, p, lowest_mask, idx)) > continue; > > - if (lowest_mask) { > - int cpu; > + if (!lowest_mask || !fitness_fn) > + return 1; > > - cpumask_and(lowest_mask, p->cpus_ptr, vec->mask); > + /* Ensure the capacity of the CPUs fit the task */ > + for_each_cpu(cpu, lowest_mask) { > + if (!fitness_fn(p, cpu)) > + cpumask_clear_cpu(cpu, lowest_mask); > + } > > + /* > + * If no CPU at the current priority can fit the task > + * continue looking > + */ > + if (cpumask_empty(lowest_mask)) { > /* > - * We have to ensure that we have at least one bit > - * still set in the array, since the map could have > - * been concurrently emptied between the first and > - * second reads of vec->mask. If we hit this > - * condition, simply act as though we never hit this > - * priority level and continue on. > + * Store our fallback priority in case we > + * didn't find a fitting CPU > */ > - if (cpumask_empty(lowest_mask)) > - continue; > + if (best_unfit_idx == -1) > + best_unfit_idx = idx; > > - if (!fitness_fn) > - return 1; > - > - /* Ensure the capacity of the CPUs fit the task */ > - for_each_cpu(cpu, lowest_mask) { > - if (!fitness_fn(p, cpu)) > - cpumask_clear_cpu(cpu, lowest_mask); > - } > - > - /* > - * If no CPU at the current priority can fit the task > - * continue looking > - */ > - if (cpumask_empty(lowest_mask)) > - continue; > + continue; > } > > return 1; > } > > + /* > + * If we failed to find a fitting lowest_mask, make sure we fall back > + * to the last known unfitting lowest_mask. > + * > + * Note that the map of the recorded idx might have changed since then, > + * so we must ensure to do the full dance to make sure that level still > + * holds a valid lowest_mask. > + * > + * As per above, the map could have been concurrently emptied while we > + * were busy searching for a fitting lowest_mask at the other priority > + * levels. > + * > + * This rule favours honouring priority over fitting the task in the > + * correct CPU (Capacity Awareness being the only user now). > + * The idea is that if a higher priority task can run, then it should > + * run even if this ends up being on unfitting CPU. > + * > + * The cost of this trade-off is not entirely clear and will probably > + * be good for some workloads and bad for others. > + * > + * The main idea here is that if some CPUs were overcommitted, we try > + * to spread which is what the scheduler traditionally did. Sys admins > + * must do proper RT planning to avoid overloading the system if they > + * really care. > + */ > + if (best_unfit_idx != -1) > + return __cpupri_find(cp, p, lowest_mask, best_unfit_idx);
Hmm, this only checks the one index, which can change and then we miss everything. I think we can do better. What about this:
for (idx = 0; idx < task_pri; idx++) { int found = -1;
if (!__cpupri_find(cp, p, lowest_mask, idx)) continue;
if (!lowest_mask || !fitness_fn) return 1;
/* Make sure we have one fit CPU before clearing */ for_each_cpu(cpu, lowest_mask) { if (fitness_fn(p, cpu)) { found = cpu; break; } }
if (found == -1) continue;
/* Ensure the capacity of the CPUs fit the task */ for_each_cpu(cpu, lowest_mask) { if (cpu < found || !fitness_fn(p, cpu)) cpumask_clear_cpu(cpu, lowest_mask); }
return 1; }
This way, if nothing fits we return the untouched lowest_mask, and only clear the lowest_mask bits if we found a fitness cpu.
-- Steve
| |