Messages in this thread | | | From | Barry Song <> | Date | Mon, 11 Oct 2021 09:24:09 +1300 | Subject | Re: [PATCH] sched/fair: Check idle_cpu in select_idle_core/cpu() |
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On Mon, Oct 11, 2021 at 3:26 AM Tao Zhou <tao.zhou@linux.dev> wrote: > > Hi Barry, > > On Mon, Oct 11, 2021 at 01:19:57AM +1300, Barry Song wrote: > > On Sun, Oct 10, 2021 at 10:45 PM Tao Zhou <tao.zhou@linux.dev> wrote: > > > > > > Hi Peter, > > > > > > On Sun, Oct 10, 2021 at 12:50:57AM +0200, Peter Zijlstra wrote: > > > > On Sun, Oct 10, 2021 at 02:09:41AM +0800, Tao Zhou wrote: > > > > > In select_idle_core(), the idle core returned may have no cpu > > > > > allowed. I think the idle core returned for the task is the one > > > > > that can be allowed to run. I insist on this semantics. > > > > > > > > > > In select_idle_cpu(), if select_idle_core() can not find the > > > > > idle core, one reason is that the core is not allowed for the > > > > > task, but the core itself is idle from the point of > > > > > sds->has_idle_cores. I insist on this semantics. > > > > > > > > > > No others, just two additional check. > > > > > --- > > > > > kernel/sched/fair.c | 4 ++-- > > > > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > > > > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > > > > index f6a05d9b5443..a44aca5095d3 100644 > > > > > --- a/kernel/sched/fair.c > > > > > +++ b/kernel/sched/fair.c > > > > > @@ -6213,7 +6213,7 @@ static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpu > > > > > *idle_cpu = cpu; > > > > > } > > > > > > > > > > - if (idle) > > > > > + if (idle && *idle_cpu != -1) > > > > > return core; > > > > > > > > In that case, core would be nr_cpu_ids (==nr_cpumask_bits), and then the caller checks: > > > > > > > > (unsigned)i < nr_cpumask_bits > > > > > > Thank you for reply. > > > > > > > > > If (1)there is no idle core or (2)the idle core has no allowed cpu, we return -1. > > > Originally, just (1) has happened, we return -1. The (2) is what I want to add. > > > > I don't understand (2). before doing > > for_each_cpu_wrap(cpu, cpus, target + 1) { > > if (has_idle_core) { > > i = select_idle_core(p, cpu, cpus, &idle_cpu); > > if ((unsigned int)i < nr_cpumask_bits) > > return i; > > > > } else { > > if (!--nr) > > return -1; > > idle_cpu = __select_idle_cpu(cpu, p); > > if ((unsigned int)idle_cpu < nr_cpumask_bits) > > break; > > } > > } > > > > to select idle core, we have already done: > > cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr); > > > > so we are only scanning allowed cpus. > > Um.. You read top down.. and you are right. > The function itself semantics is important to me. > > After a secondary recall and not thorough now, I realize that > cpus_ptr may be changed. > > > See code of this: > > static void migrate_disable_switch(struct rq *rq, struct task_struct *p) > { > if (likely(!p->migration_disabled)) > return; > > if (p->cpus_ptr != &p->cpus_mask) > return; > > /* > * Violates locking rules! see comment in __do_set_cpus_allowed(). > */ > __do_set_cpus_allowed(p, cpumask_of(rq->cpu), SCA_MIGRATE_DISABLE); > } > > > This change is under the light of ->pi_lock. > That thing is quick to forget to me.. > Not sure I am right. Thank you for remind. > > If the cpu_ptr can be changed, you can not depend on the first AND > operation there.
The explanation doesn't make any sense to me. We are scanning based on the first AND operation. select_idle_core() is returning *idle_cpu based on the cpumask after AND operation. Even though cpumask can change after select_idle_core() is done or before select_idle_core() is called, the return value is not wrong.
> > > > > > > If we find idle core and has allowed cpu in the core, is it better to return > > > @*idle_cpu. > > > > > > if (idle && *idle_cpu != -1) > > > return *idle_cpu; > > > > > > This @*idle_cpu is the allowed cpu in the idle core. We do not promise anything > > > about the @core(target) is the allowed cpu until we hit in select_task_rq() --> > > > select_fallback_rq(). And the select_fallback_rq() will return a different cpu > > > than the @core or @*idle_cpu. > > > > > > > > cpumask_andnot(cpus, cpus, cpu_smt_mask(core)); > > > > > @@ -6324,7 +6324,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool > > > > > } > > > > > } > > > > > > > > > > - if (has_idle_core) > > > > > + if (has_idle_core && *idle_cpu != -1) > > > > > set_idle_cores(target, false); > > > > > > > > And this one I'm completely failing, why shouldn't we mark the core as > > > > non-idle when there is a single idle CPU found? That's just worng. > > > > > > When @has_idle_core is true, it implies for all cpu in the core the case > > > (1) or case (2) has happened. The (1) can be mark as non-idle. I conclude > > > to contradiction myself last time. The (2) is also seemed to be non-idle. > > > > > > > > > But, I think I am totally wrong because the sds->has_idle_cores is related > > > to the cpu not task. So, the affinity should not affect the decision of > > > sds->has_idle_cores. > > > > > > > > > > > > Thanks, > > > Tao > > > > Thanks > > barry > > > > Thanks, > Tao
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