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From: Tao Zhou To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, Ingo Molnar , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira Subject: Re: [PATCH] sched/fair: Check idle_cpu in select_idle_core/cpu() Message-ID: References: <20211009180941.20458-1-tao.zhou@linux.dev> <20211009225057.GB174703@worktop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20211009225057.GB174703@worktop.programming.kicks-ass.net> X-Migadu-Flow: FLOW_OUT X-Migadu-Auth-User: tao.zhou@linux.dev Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. 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