Commit-ID: 45f5519ec55e75af3565dd737586d3b041834f71 Gitweb: https://git.kernel.org/tip/45f5519ec55e75af3565dd737586d3b041834f71 Author: Peter Zijlstra <pet...@infradead.org> AuthorDate: Thu, 5 Jul 2018 14:36:17 +0200 Committer: Ingo Molnar <mi...@kernel.org> CommitDate: Mon, 16 Jul 2018 00:16:29 +0200
sched/cpufreq: Clarify sugov_get_util() Add a few comments to (hopefully) clarifying some of the magic in sugov_get_util(). Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> Acked-by: Viresh Kumar <viresh.ku...@linaro.org> Cc: Linus Torvalds <torva...@linux-foundation.org> Cc: morten.rasmus...@arm.com Cc: Peter Zijlstra <pet...@infradead.org> Cc: Thomas Gleixner <t...@linutronix.de> Cc: Vincent Guittot <vincent.guit...@linaro.org> Cc: clau...@evidence.eu.com Cc: daniel.lezc...@linaro.org Cc: dietmar.eggem...@arm.com Cc: j...@joelfernandes.org Cc: juri.le...@redhat.com Cc: luca.ab...@santannapisa.it Cc: patrick.bell...@arm.com Cc: quentin.per...@arm.com Cc: r...@rjwysocki.net Cc: valentin.schnei...@arm.com Link: http://lkml.kernel.org/r/20180705123617.gm2...@hirez.programming.kicks-ass.net Signed-off-by: Ingo Molnar <mi...@kernel.org> --- kernel/sched/cpufreq_schedutil.c | 75 +++++++++++++++++++++++++++++----------- 1 file changed, 54 insertions(+), 21 deletions(-) diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c index c9622b3f183d..97dcd4472a0e 100644 --- a/kernel/sched/cpufreq_schedutil.c +++ b/kernel/sched/cpufreq_schedutil.c @@ -177,6 +177,26 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy, return cpufreq_driver_resolve_freq(policy, freq); } +/* + * This function computes an effective utilization for the given CPU, to be + * used for frequency selection given the linear relation: f = u * f_max. + * + * The scheduler tracks the following metrics: + * + * cpu_util_{cfs,rt,dl,irq}() + * cpu_bw_dl() + * + * Where the cfs,rt and dl util numbers are tracked with the same metric and + * synchronized windows and are thus directly comparable. + * + * The cfs,rt,dl utilization are the running times measured with rq->clock_task + * which excludes things like IRQ and steal-time. These latter are then accrued + * in the irq utilization. + * + * The DL bandwidth number otoh is not a measured metric but a value computed + * based on the task model parameters and gives the minimal utilization + * required to meet deadlines. + */ static unsigned long sugov_get_util(struct sugov_cpu *sg_cpu) { struct rq *rq = cpu_rq(sg_cpu->cpu); @@ -188,47 +208,60 @@ static unsigned long sugov_get_util(struct sugov_cpu *sg_cpu) if (rt_rq_is_runnable(&rq->rt)) return max; + /* + * Early check to see if IRQ/steal time saturates the CPU, can be + * because of inaccuracies in how we track these -- see + * update_irq_load_avg(). + */ irq = cpu_util_irq(rq); - if (unlikely(irq >= max)) return max; - /* Sum rq utilization */ + /* + * Because the time spend on RT/DL tasks is visible as 'lost' time to + * CFS tasks and we use the same metric to track the effective + * utilization (PELT windows are synchronized) we can directly add them + * to obtain the CPU's actual utilization. + */ util = cpu_util_cfs(rq); util += cpu_util_rt(rq); /* - * Interrupt time is not seen by RQS utilization so we can compare - * them with the CPU capacity + * We do not make cpu_util_dl() a permanent part of this sum because we + * want to use cpu_bw_dl() later on, but we need to check if the + * CFS+RT+DL sum is saturated (ie. no idle time) such that we select + * f_max when there is no idle time. + * + * NOTE: numerical errors or stop class might cause us to not quite hit + * saturation when we should -- something for later. */ if ((util + cpu_util_dl(rq)) >= max) return max; /* - * As there is still idle time on the CPU, we need to compute the - * utilization level of the CPU. + * There is still idle time; further improve the number by using the + * irq metric. Because IRQ/steal time is hidden from the task clock we + * need to scale the task numbers: * + * 1 - irq + * U' = irq + ------- * U + * max + */ + util *= (max - irq); + util /= max; + util += irq; + + /* * Bandwidth required by DEADLINE must always be granted while, for * FAIR and RT, we use blocked utilization of IDLE CPUs as a mechanism * to gracefully reduce the frequency when no tasks show up for longer * periods of time. * - * Ideally we would like to set util_dl as min/guaranteed freq and - * util_cfs + util_dl as requested freq. However, cpufreq is not yet - * ready for such an interface. So, we only do the latter for now. + * Ideally we would like to set bw_dl as min/guaranteed freq and util + + * bw_dl as requested freq. However, cpufreq is not yet ready for such + * an interface. So, we only do the latter for now. */ - - /* Weight RQS utilization to normal context window */ - util *= (max - irq); - util /= max; - - /* Add interrupt utilization */ - util += irq; - - /* Add DL bandwidth requirement */ - util += sg_cpu->bw_dl; - - return min(max, util); + return min(max, util + sg_cpu->bw_dl); } /**
