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) <peterz@infradead.org> Acked-by: Viresh Kumar <viresh.kumar@linaro.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Morten.Rasmussen@arm.com Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Vincent Guittot <vincent.guittot@linaro.org> Cc: claudio@evidence.eu.com Cc: daniel.lezcano@linaro.org Cc: dietmar.eggemann@arm.com Cc: joel@joelfernandes.org Cc: juri.lelli@redhat.com Cc: luca.abeni@santannapisa.it Cc: patrick.bellasi@arm.com Cc: quentin.perret@arm.com Cc: rjw@rjwysocki.net Cc: valentin.schneider@arm.com Link: http://lkml.kernel.org/r/20180705123617.GM2458@hirez.programming.kicks-ass.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
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@ -177,6 +177,26 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
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return cpufreq_driver_resolve_freq(policy, freq);
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return cpufreq_driver_resolve_freq(policy, freq);
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}
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}
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/*
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* This function computes an effective utilization for the given CPU, to be
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* used for frequency selection given the linear relation: f = u * f_max.
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*
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* The scheduler tracks the following metrics:
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*
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* cpu_util_{cfs,rt,dl,irq}()
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* cpu_bw_dl()
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*
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* Where the cfs,rt and dl util numbers are tracked with the same metric and
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* synchronized windows and are thus directly comparable.
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*
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* The cfs,rt,dl utilization are the running times measured with rq->clock_task
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* which excludes things like IRQ and steal-time. These latter are then accrued
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* in the irq utilization.
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*
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* The DL bandwidth number otoh is not a measured metric but a value computed
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* based on the task model parameters and gives the minimal utilization
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* required to meet deadlines.
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*/
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static unsigned long sugov_get_util(struct sugov_cpu *sg_cpu)
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static unsigned long sugov_get_util(struct sugov_cpu *sg_cpu)
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{
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{
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struct rq *rq = cpu_rq(sg_cpu->cpu);
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struct rq *rq = cpu_rq(sg_cpu->cpu);
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@ -188,47 +208,60 @@ static unsigned long sugov_get_util(struct sugov_cpu *sg_cpu)
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if (rt_rq_is_runnable(&rq->rt))
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if (rt_rq_is_runnable(&rq->rt))
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return max;
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return max;
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/*
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* Early check to see if IRQ/steal time saturates the CPU, can be
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* because of inaccuracies in how we track these -- see
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* update_irq_load_avg().
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*/
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irq = cpu_util_irq(rq);
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irq = cpu_util_irq(rq);
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if (unlikely(irq >= max))
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if (unlikely(irq >= max))
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return max;
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return max;
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/* Sum rq utilization */
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/*
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* Because the time spend on RT/DL tasks is visible as 'lost' time to
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* CFS tasks and we use the same metric to track the effective
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* utilization (PELT windows are synchronized) we can directly add them
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* to obtain the CPU's actual utilization.
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*/
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util = cpu_util_cfs(rq);
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util = cpu_util_cfs(rq);
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util += cpu_util_rt(rq);
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util += cpu_util_rt(rq);
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/*
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/*
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* Interrupt time is not seen by RQS utilization so we can compare
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* We do not make cpu_util_dl() a permanent part of this sum because we
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* them with the CPU capacity
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* want to use cpu_bw_dl() later on, but we need to check if the
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* CFS+RT+DL sum is saturated (ie. no idle time) such that we select
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* f_max when there is no idle time.
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*
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* NOTE: numerical errors or stop class might cause us to not quite hit
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* saturation when we should -- something for later.
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*/
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*/
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if ((util + cpu_util_dl(rq)) >= max)
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if ((util + cpu_util_dl(rq)) >= max)
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return max;
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return max;
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/*
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/*
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* As there is still idle time on the CPU, we need to compute the
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* There is still idle time; further improve the number by using the
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* utilization level of the CPU.
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* irq metric. Because IRQ/steal time is hidden from the task clock we
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* need to scale the task numbers:
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*
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*
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* 1 - irq
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* U' = irq + ------- * U
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* max
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*/
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util *= (max - irq);
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util /= max;
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util += irq;
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/*
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* Bandwidth required by DEADLINE must always be granted while, for
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* Bandwidth required by DEADLINE must always be granted while, for
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* FAIR and RT, we use blocked utilization of IDLE CPUs as a mechanism
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* FAIR and RT, we use blocked utilization of IDLE CPUs as a mechanism
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* to gracefully reduce the frequency when no tasks show up for longer
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* to gracefully reduce the frequency when no tasks show up for longer
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* periods of time.
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* periods of time.
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*
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*
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* Ideally we would like to set util_dl as min/guaranteed freq and
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* Ideally we would like to set bw_dl as min/guaranteed freq and util +
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* util_cfs + util_dl as requested freq. However, cpufreq is not yet
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* bw_dl as requested freq. However, cpufreq is not yet ready for such
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* ready for such an interface. So, we only do the latter for now.
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* an interface. So, we only do the latter for now.
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*/
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*/
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return min(max, util + sg_cpu->bw_dl);
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/* Weight RQS utilization to normal context window */
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util *= (max - irq);
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util /= max;
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/* Add interrupt utilization */
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util += irq;
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/* Add DL bandwidth requirement */
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util += sg_cpu->bw_dl;
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return min(max, util);
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}
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}
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/**
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/**
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