Hi Rohit, Just some comments:
On 09/25/2017 05:02 PM, Rohit Jain wrote: > While looking for idle CPUs for a waking task, we should also account > for the delays caused due to the bandwidth reduction by RT/IRQ tasks. > > This patch does that by trying to find a higher capacity CPU with > minimum wake up latency. > > > Signed-off-by: Rohit Jain <[email protected]> > --- > kernel/sched/fair.c | 27 ++++++++++++++++++++++++--- > 1 file changed, 24 insertions(+), 3 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index eca6a57..afb701f 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -5590,6 +5590,11 @@ static unsigned long capacity_orig_of(int cpu) > return cpu_rq(cpu)->cpu_capacity_orig; > } > > +static inline bool full_capacity(int cpu) > +{ > + return (capacity_of(cpu) >= (capacity_orig_of(cpu)*819 >> 10)); Wouldn't 768 be better for multiplication? gcc converts the expression to shifts and adds then. > +} > + > static unsigned long cpu_avg_load_per_task(int cpu) > { > struct rq *rq = cpu_rq(cpu); > @@ -5916,8 +5921,10 @@ find_idlest_cpu(struct sched_group *group, struct > task_struct *p, int this_cpu) > unsigned long load, min_load = ULONG_MAX; > unsigned int min_exit_latency = UINT_MAX; > u64 latest_idle_timestamp = 0; > + unsigned int backup_cap = 0; > int least_loaded_cpu = this_cpu; > int shallowest_idle_cpu = -1; > + int shallowest_idle_cpu_backup = -1; > int i; > > /* Check if we have any choice: */ > @@ -5937,7 +5944,12 @@ find_idlest_cpu(struct sched_group *group, struct > task_struct *p, int this_cpu) > */ > min_exit_latency = idle->exit_latency; > latest_idle_timestamp = rq->idle_stamp; > - shallowest_idle_cpu = i; > + if (full_capacity(i)) { > + shallowest_idle_cpu = i; > + } else if (capacity_of(i) > backup_cap) { > + shallowest_idle_cpu_backup = i; > + backup_cap = capacity_of(i); > + } I'm a bit skeptical about this - if the CPU is idle, then is it likely that the capacity of the CPU is reduced due to RT pressure? I can see that it can matter, but I am wondering if you have any data for your usecase to show that it does (that is if you didn't consider RT pressure for idle CPUs, are you still seeing a big enough performance improvement to warrant the change? > } else if ((!idle || idle->exit_latency == > min_exit_latency) && > rq->idle_stamp > latest_idle_timestamp) { > /* > @@ -5946,7 +5958,12 @@ find_idlest_cpu(struct sched_group *group, struct > task_struct *p, int this_cpu) > * a warmer cache. > */ > latest_idle_timestamp = rq->idle_stamp; > - shallowest_idle_cpu = i; > + if (full_capacity(i)) { > + shallowest_idle_cpu = i; > + } else if (capacity_of(i) > backup_cap) { > + shallowest_idle_cpu_backup = i; > + backup_cap = capacity_of(i); > + } > } > } else if (shallowest_idle_cpu == -1) { > load = weighted_cpuload(cpu_rq(i)); > @@ -5957,7 +5974,11 @@ find_idlest_cpu(struct sched_group *group, struct > task_struct *p, int this_cpu) > } > } > > - return shallowest_idle_cpu != -1 ? shallowest_idle_cpu : > least_loaded_cpu; > + if (shallowest_idle_cpu != -1) > + return shallowest_idle_cpu; > + > + return (shallowest_idle_cpu_backup != -1 ? > + shallowest_idle_cpu_backup : least_loaded_cpu); > } > > #ifdef CONFIG_SCHED_SMT > I see code duplication here which can be reduced by 7 lines compared to your original patch: --- kernel/sched/fair.c | 20 +++++++++++++++++--- 1 file changed, 17 insertions(+), 3 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index c95880e216f6..72fc8d18b251 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5528,6 +5528,7 @@ find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu) /* Traverse only the allowed CPUs */ for_each_cpu_and(i, sched_group_span(group), &p->cpus_allowed) { if (idle_cpu(i)) { + int idle_candidate = -1; struct rq *rq = cpu_rq(i); struct cpuidle_state *idle = idle_get_state(rq); if (idle && idle->exit_latency < min_exit_latency) { @@ -5538,7 +5539,7 @@ find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu) */ min_exit_latency = idle->exit_latency; latest_idle_timestamp = rq->idle_stamp; - shallowest_idle_cpu = i; + idle_candidate = i; } else if ((!idle || idle->exit_latency == min_exit_latency) && rq->idle_stamp > latest_idle_timestamp) { /* @@ -5547,7 +5548,16 @@ find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu) * a warmer cache. */ latest_idle_timestamp = rq->idle_stamp; - shallowest_idle_cpu = i; + idle_candidate = i; + } + + if (idle_candidate != -1) { + if (full_capacity(idle_candidate)) { + shallowest_idle_cpu = idle_candidate; + } else if (capacity_of(idle_candidate) > backup_cap) { + shallowest_idle_cpu_backup = idle_candidate; + backup_cap = capacity_of(idle_candidate); + } } } else if (shallowest_idle_cpu == -1) { load = weighted_cpuload(i); @@ -5558,7 +5568,11 @@ find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu) } } - return shallowest_idle_cpu != -1 ? shallowest_idle_cpu : least_loaded_cpu; + if (shallowest_idle_cpu != -1) + return shallowest_idle_cpu; + + return (shallowest_idle_cpu_backup != -1 ? + shallowest_idle_cpu_backup : least_loaded_cpu); } #ifdef CONFIG_SCHED_SMT -- 2.14.1.821.g8fa685d3b7-goog
