Messages in this thread | | | Subject | Re: [RFC v5 4/6] sched/fair: Tune task wake-up logic to pack small background tasks on fewer cores | From | Parth Shah <> | Date | Mon, 7 Oct 2019 22:23:58 +0530 |
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On 10/7/19 5:49 PM, Vincent Guittot wrote: > On Mon, 7 Oct 2019 at 10:31, Parth Shah <parth@linux.ibm.com> wrote: >> >> The algorithm finds the first non idle core in the system and tries to >> place a task in the idle CPU in the chosen core. To maintain >> cache hotness, work of finding non idle core starts from the prev_cpu, >> which also reduces task ping-pong behaviour inside of the core. >> >> Define a new method to select_non_idle_core which keep tracks of the idle >> and non-idle CPUs in the core and based on the heuristics determines if the >> core is sufficiently busy to place the incoming backgroung task. The >> heuristic further defines the non-idle CPU into either busy (>12.5% util) >> CPU and overutilized (>80% util) CPU. >> - The core containing more idle CPUs and no busy CPUs is not selected for >> packing >> - The core if contains more than 1 overutilized CPUs are exempted from >> task packing >> - Pack if there is atleast one busy CPU and overutilized CPUs count is <2 >> >> Value of 12.5% utilization for busy CPU gives sufficient heuristics for CPU >> doing enough work and not become idle in nearby timeframe. >> >> Signed-off-by: Parth Shah <parth@linux.ibm.com> >> --- >> kernel/sched/core.c | 3 ++ >> kernel/sched/fair.c | 95 ++++++++++++++++++++++++++++++++++++++++++++- >> 2 files changed, 97 insertions(+), 1 deletion(-) >> >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >> index 6e1ae8046fe0..7e3aff59540a 100644 >> --- a/kernel/sched/core.c >> +++ b/kernel/sched/core.c >> @@ -6402,6 +6402,7 @@ static struct kmem_cache *task_group_cache __read_mostly; >> >> DECLARE_PER_CPU(cpumask_var_t, load_balance_mask); >> DECLARE_PER_CPU(cpumask_var_t, select_idle_mask); >> +DECLARE_PER_CPU(cpumask_var_t, turbo_sched_mask); >> >> void __init sched_init(void) >> { >> @@ -6442,6 +6443,8 @@ void __init sched_init(void) >> cpumask_size(), GFP_KERNEL, cpu_to_node(i)); >> per_cpu(select_idle_mask, i) = (cpumask_var_t)kzalloc_node( >> cpumask_size(), GFP_KERNEL, cpu_to_node(i)); >> + per_cpu(turbo_sched_mask, i) = (cpumask_var_t)kzalloc_node( >> + cpumask_size(), GFP_KERNEL, cpu_to_node(i)); >> } >> #endif /* CONFIG_CPUMASK_OFFSTACK */ >> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> index b798fe7ff7cd..d4a1b6474338 100644 >> --- a/kernel/sched/fair.c >> +++ b/kernel/sched/fair.c >> @@ -5353,6 +5353,8 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags) >> /* Working cpumask for: load_balance, load_balance_newidle. */ >> DEFINE_PER_CPU(cpumask_var_t, load_balance_mask); >> DEFINE_PER_CPU(cpumask_var_t, select_idle_mask); >> +/* A cpumask to find active cores in the system. */ >> +DEFINE_PER_CPU(cpumask_var_t, turbo_sched_mask); >> >> #ifdef CONFIG_NO_HZ_COMMON >> >> @@ -5964,6 +5966,76 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t >> return cpu; >> } >> >> +#ifdef CONFIG_SCHED_SMT >> +static inline bool is_background_task(struct task_struct *p) >> +{ >> + if (p->flags & PF_CAN_BE_PACKED) >> + return true; >> + >> + return false; >> +} >> + >> +#define busyness_threshold (100 >> 3) >> +#define is_cpu_busy(util) ((util) > busyness_threshold) >> + >> +/* >> + * Try to find a non idle core in the system based on few heuristics: >> + * - Keep track of overutilized (>80% util) and busy (>12.5% util) CPUs >> + * - If none CPUs are busy then do not select the core for task packing >> + * - If atleast one CPU is busy then do task packing unless overutilized CPUs >> + * count is < busy/2 CPU count >> + * - Always select idle CPU for task packing >> + */ >> +static int select_non_idle_core(struct task_struct *p, int prev_cpu, int target) >> +{ >> + struct cpumask *cpus = this_cpu_cpumask_var_ptr(turbo_sched_mask); >> + int iter_cpu, sibling; >> + >> + cpumask_and(cpus, cpu_online_mask, p->cpus_ptr); >> + >> + for_each_cpu_wrap(iter_cpu, cpus, prev_cpu) { >> + int idle_cpu_count = 0, non_idle_cpu_count = 0; >> + int overutil_cpu_count = 0; >> + int busy_cpu_count = 0; >> + int best_cpu = iter_cpu; >> + >> + for_each_cpu(sibling, cpu_smt_mask(iter_cpu)) { >> + __cpumask_clear_cpu(sibling, cpus); >> + if (idle_cpu(iter_cpu)) { >> + idle_cpu_count++; >> + best_cpu = iter_cpu; >> + } else { >> + non_idle_cpu_count++; >> + if (cpu_overutilized(iter_cpu)) >> + overutil_cpu_count++; >> + if (is_cpu_busy(cpu_util(iter_cpu))) >> + busy_cpu_count++; >> + } >> + } >> + >> + /* >> + * Pack tasks to this core if >> + * 1. Idle CPU count is higher and atleast one is busy >> + * 2. If idle_cpu_count < non_idle_cpu_count then ideally do >> + * packing but if there are more CPUs overutilized then don't >> + * overload it. > > Could you give details about the rationale behind these conditions ?
sure. but first maybe some background is required for busy_cpu. Task packing needs to be done across cores minimizing number of busy cores in the chip. Hence when picking a core for packing a background task it will be good to not select a core which is in deeper idle-states.
Usually deeper idle states have target_residency >= 10ms which are really power saving states and saved power can be channeled to active cores. A CPU with utilization of 12.5% is most probably not going to those deeper idle states and picking a CPU with >= 12.5% util seems to be a good approximation.
Now going to the _main point_, task packing needs to take care of the following scenarios: 1. Not select a core having all the CPUs idle or <= 12.5% util 2. Do not select a core with 2 or more CPUs overloaded (>=80% util) 3. Select a core even if 1 CPU is overloaded as background tasks are usually short running and spending time for selecting better alternative is not worth the investment here 4. Select a core if at least one CPU is busy (>=12.5% util) 5. On selecting a core, select an idle CPU in it.
Hence to satisfy this scenarios for SMT-1/2/4 (POWER9) or 8 (POWER8 has 8-threads per core/ POWER9 has feature to make fake SMT-8), the approach keeps track of idle, non-idle, busy and overloaded CPU count in the core and uses above approach to find _sufficiently_ non-idle core, which seems to be a good heuristics to do task packing without much of regression on CPU intensive threads.
So as per the comments in this patch, first point covers the scenario 1 and 4 (if part in the code), and second point covers scenario 2 and 3 (else part in the code).
>> + */ >> + if (idle_cpu_count > non_idle_cpu_count) { >> + if (busy_cpu_count) >> + return best_cpu; >> + } else { >> + /* >> + * Pack tasks if at max 1 CPU is overutilized >> + */ >> + if (overutil_cpu_count < 2) >> + return best_cpu; >> + } >> + } >> + >> + return select_idle_sibling(p, prev_cpu, target); >> +} >> +#endif /* CONFIG_SCHED_SMT */ >> + >> /* >> * Try and locate an idle core/thread in the LLC cache domain. >> */ >> @@ -6418,6 +6490,23 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) >> return -1; >> } >> >> +#ifdef CONFIG_SCHED_SMT >> +/* >> + * Select all classified background tasks for task packing >> + */ >> +static inline int turbosched_select_non_idle_core(struct task_struct *p, >> + int prev_cpu, int target) >> +{ >> + return select_non_idle_core(p, prev_cpu, target); >> +} >> +#else >> +static inline int turbosched_select_non_idle_core(struct task_struct *p, >> + int prev_cpu, int target) >> +{ >> + return select_idle_sibling(p, prev_cpu, target); > > should be better to make turbosched_select_non_idle_core empty and > make sure that __turbo_sched_enabled is never enabled if > CONFIG_SCHED_SMT is disabled >
Totally agreed. I thought keeping like this so as to not have any "#def.." in select_task_rq_fair method. So can I do this by adding a new method like __select_idle_sibling() which will call turbosched_select_non_idle_core() in case of SCHED_SMT present and otherwise will call the regular select_idle_sibling()?
>> +} >> +#endif >> + >> /* >> * select_task_rq_fair: Select target runqueue for the waking task in domains >> * that have the 'sd_flag' flag set. In practice, this is SD_BALANCE_WAKE, >> @@ -6483,7 +6572,11 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f >> } else if (sd_flag & SD_BALANCE_WAKE) { /* XXX always ? */ >> /* Fast path */ >> >> - new_cpu = select_idle_sibling(p, prev_cpu, new_cpu); >> + if (is_turbosched_enabled() && unlikely(is_background_task(p))) >> + new_cpu = turbosched_select_non_idle_core(p, prev_cpu, >> + new_cpu); > > Could you add turbosched_select_non_idle_core() similarly to > find_energy_efficient_cpu() ? > Add it at the beg select_task_rq_fair() > Return immediately with theCPU if you have found one > Or let the normal path select a CPU if the > turbosched_select_non_idle_core() has not been able to find a suitable > CPU for packing >
of course. I can do that. I was just not aware about the effect of wake_affine and so was waiting for such comments to be sure of. Thanks for this. Maybe I can add just below the sched_energy_present(){...} construct giving precedence to EAS? I'm asking this because I remember Patrick telling me to leverage task packing for android as well?
Thank you very much for looking at the patches. Parth
> >> + else >> + new_cpu = select_idle_sibling(p, prev_cpu, new_cpu); >> >> if (want_affine) >> current->recent_used_cpu = cpu; >> -- >> 2.17.1 >>
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