Messages in this thread | | | Subject | Re: [PATCH 2/2] sched/fair: Relax task_hot() for misfit tasks | From | Dietmar Eggemann <> | Date | Thu, 22 Apr 2021 19:29:27 +0200 |
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On 21/04/2021 12:52, Valentin Schneider wrote: > On 20/04/21 16:33, Vincent Guittot wrote: >> On Mon, 19 Apr 2021 at 19:13, Valentin Schneider >> <valentin.schneider@arm.com> wrote: >>> >>> On 16/04/21 15:51, Vincent Guittot wrote: >>>> Le jeudi 15 avril 2021 � 18:58:46 (+0100), Valentin Schneider a �crit : >>>>> + >>>>> +/* >>>>> + * What does migrating this task do to our capacity-aware scheduling criterion? >>>>> + * >>>>> + * Returns 1, if the task needs more capacity than the dst CPU can provide. >>>>> + * Returns 0, if the task needs the extra capacity provided by the dst CPU >>>>> + * Returns -1, if the task isn't impacted by the migration wrt capacity. >>>>> + */ >>>>> +static int migrate_degrades_capacity(struct task_struct *p, struct lb_env *env) >>>>> +{ >>>>> + if (!(env->sd->flags & SD_ASYM_CPUCAPACITY)) >>>>> + return -1; >>>>> + >>>>> + if (!task_fits_capacity(p, capacity_of(env->src_cpu))) { >>>>> + if (cpu_capacity_greater(env->dst_cpu, env->src_cpu)) >>>>> + return 0; >>>>> + else if (cpu_capacity_greater(env->src_cpu, env->dst_cpu)) >>>>> + return 1; >>>>> + else >>>>> + return -1; >>>>> + } >>>> >>>> Being there means that task fits src_cpu capacity so why testing p against dst_cpu ? >>>> >>> >>> Because if p fits on src_cpu, we don't want to move it to a dst_cpu on >>> which it *doesn't* fit. >> >> OK. I was confused because I thought that this was only to force >> migration in case of group_misfit_task but you tried to extend to >> other cases... I'm not convinced that you succeeded to cover all cases >> >> Also I found this function which returns 3 values a bit disturbing. >> IIUC you tried to align to migrate_degrades_capacity but you should >> have better aligned to task_hot and return only 0 or 1. -1 is not used >> > > Ack, will do. > >>>>> @@ -7672,6 +7698,15 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env) >>>>> if (tsk_cache_hot == -1) >>>>> tsk_cache_hot = task_hot(p, env); >>>>> >>>>> + /* >>>>> + * On a (sane) asymmetric CPU capacity system, the increase in compute >>>>> + * capacity should offset any potential performance hit caused by a >>>>> + * migration. >>>>> + */ >>>>> + if ((env->dst_grp_type == group_has_spare) && >>>> >>>> Shouldn't it be env->src_grp_type == group_misfit_task to only care of misfit task case as >>>> stated in $subject >>>> >>> >>> Previously this was env->idle != CPU_NOT_IDLE, but I figured dst_grp_type >>> could give us a better picture. Staring at this some more, this isn't so >>> true when the group size goes up - there's no guarantees the dst_cpu is the >>> one that has spare cycles, and the other CPUs might not be able to grant >>> the capacity uplift dst_cpu can. >> >> yeah you have to keep checking for env->idle != CPU_NOT_IDLE >> >>> >>> As for not using src_grp_type == group_misfit_task, this is pretty much the >>> same as [1]. CPU-bound (misfit) task + some other task on the same rq >>> implies group_overloaded classification when balancing at MC level (no SMT, >>> so one group per CPU). >> >> Is it something that happens often or just a sporadic/transient state >> ? I mean does it really worth the extra complexity and do you see >> performance improvement ? >> > > "Unfortunately" yes, this is a relatively common scenario when running "1 > big task per CPU" types of workloads. The expected behaviour for big.LITTLE > systems is to upmigrate tasks stuck on the LITTLE CPUs as soon as a big CPU > becomes free, usually via newidle balance (which, since they process work > faster than the LITTLEs, is bound to happen), and an extra task being > enqueued at "the wrong time" can prevent this from happening. > > This usually means a misfit task can take a few dozen extra ms than it > should to be migrated - in the tests I run (which are pretty much this 1 > hog per CPU workload) this happens about ~20% of the time. > >> You should better focus on fixing the simple case of group_misfit_task >> task. This other cases looks far more complex with lot of corner cases >> >>> >>> [1]: http://lore.kernel.org/r/jhjblcuv2mo.mognet@arm.com
Just to make sure I can follow the conversation ...
In case you:
(1) return 1 instead of -1
(2) keep the `env->idle != CPU_NOT_IDLE` check
(3) and remove the `dst_grp_type == group_has_spare` check
you are pretty much back to what you had in [PATCH v3 7/7] directly in task_hot() except:
(4) the 'if (p fits on src_cpu && p !fits dst_cpu) => tsk_cache_hot) check?
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