Messages in this thread | | | From | Vincent Guittot <> | Date | Fri, 30 Apr 2021 08:58:42 +0200 | Subject | Re: [PATCH 2/2] sched/fair: Relax task_hot() for misfit tasks |
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On Wed, 21 Apr 2021 at 12:52, Valentin Schneider <valentin.schneider@arm.com> 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
A few dozens is quite long. With a big core being idle, it should try every 8ms on a quad x quad system and I suspect the next try will be during the next tick. Would be good to understand why it has to wait so much
> 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
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