Messages in this thread Patch in this message | | | Date | Tue, 10 Oct 2017 03:59:06 -0700 | From | tip-bot for Brendan Jackman <> | Subject | [tip:sched/core] sched/fair: Force balancing on NOHZ balance if local group has capacity |
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Commit-ID: 583ffd99d7657755736d831bbc182612d1d2697d Gitweb: https://git.kernel.org/tip/583ffd99d7657755736d831bbc182612d1d2697d Author: Brendan Jackman <brendan.jackman@arm.com> AuthorDate: Thu, 5 Oct 2017 11:58:54 +0100 Committer: Ingo Molnar <mingo@kernel.org> CommitDate: Tue, 10 Oct 2017 11:45:32 +0200
sched/fair: Force balancing on NOHZ balance if local group has capacity
The "goto force_balance" here is intended to mitigate the fact that avg_load calculations can result in bad placement decisions when priority is asymmetrical.
The original commit that adds it:
fab476228ba3 ("sched: Force balancing on newidle balance if local group has capacity")
explains:
Under certain situations, such as a niced down task (i.e. nice = -15) in the presence of nr_cpus NICE0 tasks, the niced task lands on a sched group and kicks away other tasks because of its large weight. This leads to sub-optimal utilization of the machine. Even though the sched group has capacity, it does not pull tasks because sds.this_load >> sds.max_load, and f_b_g() returns NULL.
A similar but inverted issue also affects ARM big.LITTLE (asymmetrical CPU capacity) systems - consider 8 always-running, same-priority tasks on a system with 4 "big" and 4 "little" CPUs. Suppose that 5 of them end up on the "big" CPUs (which will be represented by one sched_group in the DIE sched_domain) and 3 on the "little" (the other sched_group in DIE), leaving one CPU unused. Because the "big" group has a higher group_capacity its avg_load may not present an imbalance that would cause migrating a task to the idle "little".
The force_balance case here solves the problem but currently only for CPU_NEWLY_IDLE balances, which in theory might never happen on the unused CPU. Including CPU_IDLE in the force_balance case means there's an upper bound on the time before we can attempt to solve the underutilization: after DIE's sd->balance_interval has passed the next nohz balance kick will help us out.
Signed-off-by: Brendan Jackman <brendan.jackman@arm.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Mike Galbraith <efault@gmx.de> Cc: Morten Rasmussen <morten.rasmussen@arm.com> Cc: Paul Turner <pjt@google.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/20170807163900.25180-1-brendan.jackman@arm.com Signed-off-by: Ingo Molnar <mingo@kernel.org> --- kernel/sched/fair.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index c04a425..cf3e816 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -8186,8 +8186,11 @@ static struct sched_group *find_busiest_group(struct lb_env *env) if (busiest->group_type == group_imbalanced) goto force_balance; - /* SD_BALANCE_NEWIDLE trumps SMP nice when underutilized */ - if (env->idle == CPU_NEWLY_IDLE && group_has_capacity(env, local) && + /* + * When dst_cpu is idle, prevent SMP nice and/or asymmetric group + * capacities from resulting in underutilization due to avg_load. + */ + if (env->idle != CPU_NOT_IDLE && group_has_capacity(env, local) && busiest->group_no_capacity) goto force_balance;
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