LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Morten Rasmussen <morten.rasmussen@arm.com>
To: peterz@infradead.org, mingo@redhat.com
Cc: valentin.schneider@arm.com, dietmar.eggemann@arm.com,
	vincent.guittot@linaro.org, gaku.inami.xh@renesas.com,
	linux-kernel@vger.kernel.org,
	Morten Rasmussen <morten.rasmussen@arm.com>
Subject: [PATCHv2 7/7] sched/fair: Set sd->overload when misfit
Date: Thu, 15 Mar 2018 14:47:04 +0000	[thread overview]
Message-ID: <1521125224-15434-8-git-send-email-morten.rasmussen@arm.com> (raw)
In-Reply-To: <1521125224-15434-1-git-send-email-morten.rasmussen@arm.com>

From: Valentin Schneider <valentin.schneider@arm.com>

Idle balance is a great opportunity to pull a misfit task. However,
there are scenarios where misfit tasks are present but idle balance is
prevented by the overload flag.

A good example of this is a workload of n identical tasks. Let's suppose
we have a 2+2 Arm big.LITTLE system. We then spawn 4 fairly
CPU-intensive tasks - for the sake of simplicity let's say they are just
CPU hogs, even when running on big CPUs.

They are identical tasks, so on an SMP system they should all end at
(roughly) the same time. However, in our case the LITTLE CPUs are less
performing than the big CPUs, so tasks running on the LITTLEs will have
a longer completion time.

This means that the big CPUs will complete their work earlier, at which
point they should pull the tasks from the LITTLEs. What we want to
happen is summarized as follows:

a,b,c,d are our CPU-hogging tasks
_ signifies idling

LITTLE_0 | a a a a _ _
LITTLE_1 | b b b b _ _
---------|-------------
  big_0  | c c c c a a
  big_1  | d d d d b b
                  ^
                  ^
    Tasks end on the big CPUs, idle balance happens
    and the misfit tasks are pulled straight away

This however won't happen, because currently the overload flag is only
set when there is any CPU that has more than one runnable task - which
may very well not be the case here if our CPU-hogging workload is all
there is to run.

As such, this commit sets the overload flag in update_sg_lb_stats when
a group is flagged as having a misfit task.

cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
---
 kernel/sched/fair.c  | 6 ++++--
 kernel/sched/sched.h | 6 +++++-
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 4e79ec8760be..7952e9491713 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7964,7 +7964,7 @@ static bool update_nohz_stats(struct rq *rq, bool force)
  * @load_idx: Load index of sched_domain of this_cpu for load calc.
  * @local_group: Does group contain this_cpu.
  * @sgs: variable to hold the statistics for this group.
- * @overload: Indicate more than one runnable task for any CPU.
+ * @overload: Indicate pullable load (e.g. >1 runnable task).
  */
 static inline void update_sg_lb_stats(struct lb_env *env,
 			struct sched_group *group, int load_idx,
@@ -8008,8 +8008,10 @@ static inline void update_sg_lb_stats(struct lb_env *env,
 			sgs->idle_cpus++;
 
 		if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
-		    !sgs->group_misfit_task_load && rq->misfit_task_load)
+		    !sgs->group_misfit_task_load && rq->misfit_task_load) {
 			sgs->group_misfit_task_load = rq->misfit_task_load;
+			*overload = 1;
+		}
 	}
 
 	/* Adjust by relative CPU capacity of the group */
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 31847b7f7d47..2981a162d9c0 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -688,7 +688,11 @@ struct root_domain {
 	cpumask_var_t		span;
 	cpumask_var_t		online;
 
-	/* Indicate more than one runnable task for any CPU */
+	/*
+	 * Indicate pullable load on at least one CPU, e.g:
+	 * - More than one runnable task
+	 * - Running task is misfit
+	 */
 	int			overload;
 
 	/*
-- 
2.7.4

  parent reply	other threads:[~2018-03-15 14:47 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-03-15 14:46 [PATCHv2 0/7] sched/fair: Migrate 'misfit' tasks on asymmetric capacity systems Morten Rasmussen
2018-03-15 14:46 ` [PATCHv2 1/7] sched: Add static_key for asymmetric cpu capacity optimizations Morten Rasmussen
2018-04-27 14:04   ` [PATCHv2,1/7] " Jiada Wang
2018-04-29 19:09     ` Valentin Schneider
2018-03-15 14:46 ` [PATCHv2 2/7] sched/fair: Add group_misfit_task load-balance type Morten Rasmussen
2018-04-11 10:45   ` Peter Zijlstra
2018-03-15 14:47 ` [PATCHv2 3/7] sched/fair: Consider misfit tasks when load-balancing Morten Rasmussen
2018-04-11 10:46   ` Peter Zijlstra
2018-04-11 10:53   ` Peter Zijlstra
2018-03-15 14:47 ` [PATCHv2 4/7] sched/fair: Kick nohz balance if rq->misfit_task Morten Rasmussen
2018-03-15 14:47 ` [PATCHv2 5/7] sched: Change root_domain->overload type to int Morten Rasmussen
2018-03-15 14:47 ` [PATCHv2 6/7] sched: Wrap rq->rd->overload accesses with READ/WRITE_ONCE Morten Rasmussen
2018-03-15 14:47 ` Morten Rasmussen [this message]
2018-03-20  5:30 ` [PATCHv2 0/7] sched/fair: Migrate 'misfit' tasks on asymmetric capacity systems Gaku Inami
2018-03-20  9:17   ` Morten Rasmussen

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=1521125224-15434-8-git-send-email-morten.rasmussen@arm.com \
    --to=morten.rasmussen@arm.com \
    --cc=dietmar.eggemann@arm.com \
    --cc=gaku.inami.xh@renesas.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mingo@redhat.com \
    --cc=peterz@infradead.org \
    --cc=valentin.schneider@arm.com \
    --cc=vincent.guittot@linaro.org \
    --subject='Re: [PATCHv2 7/7] sched/fair: Set sd->overload when misfit' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).