archive mirror
 help / color / mirror / Atom feed
From: "tip-bot2 for Valentin Schneider" <>
Cc: Valentin Schneider <>,
	"Peter Zijlstra (Intel)" <>,
	Dietmar Eggemann <>,,
	Linus Torvalds <>,
	Thomas Gleixner <>,,,,,,,
	Ingo Molnar <>, Borislav Petkov <>,
Subject: [tip: sched/urgent] sched/topology: Allow sched_asym_cpucapacity to be disabled
Date: Tue, 29 Oct 2019 09:52:17 -0000	[thread overview]
Message-ID: <157234273730.29376.11861956477066450978.tip-bot2@tip-bot2> (raw)
In-Reply-To: <>

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     e284df705cf1eeedb5ec3a66ed82d17a64659150
Author:        Valentin Schneider <>
AuthorDate:    Wed, 23 Oct 2019 16:37:45 +01:00
Committer:     Ingo Molnar <>
CommitterDate: Tue, 29 Oct 2019 09:58:46 +01:00

sched/topology: Allow sched_asym_cpucapacity to be disabled

While the static key is correctly initialized as being disabled, it will
remain forever enabled once turned on. This means that if we start with an
asymmetric system and hotplug out enough CPUs to end up with an SMP system,
the static key will remain set - which is obviously wrong. We should detect
this and turn off things like misfit migration and capacity aware wakeups.

As Quentin pointed out, having separate root domains makes this slightly
trickier. We could have exclusive cpusets that create an SMP island - IOW,
the domains within this root domain will not see any asymmetry. This means
we can't just disable the key on domain destruction, we need to count how
many asymmetric root domains we have.

Consider the following example using Juno r0 which is 2+4 big.LITTLE, where
two identical cpusets are created: they both span both big and LITTLE CPUs:

    asym0    asym1
  [       ][       ]
   L  L  B  L  L  B

  $ cgcreate -g cpuset:asym0
  $ cgset -r cpuset.cpus=0,1,3 asym0
  $ cgset -r cpuset.mems=0 asym0
  $ cgset -r cpuset.cpu_exclusive=1 asym0

  $ cgcreate -g cpuset:asym1
  $ cgset -r cpuset.cpus=2,4,5 asym1
  $ cgset -r cpuset.mems=0 asym1
  $ cgset -r cpuset.cpu_exclusive=1 asym1

  $ cgset -r cpuset.sched_load_balance=0 .

(the CPU numbering may look odd because on the Juno LITTLEs are CPUs 0,3-5
and bigs are CPUs 1-2)

If we make one of those SMP (IOW remove asymmetry) by e.g. hotplugging its
big core, we would end up with an SMP cpuset and an asymmetric cpuset - the
static key must remain set, because we still have one asymmetric root domain.

With the above example, this could be done with:

  $ echo 0 > /sys/devices/system/cpu/cpu2/online

Which would result in:

    asym0   asym1
  [       ][    ]
   L  L  B  L  L

When both SMP and asymmetric cpusets are present, all CPUs will observe
sched_asym_cpucapacity being set (it is system-wide), but not all CPUs
observe asymmetry in their sched domain hierarchy:

  per_cpu(sd_asym_cpucapacity, <any CPU in asym0>) == <some SD at DIE level>
  per_cpu(sd_asym_cpucapacity, <any CPU in asym1>) == NULL

Change the simple key enablement to an increment, and decrement the key
counter when destroying domains that cover asymmetric CPUs.

Signed-off-by: Valentin Schneider <>
Signed-off-by: Peter Zijlstra (Intel) <>
Reviewed-by: Dietmar Eggemann <>
Cc: Linus Torvalds <>
Cc: Peter Zijlstra <>
Cc: Thomas Gleixner <>
Fixes: df054e8445a4 ("sched/topology: Add static_key for asymmetric CPU capacity optimizations")
Signed-off-by: Ingo Molnar <>
 kernel/sched/topology.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 9318acf..49b835f 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -2029,7 +2029,7 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
 	if (has_asym)
-		static_branch_enable_cpuslocked(&sched_asym_cpucapacity);
+		static_branch_inc_cpuslocked(&sched_asym_cpucapacity);
 	if (rq && sched_debug_enabled) {
 		pr_info("root domain span: %*pbl (max cpu_capacity = %lu)\n",
@@ -2124,8 +2124,12 @@ int sched_init_domains(const struct cpumask *cpu_map)
 static void detach_destroy_domains(const struct cpumask *cpu_map)
+	unsigned int cpu = cpumask_any(cpu_map);
 	int i;
+	if (rcu_access_pointer(per_cpu(sd_asym_cpucapacity, cpu)))
+		static_branch_dec_cpuslocked(&sched_asym_cpucapacity);
 	for_each_cpu(i, cpu_map)
 		cpu_attach_domain(NULL, &def_root_domain, i);

      reply	other threads:[~2019-10-29  9:52 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-23 15:37 [PATCH v4 0/2] sched/topology: Asymmetric topologies fixes Valentin Schneider
2019-10-23 15:37 ` [PATCH v4 1/2] sched/topology: Don't try to build empty sched domains Valentin Schneider
2019-10-24 16:19   ` Dietmar Eggemann
2019-10-24 16:45     ` Valentin Schneider
2019-10-29  9:52   ` [tip: sched/urgent] " tip-bot2 for Valentin Schneider
2019-10-31 16:23   ` [PATCH v4 1/2] " Michal Koutný
2019-10-31 17:23     ` Valentin Schneider
2019-11-01 10:08       ` Michal Koutný
2019-10-23 15:37 ` [PATCH v4 2/2] sched/topology: Allow sched_asym_cpucapacity to be disabled Valentin Schneider
2019-10-29  9:52   ` tip-bot2 for Valentin Schneider [this message]

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:

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

  git send-email \
    --in-reply-to=157234273730.29376.11861956477066450978.tip-bot2@tip-bot2 \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
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).