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Message-ID: <20120725115415.3900.87325.stgit@srivatsabhat.in.ibm.com>
Date: Wed, 25 Jul 2012 17:24:34 +0530
From: "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To: tglx@...utronix.de, mingo@...nel.org, peterz@...radead.org,
rusty@...tcorp.com.au, paulmck@...ux.vnet.ibm.com,
namhyung@...nel.org, tj@...nel.org
Cc: rjw@...k.pl, srivatsa.bhat@...ux.vnet.ibm.com,
nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: [RFC PATCH 4/6] sched,
cpuset: Prepare scheduler and cpuset CPU hotplug callbacks for
reverse invocation
Some of the CPU hotplug callbacks of the scheduler and cpuset infrastructure are
intertwined in an interesting way. The scheduler's sched_cpu_[in]active()
callbacks and cpuset's cpuset_cpu_[in]active() callbacks have the following
documented dependency:
The sched_cpu_active() callback must be the first callback to run, and
should be immediately followed by cpuset_cpu_active() to update the
cpusets and the sched domains. This ordering (sched followed by cpuset)
needs to be honored in both the CPU online *and* the CPU offline paths.
Hence its not straightforward to convert these callbacks to the reverse
invocation model, because, a plain conversion would result in the problem
explained below.
In general, if 2 notifiers A and B expect to be -always- called in the order
A followed by B, ie., during both CPU online and CPU offline, then we can't
ensure that easily because, when we do reverse invocation, we get the
following call path:
Event | Invocation order
-------------|---------------------
CPU online: | A (high priority), B (low priority)
CPU offline: | B (low priority), A (high priority)
So this breaks the requirement for A and B. We see this ordering requirement
in the case of the scheduler and cpusets.
So, to solve this, club the 2 callbacks together as a unit, so that they
are always invoked as a unit, which means, forward or reverse, the requirement
is satisfied. In this case, since the 2 callbacks are quite related, it
doesn't break semantics/readability if we club them together, which is a good
thing!
There is a one more aspect that we need to take care of while clubbing the two
callbacks. During boot, the scheduler is initialized in two phases:
sched_init(), which happens before SMP initialization (and hence *before* the
non-boot CPUs are booted up), and sched_init_smp(), which happens after SMP
initialization (and hence *after* the non-boot CPUs are booted).
In the original code, the cpuset callbacks are registered during
sched_init_smp(), which means that while starting the non-boot CPUs, only the
scheduler callbacks are invoked, not the cpuset ones. So in order to keep
this intact even after clubbing the 2 callbacks, we need to be able to find out
if we are running post-SMP init or if we are running pre-SMP early boot code,
to decide whether to pass on control to the cpuset callback or not. So introduce
a flag 'sched_smp_init_complete' that gets set after the scheduler is
initialized for SMP. This would help us in making that decision.
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
---
include/linux/cpu.h | 16 ++++-----
kernel/sched/core.c | 89 ++++++++++++++++++++++++++++++---------------------
2 files changed, 59 insertions(+), 46 deletions(-)
diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index ce7a074..255b889 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -55,20 +55,18 @@ extern ssize_t arch_print_cpu_modalias(struct device *dev,
*/
enum {
/*
- * SCHED_ACTIVE marks a cpu which is coming up active during
- * CPU_ONLINE and CPU_DOWN_FAILED and must be the first
- * notifier. CPUSET_ACTIVE adjusts cpuset according to
- * cpu_active mask right after SCHED_ACTIVE. During
- * CPU_DOWN_PREPARE, SCHED_INACTIVE and CPUSET_INACTIVE are
- * ordered in the similar way.
+ * SCHED_ACTIVE marks a cpu which is coming up active during CPU_ONLINE
+ * and CPU_DOWN_FAILED and must be the first notifier. It then passes
+ * control to the cpuset_cpu_active() notifier which adjusts cpusets
+ * according to cpu_active mask. During CPU_DOWN_PREPARE, SCHED_INACTIVE
+ * marks the cpu as inactive and passes control to the
+ * cpuset_cpu_inactive() notifier in a similar way.
*
* This ordering guarantees consistent cpu_active mask and
* migration behavior to all cpu notifiers.
*/
CPU_PRI_SCHED_ACTIVE = INT_MAX,
- CPU_PRI_CPUSET_ACTIVE = INT_MAX - 1,
- CPU_PRI_SCHED_INACTIVE = INT_MIN + 1,
- CPU_PRI_CPUSET_INACTIVE = INT_MIN,
+ CPU_PRI_SCHED_INACTIVE = INT_MIN,
/* migration should happen before other stuff but after perf */
CPU_PRI_PERF = 20,
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d5594a4..9ccebdd 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -280,6 +280,7 @@ const_debug unsigned int sysctl_sched_time_avg = MSEC_PER_SEC;
unsigned int sysctl_sched_rt_period = 1000000;
__read_mostly int scheduler_running;
+static bool __read_mostly sched_smp_init_complete;
/*
* part of the period that we allow rt tasks to run in us.
@@ -5505,29 +5506,75 @@ static struct notifier_block __cpuinitdata migration_notifier = {
.priority = CPU_PRI_MIGRATION,
};
+/*
+ * Update cpusets according to cpu_active mask. If cpusets are
+ * disabled, cpuset_update_active_cpus() becomes a simple wrapper
+ * around partition_sched_domains().
+ */
+static int cpuset_cpu_active(struct notifier_block *nfb, unsigned long action,
+ void *hcpu)
+{
+ switch (action & ~CPU_TASKS_FROZEN) {
+ case CPU_ONLINE:
+ case CPU_DOWN_FAILED:
+ cpuset_update_active_cpus();
+ return NOTIFY_OK;
+ default:
+ return NOTIFY_DONE;
+ }
+}
+
+static int cpuset_cpu_inactive(struct notifier_block *nfb, unsigned long action,
+ void *hcpu)
+{
+ switch (action & ~CPU_TASKS_FROZEN) {
+ case CPU_DOWN_PREPARE:
+ cpuset_update_active_cpus();
+ return NOTIFY_OK;
+ default:
+ return NOTIFY_DONE;
+ }
+}
+
static int __cpuinit sched_cpu_active(struct notifier_block *nfb,
unsigned long action, void *hcpu)
{
+ int ret;
+
switch (action & ~CPU_TASKS_FROZEN) {
case CPU_STARTING:
case CPU_DOWN_FAILED:
set_cpu_active((long)hcpu, true);
- return NOTIFY_OK;
+ ret = NOTIFY_OK;
+ break;
default:
- return NOTIFY_DONE;
+ ret = NOTIFY_DONE;
}
+
+ if (likely(sched_smp_init_complete))
+ return cpuset_cpu_active(nfb, action, hcpu);
+
+ return ret;
}
static int __cpuinit sched_cpu_inactive(struct notifier_block *nfb,
unsigned long action, void *hcpu)
{
+ int ret;
+
switch (action & ~CPU_TASKS_FROZEN) {
case CPU_DOWN_PREPARE:
set_cpu_active((long)hcpu, false);
- return NOTIFY_OK;
+ ret = NOTIFY_OK;
+ break;
default:
- return NOTIFY_DONE;
+ ret = NOTIFY_DONE;
}
+
+ if (likely(sched_smp_init_complete))
+ return cpuset_cpu_inactive(nfb, action, hcpu);
+
+ return ret;
}
static int __init migration_init(void)
@@ -6967,36 +7014,6 @@ match2:
mutex_unlock(&sched_domains_mutex);
}
-/*
- * Update cpusets according to cpu_active mask. If cpusets are
- * disabled, cpuset_update_active_cpus() becomes a simple wrapper
- * around partition_sched_domains().
- */
-static int cpuset_cpu_active(struct notifier_block *nfb, unsigned long action,
- void *hcpu)
-{
- switch (action & ~CPU_TASKS_FROZEN) {
- case CPU_ONLINE:
- case CPU_DOWN_FAILED:
- cpuset_update_active_cpus();
- return NOTIFY_OK;
- default:
- return NOTIFY_DONE;
- }
-}
-
-static int cpuset_cpu_inactive(struct notifier_block *nfb, unsigned long action,
- void *hcpu)
-{
- switch (action & ~CPU_TASKS_FROZEN) {
- case CPU_DOWN_PREPARE:
- cpuset_update_active_cpus();
- return NOTIFY_OK;
- default:
- return NOTIFY_DONE;
- }
-}
-
void __init sched_init_smp(void)
{
cpumask_var_t non_isolated_cpus;
@@ -7015,9 +7032,6 @@ void __init sched_init_smp(void)
mutex_unlock(&sched_domains_mutex);
put_online_cpus();
- hotcpu_notifier(cpuset_cpu_active, CPU_PRI_CPUSET_ACTIVE);
- hotcpu_notifier(cpuset_cpu_inactive, CPU_PRI_CPUSET_INACTIVE);
-
/* RT runtime code needs to handle some hotplug events */
hotcpu_notifier(update_runtime, 0);
@@ -7030,6 +7044,7 @@ void __init sched_init_smp(void)
free_cpumask_var(non_isolated_cpus);
init_sched_rt_class();
+ sched_smp_init_complete = true;
}
#else
void __init sched_init_smp(void)
--
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