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Date: Sat, 28 Sep 2013 16:47:20 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
Ingo Molnar <mingo@...nel.org>,
Andrea Arcangeli <aarcange@...hat.com>,
Johannes Weiner <hannes@...xchg.org>,
Linux-MM <linux-mm@...ck.org>,
LKML <linux-kernel@...r.kernel.org>,
Thomas Gleixner <tglx@...utronix.de>,
Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()
On Sat, Sep 28, 2013 at 02:48:59PM +0200, Oleg Nesterov wrote:
> > > > void cpu_hotplug_done(void)
> > > > {
> ...
> > > > + /*
> > > > + * Wait for any pending readers to be running. This ensures readers
> > > > + * after writer and avoids writers starving readers.
> > > > + */
> > > > + wait_event(cpuhp_writer, !atomic_read(&cpuhp_waitcount));
> > > > }
> > >
> > > OK, to some degree I can understand "avoids writers starving readers"
> > > part (although the next writer should do synchronize_sched() first),
> > > but could you explain "ensures readers after writer" ?
> >
> > Suppose reader A sees state == BLOCK and goes to sleep; our writer B
> > does cpu_hotplug_done() and wakes all pending readers. If for some
> > reason A doesn't schedule to inc ref until B again executes
> > cpu_hotplug_begin() and state is once again BLOCK, A will not have made
> > any progress.
>
> Yes, yes, thanks, this is clear. But this explains "writers starving readers".
> And let me repeat, if B again executes cpu_hotplug_begin() it will do
> another synchronize_sched() before it sets BLOCK, so I am not sure we
> need this "in practice".
>
> I was confused by "ensures readers after writer", I thought this means
> we need the additional synchronization with the readers which are going
> to increment cpuhp_waitcount, say, some sort of barries.
Ah no; I just wanted to guarantee that any pending readers did get a
chance to run. And yes due to the two sync_sched() calls it seems
somewhat unlikely in practise.
> Please note that this wait_event() adds a problem... it doesn't allow
> to "offload" the final synchronize_sched(). Suppose a 4k cpu machine
> does disable_nonboot_cpus(), we do not want 2 * 4k * synchronize_sched's
> in this case. We can solve this, but this wait_event() complicates
> the problem.
That seems like a particularly easy fix; something like so?
---
include/linux/cpu.h | 1
kernel/cpu.c | 84 ++++++++++++++++++++++++++++++++++------------------
2 files changed, 56 insertions(+), 29 deletions(-)
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -109,6 +109,7 @@ enum {
#define CPU_DOWN_FAILED_FROZEN (CPU_DOWN_FAILED | CPU_TASKS_FROZEN)
#define CPU_DEAD_FROZEN (CPU_DEAD | CPU_TASKS_FROZEN)
#define CPU_DYING_FROZEN (CPU_DYING | CPU_TASKS_FROZEN)
+#define CPU_POST_DEAD_FROZEN (CPU_POST_DEAD | CPU_TASKS_FROZEN)
#define CPU_STARTING_FROZEN (CPU_STARTING | CPU_TASKS_FROZEN)
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -364,8 +364,7 @@ static int __ref take_cpu_down(void *_pa
return 0;
}
-/* Requires cpu_add_remove_lock to be held */
-static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
+static int __ref __cpu_down(unsigned int cpu, int tasks_frozen)
{
int err, nr_calls = 0;
void *hcpu = (void *)(long)cpu;
@@ -375,21 +374,13 @@ static int __ref _cpu_down(unsigned int
.hcpu = hcpu,
};
- if (num_online_cpus() == 1)
- return -EBUSY;
-
- if (!cpu_online(cpu))
- return -EINVAL;
-
- cpu_hotplug_begin();
-
err = __cpu_notify(CPU_DOWN_PREPARE | mod, hcpu, -1, &nr_calls);
if (err) {
nr_calls--;
__cpu_notify(CPU_DOWN_FAILED | mod, hcpu, nr_calls, NULL);
printk("%s: attempt to take down CPU %u failed\n",
__func__, cpu);
- goto out_release;
+ return err;
}
smpboot_park_threads(cpu);
@@ -398,7 +389,7 @@ static int __ref _cpu_down(unsigned int
/* CPU didn't die: tell everyone. Can't complain. */
smpboot_unpark_threads(cpu);
cpu_notify_nofail(CPU_DOWN_FAILED | mod, hcpu);
- goto out_release;
+ return err;
}
BUG_ON(cpu_online(cpu));
@@ -420,10 +411,27 @@ static int __ref _cpu_down(unsigned int
check_for_tasks(cpu);
-out_release:
+ return err;
+}
+
+/* Requires cpu_add_remove_lock to be held */
+static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
+{
+ unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0;
+ int err;
+
+ if (num_online_cpus() == 1)
+ return -EBUSY;
+
+ if (!cpu_online(cpu))
+ return -EINVAL;
+
+ cpu_hotplug_begin();
+ err = __cpu_down(cpu, tasks_frozen);
cpu_hotplug_done();
+
if (!err)
- cpu_notify_nofail(CPU_POST_DEAD | mod, hcpu);
+ cpu_notify_nofail(CPU_POST_DEAD | mod, (void *)(long)cpu);
return err;
}
@@ -447,30 +455,22 @@ int __ref cpu_down(unsigned int cpu)
EXPORT_SYMBOL(cpu_down);
#endif /*CONFIG_HOTPLUG_CPU*/
-/* Requires cpu_add_remove_lock to be held */
-static int _cpu_up(unsigned int cpu, int tasks_frozen)
+static int ___cpu_up(unsigned int cpu, int tasks_frozen)
{
int ret, nr_calls = 0;
void *hcpu = (void *)(long)cpu;
unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0;
struct task_struct *idle;
- cpu_hotplug_begin();
-
- if (cpu_online(cpu) || !cpu_present(cpu)) {
- ret = -EINVAL;
- goto out;
- }
-
idle = idle_thread_get(cpu);
if (IS_ERR(idle)) {
ret = PTR_ERR(idle);
- goto out;
+ return ret;
}
ret = smpboot_create_threads(cpu);
if (ret)
- goto out;
+ return ret;
ret = __cpu_notify(CPU_UP_PREPARE | mod, hcpu, -1, &nr_calls);
if (ret) {
@@ -492,9 +492,24 @@ static int _cpu_up(unsigned int cpu, int
/* Now call notifier in preparation. */
cpu_notify(CPU_ONLINE | mod, hcpu);
+ return 0;
+
out_notify:
- if (ret != 0)
- __cpu_notify(CPU_UP_CANCELED | mod, hcpu, nr_calls, NULL);
+ __cpu_notify(CPU_UP_CANCELED | mod, hcpu, nr_calls, NULL);
+ return ret;
+}
+
+/* Requires cpu_add_remove_lock to be held */
+static int _cpu_up(unsigned int cpu, int tasks_frozen)
+{
+ cpu_hotplug_begin();
+
+ if (cpu_online(cpu) || !cpu_present(cpu)) {
+ ret = -EINVAL;
+ goto out;
+ }
+
+ ret = ___cpu_up(cpu, tasks_frozen);
out:
cpu_hotplug_done();
@@ -572,11 +587,13 @@ int disable_nonboot_cpus(void)
*/
cpumask_clear(frozen_cpus);
+ cpu_hotplug_begin();
+
printk("Disabling non-boot CPUs ...\n");
for_each_online_cpu(cpu) {
if (cpu == first_cpu)
continue;
- error = _cpu_down(cpu, 1);
+ error = __cpu_down(cpu, 1);
if (!error)
cpumask_set_cpu(cpu, frozen_cpus);
else {
@@ -586,6 +603,11 @@ int disable_nonboot_cpus(void)
}
}
+ cpu_hotplug_done();
+
+ for_each_cpu(cpu, frozen_cpus)
+ cpu_notify_nofail(CPU_POST_DEAD_FROZEN, (void*)(long)cpu);
+
if (!error) {
BUG_ON(num_online_cpus() > 1);
/* Make sure the CPUs won't be enabled by someone else */
@@ -619,8 +641,10 @@ void __ref enable_nonboot_cpus(void)
arch_enable_nonboot_cpus_begin();
+ cpu_hotplug_begin();
+
for_each_cpu(cpu, frozen_cpus) {
- error = _cpu_up(cpu, 1);
+ error = ___cpu_up(cpu, 1);
if (!error) {
printk(KERN_INFO "CPU%d is up\n", cpu);
continue;
@@ -628,6 +652,8 @@ void __ref enable_nonboot_cpus(void)
printk(KERN_WARNING "Error taking CPU%d up: %d\n", cpu, error);
}
+ cpu_hotplug_done();
+
arch_enable_nonboot_cpus_end();
cpumask_clear(frozen_cpus);
--
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