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Date: Wed, 02 Oct 2013 15:44:40 +0530
From: "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To: Peter Zijlstra <peterz@...radead.org>
CC: "Rafael J. Wysocki" <rjw@...ysocki.net>,
Oleg Nesterov <oleg@...hat.com>,
"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>,
Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()
On 10/01/2013 11:44 PM, Srivatsa S. Bhat wrote:
> On 10/01/2013 11:06 PM, Peter Zijlstra wrote:
>> On Tue, Oct 01, 2013 at 10:41:15PM +0530, Srivatsa S. Bhat wrote:
>>> However, as Oleg said, its definitely worth considering whether this proposed
>>> change in semantics is going to hurt us in the future. CPU_POST_DEAD has certainly
>>> proved to be very useful in certain challenging situations (commit 1aee40ac9c
>>> explains one such example), so IMHO we should be very careful not to undermine
>>> its utility.
>>
>> Urgh.. crazy things. I've always understood POST_DEAD to mean 'will be
>> called at some time after the unplug' with no further guarantees. And my
>> patch preserves that.
>>
>> Its not at all clear to me why cpufreq needs more; 1aee40ac9c certainly
>> doesn't explain it.
>>
>
> Sorry if I was unclear - I didn't mean to say that cpufreq needs more guarantees
> than that. I was just saying that the cpufreq code would need certain additional
> changes/restructuring to accommodate the change in the semantics brought about
> by this patch. IOW, it won't work as it is, but it can certainly be fixed.
>
Ok, so I thought a bit more about the changes you are proposing, and I agree
that they would be beneficial in the long run, especially given that it can
eventually lead to a more stream-lined hotplug process where different CPUs
can be hotplugged independently without waiting on each other, like you
mentioned in your other mail. So I'm fine with the new POST_DEAD guarantees
you are proposing - that they are run after unplug, and will be completed
before UP_PREPARE of the same CPU. And its also very convenient that we need
to fix only cpufreq to accommodate this change.
So below is a quick untested patch that modifies the cpufreq hotplug
callbacks appropriately. With this, cpufreq should be able to handle the
POST_DEAD changes, irrespective of whether we do that in the regular path
or in the suspend/resume path. (Because, I've restructured it in such a way
that the races that I had mentioned earlier are totally avoided. That is,
the POST_DEAD handler now performs only the bare-minimal final cleanup, which
doesn't race with or depend on anything else).
diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 04548f7..0a33c1a 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -1165,7 +1165,7 @@ static int __cpufreq_remove_dev_prepare(struct device *dev,
bool frozen)
{
unsigned int cpu = dev->id, cpus;
- int new_cpu, ret;
+ int new_cpu, ret = 0;
unsigned long flags;
struct cpufreq_policy *policy;
@@ -1200,9 +1200,10 @@ static int __cpufreq_remove_dev_prepare(struct device *dev,
policy->governor->name, CPUFREQ_NAME_LEN);
#endif
- lock_policy_rwsem_read(cpu);
+ lock_policy_rwsem_write(cpu);
cpus = cpumask_weight(policy->cpus);
- unlock_policy_rwsem_read(cpu);
+ cpumask_clear_cpu(cpu, policy->cpus);
+ unlock_policy_rwsem_write(cpu);
if (cpu != policy->cpu) {
if (!frozen)
@@ -1220,7 +1221,23 @@ static int __cpufreq_remove_dev_prepare(struct device *dev,
}
}
- return 0;
+ /* If no target, nothing more to do */
+ if (!cpufreq_driver->target)
+ return 0;
+
+ /* If cpu is last user of policy, cleanup the policy governor */
+ if (cpus == 1) {
+ ret = __cpufreq_governor(policy, CPUFREQ_GOV_POLICY_EXIT);
+ if (ret)
+ pr_err("%s: Failed to exit governor\n", __func__);
+ } else {
+ if ((ret = __cpufreq_governor(policy, CPUFREQ_GOV_START)) ||
+ (ret = __cpufreq_governor(policy, CPUFREQ_GOV_LIMITS))) {
+ pr_err("%s: Failed to start governor\n", __func__);
+ }
+ }
+
+ return ret;
}
static int __cpufreq_remove_dev_finish(struct device *dev,
@@ -1243,25 +1260,12 @@ static int __cpufreq_remove_dev_finish(struct device *dev,
return -EINVAL;
}
- WARN_ON(lock_policy_rwsem_write(cpu));
+ WARN_ON(lock_policy_rwsem_read(cpu));
cpus = cpumask_weight(policy->cpus);
-
- if (cpus > 1)
- cpumask_clear_cpu(cpu, policy->cpus);
- unlock_policy_rwsem_write(cpu);
+ unlock_policy_rwsem_read(cpu);
/* If cpu is last user of policy, free policy */
- if (cpus == 1) {
- if (cpufreq_driver->target) {
- ret = __cpufreq_governor(policy,
- CPUFREQ_GOV_POLICY_EXIT);
- if (ret) {
- pr_err("%s: Failed to exit governor\n",
- __func__);
- return ret;
- }
- }
-
+ if (cpus == 0) {
if (!frozen) {
lock_policy_rwsem_read(cpu);
kobj = &policy->kobj;
@@ -1294,15 +1298,6 @@ static int __cpufreq_remove_dev_finish(struct device *dev,
if (!frozen)
cpufreq_policy_free(policy);
- } else {
- if (cpufreq_driver->target) {
- if ((ret = __cpufreq_governor(policy, CPUFREQ_GOV_START)) ||
- (ret = __cpufreq_governor(policy, CPUFREQ_GOV_LIMITS))) {
- pr_err("%s: Failed to start governor\n",
- __func__);
- return ret;
- }
- }
}
per_cpu(cpufreq_cpu_data, cpu) = NULL;
Regards,
Srivatsa S. Bhat
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