[ 012/108] cpufreq/powernow-k8: workqueue user shouldnt migrate the kworker to another CPU

Ben Hutchings
2012-10-07T23:47:47Z

From: Tejun Heo <tj@kernel.org>

commit 6889125b8b4e09c5e53e6ecab3433bed1ce198c9 upstream.

powernowk8_target() runs off a per-cpu work item and if the
cpufreq_policy->cpu is different from the current one, it migrates the
kworker to the target CPU by manipulating current->cpus_allowed. The
function migrates the kworker back to the original CPU but this is
still broken. Workqueue concurrency management requires the kworkers
to stay on the same CPU and powernowk8_target() ends up triggerring
BUG_ON(rq != this_rq()) in try_to_wake_up_local() if it contends on
fidvid_mutex and sleeps.

It is unclear why this bug is being reported now. Duncan says it
appeared to be a regression of 3.6-rc1 and couldn't reproduce it on
3.5. Bisection seemed to point to 63d95a91 "workqueue: use @pool
instead of @gcwq or @cpu where applicable" which is an non-functional
change. Given that the reproduce case sometimes took upto days to
trigger, it's easy to be misled while bisecting. Maybe something made
contention on fidvid_mutex more likely? I don't know.

This patch fixes the bug by using work_on_cpu() instead if @pol->cpu
isn't the same as the current one. The code assumes that
cpufreq_policy->cpu is kept online by the caller, which Rafael tells
me is the case.

stable: ed48ece27c ("workqueue: reimplement work_on_cpu() using
system_wq") should be applied before this; otherwise, the
behavior could be horrible.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Duncan <1i5t5.duncan@cox.net>
Tested-by: Duncan <1i5t5.duncan@cox.net>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
Cc: Andreas Herrmann <andreas.herrmann3@amd.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=47301
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/cpufreq/powernow-k8.c | 63 ++++++++++++++++++++++-------------------
1 file changed, 34 insertions(+), 29 deletions(-)

diff --git a/drivers/cpufreq/powernow-k8.c b/drivers/cpufreq/powernow-k8.c
index c0e8164..1a40935 100644
--- a/drivers/cpufreq/powernow-k8.c
+++ b/drivers/cpufreq/powernow-k8.c
@@ -35,7 +35,6 @@
#include <linux/slab.h>
#include <linux/string.h>
#include <linux/cpumask.h>
-#include <linux/sched.h> /* for current / set_cpus_allowed() */
#include <linux/io.h>
#include <linux/delay.h>

@@ -1139,16 +1138,23 @@ static int transition_frequency_pstate(struct powernow_k8_data *data,
return res;
}

-/* Driver entry point to switch to the target frequency */
-static int powernowk8_target(struct cpufreq_policy *pol,
- unsigned targfreq, unsigned relation)
+struct powernowk8_target_arg {
+ struct cpufreq_policy *pol;
+ unsigned targfreq;
+ unsigned relation;
+};
+
+static long powernowk8_target_fn(void *arg)
{
- cpumask_var_t oldmask;
+ struct powernowk8_target_arg *pta = arg;
+ struct cpufreq_policy *pol = pta->pol;
+ unsigned targfreq = pta->targfreq;
+ unsigned relation = pta->relation;
struct powernow_k8_data *data = per_cpu(powernow_data, pol->cpu);
u32 checkfid;
u32 checkvid;
unsigned int newstate;
- int ret = -EIO;
+ int ret;

if (!data)
return -EINVAL;
@@ -1156,29 +1162,16 @@ static int powernowk8_target(struct cpufreq_policy *pol,
checkfid = data->currfid;
checkvid = data->currvid;

- /* only run on specific CPU from here on. */
- /* This is poor form: use a workqueue or smp_call_function_single */
- if (!alloc_cpumask_var(&oldmask, GFP_KERNEL))
- return -ENOMEM;
-
- cpumask_copy(oldmask, tsk_cpus_allowed(current));
- set_cpus_allowed_ptr(current, cpumask_of(pol->cpu));
-
- if (smp_processor_id() != pol->cpu) {
- printk(KERN_ERR PFX "limiting to cpu %u failed\n", pol->cpu);
- goto err_out;
- }
-
if (pending_bit_stuck()) {
printk(KERN_ERR PFX "failing targ, change pending bit set\n");
- goto err_out;
+ return -EIO;
}

pr_debug("targ: cpu %d, %d kHz, min %d, max %d, relation %d\n",
pol->cpu, targfreq, pol->min, pol->max, relation);

if (query_current_values_with_pending_wait(data))
- goto err_out;
+ return -EIO;

if (cpu_family != CPU_HW_PSTATE) {
pr_debug("targ: curr fid 0x%x, vid 0x%x\n",
@@ -1196,7 +1189,7 @@ static int powernowk8_target(struct cpufreq_policy *pol,

if (cpufreq_frequency_table_target(pol, data->powernow_table,
targfreq, relation, &newstate))
- goto err_out;
+ return -EIO;

mutex_lock(&fidvid_mutex);

@@ -1209,9 +1202,8 @@ static int powernowk8_target(struct cpufreq_policy *pol,
ret = transition_frequency_fidvid(data, newstate);
if (ret) {
printk(KERN_ERR PFX "transition frequency failed\n");
- ret = 1;
mutex_unlock(&fidvid_mutex);
- goto err_out;
+ return 1;
}
mutex_unlock(&fidvid_mutex);

@@ -1220,12 +1212,25 @@ static int powernowk8_target(struct cpufreq_policy *pol,
data->powernow_table[newstate].index);
else
pol->cur = find_khz_freq_from_fid(data->currfid);
- ret = 0;

-err_out:
- set_cpus_allowed_ptr(current, oldmask);
- free_cpumask_var(oldmask);
- return ret;
+ return 0;
+}
+
+/* Driver entry point to switch to the target frequency */
+static int powernowk8_target(struct cpufreq_policy *pol,
+ unsigned targfreq, unsigned relation)
+{
+ struct powernowk8_target_arg pta = { .pol = pol, .targfreq = targfreq,
+ .relation = relation };
+
+ /*
+ * Must run on @pol->cpu. cpufreq core is responsible for ensuring
+ * that we're bound to the current CPU and pol->cpu stays online.
+ */
+ if (smp_processor_id() == pol->cpu)
+ return powernowk8_target_fn(&pta);
+ else
+ return work_on_cpu(pol->cpu, powernowk8_target_fn, &pta);
}

/* Driver entry point to verify the policy and range of frequencies */