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Subject[PATCH 06/15] perf_counter: powerpc: only reserve PMU hardware when we need it
From: Paul Mackerras <paulus@samba.org>

Impact: cooperate with oprofile

At present, on PowerPC, if you have perf_counters compiled in, oprofile
doesn't work. There is code to allow the PMU to be shared between
competing subsystems, such as perf_counters and oprofile, but currently
the perf_counter subsystem reserves the PMU for itself at boot time,
and never releases it.

This makes perf_counter play nicely with oprofile. Now we keep a count
of how many perf_counter instances are counting hardware events, and
reserve the PMU when that count becomes non-zero, and release the PMU
when that count becomes zero. This means that it is possible to have
perf_counters compiled in and still use oprofile, as long as there are
no hardware perf_counters active. This also means that if oprofile is
active, sys_perf_counter_open will fail if the hw_event specifies a
hardware event.

To avoid races with other tasks creating and destroying perf_counters,
we use a mutex. We use atomic_inc_not_zero and atomic_add_unless to
avoid having to take the mutex unless there is a possibility of the
count going between 0 and 1.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---

arch/powerpc/kernel/perf_counter.c | 47 +++++++++++++++++++++++++++++++++----
1 file changed, 42 insertions(+), 5 deletions(-)

Index: linux-2.6/arch/powerpc/kernel/perf_counter.c
===================================================================
--- linux-2.6.orig/arch/powerpc/kernel/perf_counter.c
+++ linux-2.6/arch/powerpc/kernel/perf_counter.c
@@ -41,6 +41,8 @@ struct power_pmu *ppmu;
*/
static unsigned int freeze_counters_kernel = MMCR0_FCS;

+static void perf_counter_interrupt(struct pt_regs *regs);
+
void perf_counter_print_debug(void)
{
}
@@ -594,6 +596,24 @@ struct hw_perf_counter_ops power_perf_op
.read = power_perf_read
};

+/* Number of perf_counters counting hardware events */
+static atomic_t num_counters;
+/* Used to avoid races in calling reserve/release_pmc_hardware */
+static DEFINE_MUTEX(pmc_reserve_mutex);
+
+/*
+ * Release the PMU if this is the last perf_counter.
+ */
+static void hw_perf_counter_destroy(struct perf_counter *counter)
+{
+ if (!atomic_add_unless(&num_counters, -1, 1)) {
+ mutex_lock(&pmc_reserve_mutex);
+ if (atomic_dec_return(&num_counters) == 0)
+ release_pmc_hardware();
+ mutex_unlock(&pmc_reserve_mutex);
+ }
+}
+
const struct hw_perf_counter_ops *
hw_perf_counter_init(struct perf_counter *counter)
{
@@ -601,6 +621,7 @@ hw_perf_counter_init(struct perf_counter
struct perf_counter *ctrs[MAX_HWCOUNTERS];
unsigned int events[MAX_HWCOUNTERS];
int n;
+ int err;

if (!ppmu)
return NULL;
@@ -646,6 +667,27 @@ hw_perf_counter_init(struct perf_counter

counter->hw.config = events[n];
atomic64_set(&counter->hw.period_left, counter->hw_event.irq_period);
+
+ /*
+ * See if we need to reserve the PMU.
+ * If no counters are currently in use, then we have to take a
+ * mutex to ensure that we don't race with another task doing
+ * reserve_pmc_hardware or release_pmc_hardware.
+ */
+ err = 0;
+ if (!atomic_inc_not_zero(&num_counters)) {
+ mutex_lock(&pmc_reserve_mutex);
+ if (atomic_read(&num_counters) == 0 &&
+ reserve_pmc_hardware(perf_counter_interrupt))
+ err = -EBUSY;
+ else
+ atomic_inc(&num_counters);
+ mutex_unlock(&pmc_reserve_mutex);
+ }
+ counter->destroy = hw_perf_counter_destroy;
+
+ if (err)
+ return NULL;
return &power_perf_ops;
}

@@ -756,11 +798,6 @@ static int init_perf_counters(void)
{
unsigned long pvr;

- if (reserve_pmc_hardware(perf_counter_interrupt)) {
- printk(KERN_ERR "Couldn't init performance monitor subsystem\n");
- return -EBUSY;
- }
-
/* XXX should get this from cputable */
pvr = mfspr(SPRN_PVR);
switch (PVR_VER(pvr)) {
--



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