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perf_counter: powerpc: only reserve PMU hardware when we need it

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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>
Orig-LKML-Reference: <20090330171023.627912475@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Paul Mackerras 2009-03-30 19:07:07 +02:00 committed by Ingo Molnar
parent 3c1ba6fafe
commit 7595d63b3a
1 changed files with 42 additions and 5 deletions
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47
arch/powerpc/kernel/perf_counter.c
View File

@ -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_ops = {
.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 *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)
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;
}
@ -769,11 +811,6 @@ 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)) {