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Commit 7595d63b authored by Paul Mackerras's avatar Paul Mackerras Committed by Ingo Molnar
Browse files

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



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: default avatarPaul Mackerras <paulus@samba.org>
Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
Orig-LKML-Reference: <20090330171023.627912475@chello.nl>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 3c1ba6fa

arch/powerpc/kernel/perf_counter.c

+42 −5
Original line number Diff line number Diff line
@@ -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)) {