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Commit 37d81828 authored by Paul Mackerras's avatar Paul Mackerras Committed by Ingo Molnar
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perf_counter: add an mmap method to allow userspace to read hardware counters 



Impact: new feature giving performance improvement

This adds the ability for userspace to do an mmap on a hardware counter
fd and get access to a read-only page that contains the information
needed to translate a hardware counter value to the full 64-bit
counter value that would be returned by a read on the fd.  This is
useful on architectures that allow user programs to read the hardware
counters, such as PowerPC.

The mmap will only succeed if the counter is a hardware counter
monitoring the current process.

On my quad 2.5GHz PowerPC 970MP machine, userspace can read a counter
and translate it to the full 64-bit value in about 30ns using the
mmapped page, compared to about 830ns for the read syscall on the
counter, so this does give a significant performance improvement.

Signed-off-by: default avatarPaul Mackerras <paulus@samba.org>
Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
Orig-LKML-Reference: <20090323172417.297057964@chello.nl>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 96f6d444

arch/powerpc/kernel/perf_counter.c

+6 −0
Original line number Diff line number Diff line
@@ -417,6 +417,8 @@ void hw_perf_restore(u64 disable) 
		atomic64_set(&counter->hw.prev_count, val);

		counter->hw.idx = hwc_index[i] + 1;

		write_pmc(counter->hw.idx, val);

		if (counter->user_page)

			perf_counter_update_userpage(counter);

	}

	mb();

	cpuhw->mmcr[0] |= MMCR0_PMXE | MMCR0_FCECE;
@@ -572,6 +574,8 @@ static void power_perf_disable(struct perf_counter *counter) 
			ppmu->disable_pmc(counter->hw.idx - 1, cpuhw->mmcr);

			write_pmc(counter->hw.idx, 0);

			counter->hw.idx = 0;

			if (counter->user_page)

				perf_counter_update_userpage(counter);

			break;

		}

	}
@@ -698,6 +702,8 @@ static void record_and_restart(struct perf_counter *counter, long val, 
	write_pmc(counter->hw.idx, val);

	atomic64_set(&counter->hw.prev_count, val);

	atomic64_set(&counter->hw.period_left, left);

	if (counter->user_page)

		perf_counter_update_userpage(counter);



	/*

	 * Finally record data if requested.

include/linux/perf_counter.h

+15 −0
Original line number Diff line number Diff line
@@ -143,6 +143,17 @@ struct perf_counter_hw_event { 
#define PERF_COUNTER_IOC_ENABLE		_IO('$', 0)

#define PERF_COUNTER_IOC_DISABLE	_IO('$', 1)



/*

 * Structure of the page that can be mapped via mmap

 */

struct perf_counter_mmap_page {

	__u32	version;		/* version number of this structure */

	__u32	compat_version;		/* lowest version this is compat with */

	__u32	lock;			/* seqlock for synchronization */

	__u32	index;			/* hardware counter identifier */

	__s64	offset;			/* add to hardware counter value */

};



#ifdef __KERNEL__

/*

 * Kernel-internal data types and definitions:
@@ -278,6 +289,9 @@ struct perf_counter { 
	int				oncpu;

	int				cpu;



	/* pointer to page shared with userspace via mmap */

	unsigned long			user_page;



	/* read() / irq related data */

	wait_queue_head_t		waitq;

	/* optional: for NMIs */
@@ -361,6 +375,7 @@ extern int perf_counter_task_enable(void); 
extern int hw_perf_group_sched_in(struct perf_counter *group_leader,

	       struct perf_cpu_context *cpuctx,

	       struct perf_counter_context *ctx, int cpu);

extern void perf_counter_update_userpage(struct perf_counter *counter);



extern void perf_counter_output(struct perf_counter *counter,

				int nmi, struct pt_regs *regs);

kernel/perf_counter.c

+76 −0
Original line number Diff line number Diff line
@@ -1177,6 +1177,7 @@ static int perf_release(struct inode *inode, struct file *file) 
	mutex_unlock(&counter->mutex);

	mutex_unlock(&ctx->mutex);



	free_page(counter->user_page);

	free_counter(counter);

	put_context(ctx);
@@ -1346,12 +1347,87 @@ static long perf_ioctl(struct file *file, unsigned int cmd, unsigned long arg) 
	return err;

}



void perf_counter_update_userpage(struct perf_counter *counter)

{

	struct perf_counter_mmap_page *userpg;



	if (!counter->user_page)

		return;

	userpg = (struct perf_counter_mmap_page *) counter->user_page;



	++userpg->lock;

	smp_wmb();

	userpg->index = counter->hw.idx;

	userpg->offset = atomic64_read(&counter->count);

	if (counter->state == PERF_COUNTER_STATE_ACTIVE)

		userpg->offset -= atomic64_read(&counter->hw.prev_count);

	smp_wmb();

	++userpg->lock;

}



static int perf_mmap_fault(struct vm_area_struct *vma, struct vm_fault *vmf)

{

	struct perf_counter *counter = vma->vm_file->private_data;



	if (!counter->user_page)

		return VM_FAULT_SIGBUS;



	vmf->page = virt_to_page(counter->user_page);

	get_page(vmf->page);

	return 0;

}



static struct vm_operations_struct perf_mmap_vmops = {

	.fault = perf_mmap_fault,

};



static int perf_mmap(struct file *file, struct vm_area_struct *vma)

{

	struct perf_counter *counter = file->private_data;

	unsigned long userpg;



	if (!(vma->vm_flags & VM_SHARED) || (vma->vm_flags & VM_WRITE))

		return -EINVAL;

	if (vma->vm_end - vma->vm_start != PAGE_SIZE)

		return -EINVAL;



	/*

	 * For now, restrict to the case of a hardware counter

	 * on the current task.

	 */

	if (is_software_counter(counter) || counter->task != current)

		return -EINVAL;



	userpg = counter->user_page;

	if (!userpg) {

		userpg = get_zeroed_page(GFP_KERNEL);

		mutex_lock(&counter->mutex);

		if (counter->user_page) {

			free_page(userpg);

			userpg = counter->user_page;

		} else {

			counter->user_page = userpg;

		}

		mutex_unlock(&counter->mutex);

		if (!userpg)

			return -ENOMEM;

	}



	perf_counter_update_userpage(counter);



	vma->vm_flags &= ~VM_MAYWRITE;

	vma->vm_flags |= VM_RESERVED;

	vma->vm_ops = &perf_mmap_vmops;

	return 0;

}



static const struct file_operations perf_fops = {

	.release		= perf_release,

	.read			= perf_read,

	.poll			= perf_poll,

	.unlocked_ioctl		= perf_ioctl,

	.compat_ioctl		= perf_ioctl,

	.mmap			= perf_mmap,

};



/*