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Date: Tue, 17 Mar 2009 16:42:55 +1100
From: Paul Mackerras <paulus@...ba.org>
To: Ingo Molnar <mingo@...e.hu>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC: linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH/RFC 2/2] perfcounters: 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: Paul Mackerras <paulus@...ba.org>
---
arch/powerpc/kernel/perf_counter.c | 6 +++
include/linux/perf_counter.h | 15 +++++++
kernel/perf_counter.c | 78 ++++++++++++++++++++++++++++++++++++
3 files changed, 99 insertions(+), 0 deletions(-)
diff --git a/arch/powerpc/kernel/perf_counter.c b/arch/powerpc/kernel/perf_counter.c
index 5008762..44271b6 100644
--- a/arch/powerpc/kernel/perf_counter.c
+++ b/arch/powerpc/kernel/perf_counter.c
@@ -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;
}
}
@@ -732,6 +736,8 @@ 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.
diff --git a/include/linux/perf_counter.h b/include/linux/perf_counter.h
index 08c11a6..3f9309f 100644
--- a/include/linux/perf_counter.h
+++ b/include/linux/perf_counter.h
@@ -100,6 +100,17 @@ 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:
@@ -214,6 +225,9 @@ 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 */
@@ -289,6 +303,7 @@ 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);
/*
* Return 1 for a software counter, 0 for a hardware counter
diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c
index 7c62b93..a440b9e 100644
--- a/kernel/perf_counter.c
+++ b/kernel/perf_counter.c
@@ -1165,6 +1165,7 @@ static int perf_release(struct inode *inode, struct file *file)
mutex_unlock(&counter->mutex);
mutex_unlock(&ctx->mutex);
+ free_page(counter->user_page);
call_rcu(&counter->rcu_head, free_counter_rcu);
put_context(ctx);
@@ -1331,12 +1332,87 @@ 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_intr_fops = {
.release = perf_release,
.read = perf_read_irq_data,
.poll = perf_poll,
.unlocked_ioctl = perf_ioctl,
.compat_ioctl = perf_ioctl,
+ .mmap = perf_mmap,
};
static const struct file_operations perf_value_fops = {
@@ -1345,6 +1421,7 @@ static const struct file_operations perf_value_fops = {
.poll = perf_poll,
.unlocked_ioctl = perf_ioctl,
.compat_ioctl = perf_ioctl,
+ .mmap = perf_mmap,
};
static const struct file_operations perf_clone_fops = {
@@ -1353,6 +1430,7 @@ static const struct file_operations perf_clone_fops = {
.poll = perf_poll,
.unlocked_ioctl = perf_ioctl,
.compat_ioctl = perf_ioctl,
+ .mmap = perf_mmap,
};
/*
--
1.5.5.rc3.7.gba13
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