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Date: Wed, 25 Mar 2009 15:52:54 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Ingo Molnar <mingo@...e.hu>
Cc: linux-kernel@...r.kernel.org, Paul Mackerras <paulus@...ba.org>,
Mike Galbraith <efault@....de>,
Arjan van de Ven <arjan@...radead.org>,
Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: [PATCH 5/6] perf_counter: kerneltop: mmap_pages argument
On Wed, 2009-03-25 at 13:57 +0100, Peter Zijlstra wrote:
> On Wed, 2009-03-25 at 13:54 +0100, Ingo Molnar wrote:
> > * Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> >
> > > On Wed, 2009-03-25 at 13:35 +0100, Ingo Molnar wrote:
> > >
> > > > > Also, when mixing streams (events,mmap) is a single: you missed
> > > > > 'n' events still good?
> > > >
> > > > How would such mixing work? Multiple counters streaming into the
> > > > same mmap area?
> > >
> > > No basically having overflow events and mmap-vma changed events in
> > > a single output stream.
> >
> > ah, and i missed the impact of variable size records - that too
> > makes it somewhat impractical to emit overflow records in situ. (the
> > kernel does not really know the precise start of the previous
> > record, typically.)
>
> Alternatively, we could simply not emit new events until the read
> position increases,. that's much simpler.
>
> Still don't like mapping the stuff writable though..
This is what it would look like I suppose...
Any thoughts?
Not-signed-off-by: me
---
include/linux/perf_counter.h | 4 ++
kernel/perf_counter.c | 67 +++++++++++++++++++++++++++++++++++++----
2 files changed, 64 insertions(+), 7 deletions(-)
diff --git a/include/linux/perf_counter.h b/include/linux/perf_counter.h
index 6bf67ce..d5a599c 100644
--- a/include/linux/perf_counter.h
+++ b/include/linux/perf_counter.h
@@ -165,6 +165,8 @@ struct perf_counter_mmap_page {
__s64 offset; /* add to hardware counter value */
__u32 data_head; /* head in the data section */
+ __u32 data_tail; /* user-space written tail */
+ __u32 overflow; /* number of lost events */
};
struct perf_event_header {
@@ -269,8 +271,10 @@ struct file;
struct perf_mmap_data {
struct rcu_head rcu_head;
int nr_pages;
+ int writable;
atomic_t wakeup;
atomic_t head;
+ atomic_t overflow;
struct perf_counter_mmap_page *user_page;
void *data_pages[0];
};
diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c
index 3b862a7..1f5c515 100644
--- a/kernel/perf_counter.c
+++ b/kernel/perf_counter.c
@@ -1330,6 +1330,7 @@ static void __perf_counter_update_userpage(struct perf_counter *counter,
userpg->offset -= atomic64_read(&counter->hw.prev_count);
userpg->data_head = atomic_read(&data->head);
+ userpg->overflow = atomic_read(&data->overflow);
smp_wmb();
++userpg->lock;
preempt_enable();
@@ -1375,6 +1376,28 @@ unlock:
return ret;
}
+static int perf_mmap_mkwrite(struct vm_area_struct *vma, struct page *page)
+{
+ int ret = -EINVAL;
+
+ rcu_read_lock();
+ data = rcu_dereference(counter->data);
+ if (!data)
+ goto unlock;
+
+ /*
+ * Only allow writes to the control page.
+ */
+ if (page != virt_to_page(data->user_page))
+ goto unlock;
+
+ ret = 0;
+unlock:
+ rcu_read_unlock();
+
+ return ret;
+}
+
static int perf_mmap_data_alloc(struct perf_counter *counter, int nr_pages)
{
struct perf_mmap_data *data;
@@ -1463,6 +1486,7 @@ static struct vm_operations_struct perf_mmap_vmops = {
.open = perf_mmap_open,
.close = perf_mmap_close,
.fault = perf_mmap_fault,
+ .page_mkwrite = perf_mmap_mkwrite,
};
static int perf_mmap(struct file *file, struct vm_area_struct *vma)
@@ -1473,7 +1497,7 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
unsigned long locked, lock_limit;
int ret = 0;
- if (!(vma->vm_flags & VM_SHARED) || (vma->vm_flags & VM_WRITE))
+ if (!(vma->vm_flags & VM_SHARED))
return -EINVAL;
vma_size = vma->vm_end - vma->vm_start;
@@ -1503,16 +1527,19 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
mutex_lock(&counter->mmap_mutex);
if (atomic_inc_not_zero(&counter->mmap_count))
- goto out;
+ goto unlock;
WARN_ON(counter->data);
ret = perf_mmap_data_alloc(counter, nr_pages);
- if (!ret)
- atomic_set(&counter->mmap_count, 1);
-out:
+ if (ret)
+ goto unlock;
+
+ atomic_set(&counter->mmap_count, 1);
+ if (vma->vm_flags & VM_WRITE)
+ counter->data->writable = 1;
+unlock:
mutex_unlock(&counter->mmap_mutex);
- vma->vm_flags &= ~VM_MAYWRITE;
vma->vm_flags |= VM_RESERVED;
vma->vm_ops = &perf_mmap_vmops;
@@ -1540,6 +1567,28 @@ struct perf_output_handle {
int wakeup;
};
+static int perf_output_overflow(struct perf_mmap_data *data,
+ unsigned int offset, unsigned int head)
+{
+ unsigned int tail;
+ unsigned int mask;
+
+ if (!data->writable)
+ return 0;
+
+ mask = (data->nr_pages << PAGE_SHIFT) - 1;
+ smp_rmb();
+ tail = ACCESS_ONCE(data->user_page->data_tail);
+
+ offset = (offset - tail) & mask;
+ head = (head - tail) & mask;
+
+ if ((int)(head - offset) < 0)
+ return 1;
+
+ return 0;
+}
+
static int perf_output_begin(struct perf_output_handle *handle,
struct perf_counter *counter, unsigned int size)
{
@@ -1552,11 +1601,13 @@ static int perf_output_begin(struct perf_output_handle *handle,
goto out;
if (!data->nr_pages)
- goto out;
+ goto fail;
do {
offset = head = atomic_read(&data->head);
head += size;
+ if (unlikely(perf_output_overflow(data, offset, head)))
+ goto fail;
} while (atomic_cmpxchg(&data->head, offset, head) != offset);
handle->counter = counter;
@@ -1567,6 +1618,8 @@ static int perf_output_begin(struct perf_output_handle *handle,
return 0;
+fail:
+ atomic_inc(&data->overflow);
out:
rcu_read_unlock();
--
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