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Date: Wed, 10 Jul 2013 19:37:43 +0800
From: "Yan, Zheng" <zheng.z.yan@...el.com>
To: Peter Zijlstra <peterz@...radead.org>
CC: linux-kernel@...r.kernel.org, mingo@...e.hu, eranian@...gle.com,
ak@...ux.intel.com
Subject: Re: [PATCH] perf: Update event buffer tail when overwriting old events
On 07/08/2013 08:15 PM, Peter Zijlstra wrote:
> On Thu, Jun 06, 2013 at 01:58:06PM +0800, Yan, Zheng wrote:
>> From: "Yan, Zheng" <zheng.z.yan@...el.com>
>>
>> If perf event buffer is in overwrite mode, the kernel only updates
>> the data head when it overwrites old samples. The program that owns
>> the buffer need periodically check the buffer and update a variable
>> that tracks the date tail. If the program fails to do this in time,
>> the data tail can be overwritted by new samples. The program has to
>> rewind the buffer because it does not know where is the first vaild
>> sample.
>>
>> This patch makes the kernel update the date tail when it overwrites
>> old events. So the program that owns the event buffer can always
>> read the latest samples. This is convenient for programs that use
>> perf to do branch tracing. One use case is GDB branch tracing:
>> (http://sourceware.org/ml/gdb-patches/2012-06/msg00172.html)
>> It uses perf interface to read BTS, but only cares the branches
>> before the ptrace event.
>>
>> I added code to perf_output_{begin/end} to count how many cycles
>> are spent by sample output, then ran "perf record" to profile kernel
>> compilation 10 times on IvyBridge-EP. (perf record -a make -j 60)
>> The first number is scaled to 1000, the rest numbers are scaled by
>> the same factor.
>>
>> before overwrite mode after overwrite mode
>> AVG 1000 999 1046 1044
>> STDEV 19.4 19.5 17.1 17.9
>
> OK, so I was sure I replied to this email; but apparently I didn't :/
>
> So its still adding about 5% overhead to the regular case; this is sad.
>
> What does something like the below do?
>
I re-test the patch on a different 32 core sandybridge-ep machine. the result is quite good.
origin origin overwrite modified modified overwrite
AVG 1000 1044 960 1006
STDEV 39.0 26.0 28.1 14.4
Regards
Yan, Zheng
> ---
> include/linux/perf_event.h | 2 +
> kernel/events/core.c | 60 ++++++++++++++++++++++++------
> kernel/events/internal.h | 2 +
> kernel/events/ring_buffer.c | 90 +++++++++++++++++++++++++++------------------
> 4 files changed, 106 insertions(+), 48 deletions(-)
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 8873f82..bcce98a 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -753,6 +753,8 @@ static inline bool has_branch_stack(struct perf_event *event)
>
> extern int perf_output_begin(struct perf_output_handle *handle,
> struct perf_event *event, unsigned int size);
> +extern int perf_output_begin_overwrite(struct perf_output_handle *handle,
> + struct perf_event *event, unsigned int size);
> extern void perf_output_end(struct perf_output_handle *handle);
> extern unsigned int perf_output_copy(struct perf_output_handle *handle,
> const void *buf, unsigned int len);
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 1833bc5..4d674e9 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3878,6 +3878,8 @@ static const struct vm_operations_struct perf_mmap_vmops = {
> .page_mkwrite = perf_mmap_fault,
> };
>
> +static void perf_event_set_overflow(struct perf_event *event, struct ring_buffer *rb);
> +
> static int perf_mmap(struct file *file, struct vm_area_struct *vma)
> {
> struct perf_event *event = file->private_data;
> @@ -3985,6 +3987,7 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
> vma->vm_mm->pinned_vm += extra;
>
> ring_buffer_attach(event, rb);
> + perf_event_set_overflow(event, rb);
> rcu_assign_pointer(event->rb, rb);
>
> perf_event_update_userpage(event);
> @@ -4595,9 +4598,12 @@ void perf_prepare_sample(struct perf_event_header *header,
> }
> }
>
> -static void perf_event_output(struct perf_event *event,
> - struct perf_sample_data *data,
> - struct pt_regs *regs)
> +static __always_inline void
> +__perf_event_output(struct perf_event *event,
> + struct perf_sample_data *data,
> + struct pt_regs *regs,
> + int (*output_begin)(struct perf_output_handle *,
> + struct perf_event *, unsigned int))
> {
> struct perf_output_handle handle;
> struct perf_event_header header;
> @@ -4607,7 +4613,7 @@ static void perf_event_output(struct perf_event *event,
>
> perf_prepare_sample(&header, data, event, regs);
>
> - if (perf_output_begin(&handle, event, header.size))
> + if (output_begin(&handle, event, header.size))
> goto exit;
>
> perf_output_sample(&handle, &header, data, event);
> @@ -4618,6 +4624,33 @@ static void perf_event_output(struct perf_event *event,
> rcu_read_unlock();
> }
>
> +static void perf_event_output(struct perf_event *event,
> + struct perf_sample_data *data,
> + struct pt_regs *regs)
> +{
> + __perf_event_output(event, data, regs, perf_output_begin);
> +}
> +
> +static void perf_event_output_overwrite(struct perf_event *event,
> + struct perf_sample_data *data,
> + struct pt_regs *regs)
> +{
> + __perf_event_output(event, data, regs, perf_output_begin_overwrite);
> +}
> +
> +static void
> +perf_event_set_overflow(struct perf_event *event, struct ring_buffer *rb)
> +{
> + if (event->overflow_handler != perf_event_output ||
> + event->overflow_handler != perf_event_output_overwrite)
> + return;
> +
> + if (rb->overwrite)
> + event->overflow_handler = perf_event_output_overwrite;
> + else
> + event->overflow_handler = perf_event_output;
> +}
> +
> /*
> * read event_id
> */
> @@ -5183,10 +5216,7 @@ static int __perf_event_overflow(struct perf_event *event,
> irq_work_queue(&event->pending);
> }
>
> - if (event->overflow_handler)
> - event->overflow_handler(event, data, regs);
> - else
> - perf_event_output(event, data, regs);
> + event->overflow_handler(event, data, regs);
>
> if (event->fasync && event->pending_kill) {
> event->pending_wakeup = 1;
> @@ -6501,8 +6531,13 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu,
> context = parent_event->overflow_handler_context;
> }
>
> - event->overflow_handler = overflow_handler;
> - event->overflow_handler_context = context;
> + if (overflow_handler) {
> + event->overflow_handler = overflow_handler;
> + event->overflow_handler_context = context;
> + } else {
> + event->overflow_handler = perf_event_output;
> + event->overflow_handler_context = NULL;
> + }
>
> perf_event__state_init(event);
>
> @@ -6736,9 +6771,10 @@ perf_event_set_output(struct perf_event *event, struct perf_event *output_event)
> if (old_rb)
> ring_buffer_detach(event, old_rb);
>
> - if (rb)
> + if (rb) {
> ring_buffer_attach(event, rb);
> -
> + perf_event_set_overflow(event, rb);
> + }
> rcu_assign_pointer(event->rb, rb);
>
> if (old_rb) {
> diff --git a/kernel/events/internal.h b/kernel/events/internal.h
> index ca65997..c4e4610 100644
> --- a/kernel/events/internal.h
> +++ b/kernel/events/internal.h
> @@ -20,6 +20,8 @@ struct ring_buffer {
>
> atomic_t poll; /* POLL_ for wakeups */
>
> + local_t tail; /* read position */
> + local_t next_tail; /* next read position */
> local_t head; /* write position */
> local_t nest; /* nested writers */
> local_t events; /* event limit */
> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> index cd55144..5887044 100644
> --- a/kernel/events/ring_buffer.c
> +++ b/kernel/events/ring_buffer.c
> @@ -15,28 +15,9 @@
>
> #include "internal.h"
>
> -static bool perf_output_space(struct ring_buffer *rb, unsigned long tail,
> - unsigned long offset, unsigned long head)
> +static bool perf_output_space(unsigned long tail, unsigned long offset,
> + unsigned long head, unsigned long mask)
> {
> - unsigned long sz = perf_data_size(rb);
> - unsigned long mask = sz - 1;
> -
> - /*
> - * check if user-writable
> - * overwrite : over-write its own tail
> - * !overwrite: buffer possibly drops events.
> - */
> - if (rb->overwrite)
> - return true;
> -
> - /*
> - * verify that payload is not bigger than buffer
> - * otherwise masking logic may fail to detect
> - * the "not enough space" condition
> - */
> - if ((head - offset) > sz)
> - return false;
> -
> offset = (offset - tail) & mask;
> head = (head - tail) & mask;
>
> @@ -109,11 +90,11 @@ static void perf_output_put_handle(struct perf_output_handle *handle)
> preempt_enable();
> }
>
> -int perf_output_begin(struct perf_output_handle *handle,
> - struct perf_event *event, unsigned int size)
> +static __always_inline int __perf_output_begin(struct perf_output_handle *handle,
> + struct perf_event *event, unsigned int size, bool overwrite)
> {
> struct ring_buffer *rb;
> - unsigned long tail, offset, head;
> + unsigned long tail, offset, head, max_size;
> int have_lost;
> struct perf_sample_data sample_data;
> struct {
> @@ -136,7 +117,8 @@ int perf_output_begin(struct perf_output_handle *handle,
> handle->rb = rb;
> handle->event = event;
>
> - if (!rb->nr_pages)
> + max_size = perf_data_size(rb);
> + if (size > max_size)
> goto out;
>
> have_lost = local_read(&rb->lost);
> @@ -149,19 +131,43 @@ int perf_output_begin(struct perf_output_handle *handle,
>
> perf_output_get_handle(handle);
>
> - do {
> + if (overwrite) {
> + do {
> + tail = local_read(&rb->tail);
> + offset = local_read(&rb->head);
> + head = offset + size;
> + if (unlikely(!perf_output_space(tail, offset, head,
> + max_size - 1))) {
> + tail = local_read(&rb->next_tail);
> + local_set(&rb->tail, tail);
> + rb->user_page->data_tail = tail;
> + }
> + } while (local_cmpxchg(&rb->head, offset, head) != offset);
> +
> /*
> - * Userspace could choose to issue a mb() before updating the
> - * tail pointer. So that all reads will be completed before the
> - * write is issued.
> + * Save the start of next event when half of the buffer
> + * has been filled. Later when the event buffer overflows,
> + * update the tail pointer to point to it.
> */
> - tail = ACCESS_ONCE(rb->user_page->data_tail);
> - smp_rmb();
> - offset = head = local_read(&rb->head);
> - head += size;
> - if (unlikely(!perf_output_space(rb, tail, offset, head)))
> - goto fail;
> - } while (local_cmpxchg(&rb->head, offset, head) != offset);
> + if (tail == local_read(&rb->next_tail) &&
> + head - tail >= (max_size >> 1))
> + local_cmpxchg(&rb->next_tail, tail, head);
> + } else {
> + do {
> + /*
> + * Userspace could choose to issue a mb() before
> + * updating the tail pointer. So that all reads will
> + * be completed before the write is issued.
> + */
> + tail = ACCESS_ONCE(rb->user_page->data_tail);
> + smp_rmb();
> + offset = local_read(&rb->head);
> + head = offset + size;
> + if (unlikely(!perf_output_space(tail, offset, head,
> + max_size - 1)))
> + goto fail;
> + } while (local_cmpxchg(&rb->head, offset, head) != offset);
> + }
>
> if (head - local_read(&rb->wakeup) > rb->watermark)
> local_add(rb->watermark, &rb->wakeup);
> @@ -194,6 +200,18 @@ int perf_output_begin(struct perf_output_handle *handle,
> return -ENOSPC;
> }
>
> +int perf_output_begin(struct perf_output_handle *handle,
> + struct perf_event *event, unsigned int size)
> +{
> + return __perf_output_begin(handle, event, size, false);
> +}
> +
> +int perf_output_begin_overwrite(struct perf_output_handle *handle,
> + struct perf_event *event, unsigned int size)
> +{
> + return __perf_output_begin(handle, event, size, true);
> +}
> +
> unsigned int perf_output_copy(struct perf_output_handle *handle,
> const void *buf, unsigned int len)
> {
>
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