[PATCH v3][manpages 0/2] Document new feature in perf_event_open

[PATCH v3][manpages 0/2] Document new feature in perf_event_open

[Wang Nan]

Decribe PERF_EVENT_IOC_PAUSE_OUTPUT and write_backward in man pages.

v2 -> v3:
  Correct words.
  Explain the relationship between readonly ring buffer and
   over-writable ring buffer in patch 1/2.

Wang Nan (2):
  perf_event_open.2: Document PERF_EVENT_IOC_PAUSE_OUTPUT
  perf_event_open.2: Document write_backward

 man2/perf_event_open.2 | 81 ++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 79 insertions(+), 2 deletions(-)

-- 
2.10.1

[PATCH v3][manpages 1/2] perf_event_open.2: Document PERF_EVENT_IOC_PAUSE_OUTPUT

[Wang Nan]

Linux 4.7 (86e7972f690c1017fd086cdfe53d8524e68c661c) introduces
PERF_EVENT_IOC_PAUSE_OUTPUT feature. Document it.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Reviewed-by: Vince Weaver <vincent.weaver@maine.edu>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
---
 man2/perf_event_open.2 | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/man2/perf_event_open.2 b/man2/perf_event_open.2
index fade28c..561331c 100644
--- a/man2/perf_event_open.2
+++ b/man2/perf_event_open.2
@@ -1687,6 +1687,15 @@ the
 .I data_tail
 value should be written by user space to reflect the last read data.
 In this case, the kernel will not overwrite unread data.
+
+When the mapping is read only (without
+.BR PROT_WRITE ),
+setting .I data_tail is not allowed.
+In this case, the kernel will overwrite data when sample coming, unless
+the ring buffer is paused by a
+.BR PERF_EVENT_IOC_PAUSE_OUTPUT
+.BR ioctl (2)
+system call before reading.
 .TP
 .IR data_offset " (since Linux 4.1)"
 .\" commit e8c6deac69629c0cb97c3d3272f8631ef17f8f0f
@@ -2865,6 +2874,21 @@ The argument is a BPF program file descriptor that was created by
 a previous
 .BR bpf (2)
 system call.
+.TP
+.BR PERF_EVENT_IOC_PAUSE_OUTPUT " (since Linux 4.7)"
+.\" commit 86e7972f690c1017fd086cdfe53d8524e68c661c
+This allows pausing and resuming the event's ring-buffer. A
+paused ring-buffer does not prevent generation of samples, but simply
+discards the samples. The discarded samples are considered lost,
+causing
+.BR PERF_RECORD_LOST
+to be generated when possible.
+
+The argument is an integer. A nonzero value pauses the ring-buffer,
+zero resumes the ring-buffer.
+
+Pausing a read only ring buffer before reading from it without having
+to worry about data being overwritten.
 .SS Using prctl(2)
 A process can enable or disable all the event groups that are
 attached to it using the
-- 
2.10.1

[PATCH v3][manpages 2/2] perf_event_open.2: Document write_backward

[Wang Nan]

Linux 4.7 (9ecda41acb971ebd07c8fb35faf24005c0baea12) introduces write_backward
attribute to perf_event_attr. Document this feature.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Reviewed-by: Vince Weaver <vincent.weaver@maine.edu>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
---
 man2/perf_event_open.2 | 57 ++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 55 insertions(+), 2 deletions(-)

diff --git a/man2/perf_event_open.2 b/man2/perf_event_open.2
index 561331c..fccde79 100644
--- a/man2/perf_event_open.2
+++ b/man2/perf_event_open.2
@@ -245,7 +245,8 @@ struct perf_event_attr {
           use_clockid    :  1,  /* use clockid for time fields */
           context_switch :  1,  /* context switch data */
 
-          __reserved_1   : 37;
+          write_backward :  1,  /* Write ring buffer from end to beginning */
+          __reserved_1   : 36;
 
     union {
         __u32 wakeup_events;    /* wakeup every n events */
@@ -1127,6 +1128,31 @@ The advantage of this method is that it will give full
 information even with strict
 .I perf_event_paranoid
 settings.
+.IR "write_backward" " (since Linux 4.7)"
+.\" commit 9ecda41acb971ebd07c8fb35faf24005c0baea12
+This makes the resuling event use a backward ring-buffer, which
+writes samples from the end of the ring-buffer to the beginning.
+
+It is not allowed to connect events with backward and forward
+ring-buffer settings together using
+.B PERF_EVENT_IOC_SET_OUTPUT.
+
+Backward ring-buffer is useful for ring-buffers created by readonly
+.BR mmap (2).
+In this case,
+.IR data_tail
+is useless (because user space programs are not allowed to write to it).
+.IR data_head
+points to the head of the most recent sample. In a backward
+ring-buffer, it is easy to iterate over the whole ring-buffer by reading
+samples one by one from
+.IR data_head
+because size of a sample can be found from decoding its header.
+
+For a forward read only ring-buffer in contract,
+.IR data_head
+points to the end of the most recent sample, but the size of a sample
+can't be determined from the end of it.
 .TP
 .IR "wakeup_events" ", " "wakeup_watermark"
 This union sets how many samples
@@ -1671,7 +1697,9 @@ And vice versa:
 .TP
 .I data_head
 This points to the head of the data section.
-The value continuously increases, it does not wrap.
+The value continuously increases (or decrease if
+.IR write_backward
+is set), it does not wrap.
 The value needs to be manually wrapped by the size of the mmap buffer
 before accessing the samples.
 
@@ -2736,6 +2764,24 @@ Starting with Linux 3.18,
 .B POLL_HUP
 is indicated if the event being monitored is attached to a different
 process and that process exits.
+.SS Reading from overwritable ring-buffer
+Reader is unable to update
+.IR data_tail
+if the mapping is not
+.BR PROT_WRITE .
+In this case, kernel will overwrite data without considering whether
+they are read or not, so ring-buffer is overwritable and
+behaves like a flight recorder. To read from an overwritable
+ring-buffer, setting
+.IR write_backward
+is suggested, or it would be hard to find a proper position to start
+decoding. In addition, ring-buffer should be paused before reading
+through
+.BR ioctl (2)
+with
+.B PERF_EVENT_IOC_PAUSE_OUTPUT
+to avoid racing between kernel and reader. Ring-buffer should be resumed
+after finish reading.
 .SS rdpmc instruction
 Starting with Linux 3.4 on x86, you can use the
 .\" commit c7206205d00ab375839bd6c7ddb247d600693c09
@@ -2848,6 +2894,13 @@ The file descriptors must all be on the same CPU.
 
 The argument specifies the desired file descriptor, or \-1 if
 output should be ignored.
+
+Two events with different
+.IR write_backward
+settings are not allowed to be connected together using
+.B PERF_EVENT_IOC_SET_OUTPUT.
+.B EINVAL
+is returned in this case.
 .TP
 .BR PERF_EVENT_IOC_SET_FILTER " (since Linux 2.6.33)"
 .\" commit 6fb2915df7f0747d9044da9dbff5b46dc2e20830
-- 
2.10.1

Re: [PATCH v3][manpages 1/2] perf_event_open.2: Document PERF_EVENT_IOC_PAUSE_OUTPUT

[Michael Kerrisk (man-pages)]

Hello Wang Nan,

On 10/24/2016 08:52 AM, Wang Nan wrote:
> Linux 4.7 (86e7972f690c1017fd086cdfe53d8524e68c661c) introduces
> PERF_EVENT_IOC_PAUSE_OUTPUT feature. Document it.
> 
> Signed-off-by: Wang Nan <wangnan0@huawei.com>
> Reviewed-by: Vince Weaver <vincent.weaver@maine.edu>
> Cc: Michael Kerrisk <mtk.manpages@gmail.com>
> ---
>  man2/perf_event_open.2 | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/man2/perf_event_open.2 b/man2/perf_event_open.2
> index fade28c..561331c 100644
> --- a/man2/perf_event_open.2
> +++ b/man2/perf_event_open.2
> @@ -1687,6 +1687,15 @@ the
>  .I data_tail
>  value should be written by user space to reflect the last read data.
>  In this case, the kernel will not overwrite unread data.
> +
> +When the mapping is read only (without
> +.BR PROT_WRITE ),
> +setting .I data_tail is not allowed.

Missing line breaks in the preceding line.

> +In this case, the kernel will overwrite data when sample coming, unless

I find that last line hard to understand.
s/sample coming/a sample arrives/?

> +the ring buffer is paused by a
> +.BR PERF_EVENT_IOC_PAUSE_OUTPUT
> +.BR ioctl (2)
> +system call before reading.
>  .TP
>  .IR data_offset " (since Linux 4.1)"
>  .\" commit e8c6deac69629c0cb97c3d3272f8631ef17f8f0f
> @@ -2865,6 +2874,21 @@ The argument is a BPF program file descriptor that was created by
>  a previous
>  .BR bpf (2)
>  system call.
> +.TP
> +.BR PERF_EVENT_IOC_PAUSE_OUTPUT " (since Linux 4.7)"
> +.\" commit 86e7972f690c1017fd086cdfe53d8524e68c661c
> +This allows pausing and resuming the event's ring-buffer. A
> +paused ring-buffer does not prevent generation of samples, but simply
> +discards the samples. The discarded samples are considered lost,
> +causing
> +.BR PERF_RECORD_LOST
> +to be generated when possible.
> +
> +The argument is an integer. A nonzero value pauses the ring-buffer,
> +zero resumes the ring-buffer.
> +
> +Pausing a read only ring buffer before reading from it without having
> +to worry about data being overwritten.

That last sentence seems incomplete. I can't understand what you
mean here?

Thanks,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

Re: [PATCH v3][manpages 2/2] perf_event_open.2: Document write_backward

[Michael Kerrisk (man-pages)]

Hello Wang Nan,

On 10/24/2016 08:52 AM, Wang Nan wrote:
> Linux 4.7 (9ecda41acb971ebd07c8fb35faf24005c0baea12) introduces write_backward
> attribute to perf_event_attr. Document this feature.
> 
> Signed-off-by: Wang Nan <wangnan0@huawei.com>
> Reviewed-by: Vince Weaver <vincent.weaver@maine.edu>
> Cc: Michael Kerrisk <mtk.manpages@gmail.com>
> ---
>  man2/perf_event_open.2 | 57 ++++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 55 insertions(+), 2 deletions(-)
> 
> diff --git a/man2/perf_event_open.2 b/man2/perf_event_open.2
> index 561331c..fccde79 100644
> --- a/man2/perf_event_open.2
> +++ b/man2/perf_event_open.2
> @@ -245,7 +245,8 @@ struct perf_event_attr {
>            use_clockid    :  1,  /* use clockid for time fields */
>            context_switch :  1,  /* context switch data */
>  
> -          __reserved_1   : 37;
> +          write_backward :  1,  /* Write ring buffer from end to beginning */
> +          __reserved_1   : 36;
>  
>      union {
>          __u32 wakeup_events;    /* wakeup every n events */
> @@ -1127,6 +1128,31 @@ The advantage of this method is that it will give full
>  information even with strict
>  .I perf_event_paranoid
>  settings.
> +.IR "write_backward" " (since Linux 4.7)"
> +.\" commit 9ecda41acb971ebd07c8fb35faf24005c0baea12
> +This makes the resuling event use a backward ring-buffer, which

s/reuling/resulting/

s/This/Setting this bit/ ?

> +writes samples from the end of the ring-buffer to the beginning.
> +
> +It is not allowed to connect events with backward and forward
> +ring-buffer settings together using
> +.B PERF_EVENT_IOC_SET_OUTPUT.
> +
> +Backward ring-buffer is useful for ring-buffers created by readonly
> +.BR mmap (2).
> +In this case,
> +.IR data_tail
> +is useless (because user space programs are not allowed to write to it).
> +.IR data_head
> +points to the head of the most recent sample. In a backward
> +ring-buffer, it is easy to iterate over the whole ring-buffer by reading
> +samples one by one from
> +.IR data_head
> +because size of a sample can be found from decoding its header.
> +
> +For a forward read only ring-buffer in contract,

What does "in contract" here mean? This needs to be clarified.

> +.IR data_head
> +points to the end of the most recent sample, but the size of a sample
> +can't be determined from the end of it.
>  .TP
>  .IR "wakeup_events" ", " "wakeup_watermark"
>  This union sets how many samples
> @@ -1671,7 +1697,9 @@ And vice versa:
>  .TP
>  .I data_head
>  This points to the head of the data section.
> -The value continuously increases, it does not wrap.
> +The value continuously increases (or decrease if
> +.IR write_backward
> +is set), it does not wrap.
>  The value needs to be manually wrapped by the size of the mmap buffer
>  before accessing the samples.
>  
> @@ -2736,6 +2764,24 @@ Starting with Linux 3.18,
>  .B POLL_HUP
>  is indicated if the event being monitored is attached to a different
>  process and that process exits.
> +.SS Reading from overwritable ring-buffer
> +Reader is unable to update
> +.IR data_tail
> +if the mapping is not
> +.BR PROT_WRITE .
> +In this case, kernel will overwrite data without considering whether
> +they are read or not, so ring-buffer is overwritable and
> +behaves like a flight recorder. To read from an overwritable
> +ring-buffer, setting
> +.IR write_backward
> +is suggested, or it would be hard to find a proper position to start
> +decoding. In addition, ring-buffer should be paused before reading
> +through
> +.BR ioctl (2)
> +with
> +.B PERF_EVENT_IOC_PAUSE_OUTPUT
> +to avoid racing between kernel and reader. Ring-buffer should be resumed
> +after finish reading.
>  .SS rdpmc instruction
>  Starting with Linux 3.4 on x86, you can use the
>  .\" commit c7206205d00ab375839bd6c7ddb247d600693c09
> @@ -2848,6 +2894,13 @@ The file descriptors must all be on the same CPU.
>  
>  The argument specifies the desired file descriptor, or \-1 if
>  output should be ignored.
> +
> +Two events with different
> +.IR write_backward
> +settings are not allowed to be connected together using
> +.B PERF_EVENT_IOC_SET_OUTPUT.
> +.B EINVAL
> +is returned in this case.
>  .TP
>  .BR PERF_EVENT_IOC_SET_FILTER " (since Linux 2.6.33)"
>  .\" commit 6fb2915df7f0747d9044da9dbff5b46dc2e20830

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

Re: [PATCH v3][manpages 1/2] perf_event_open.2: Document PERF_EVENT_IOC_PAUSE_OUTPUT

[Michael Kerrisk (man-opages)]

Hello Wangnan,

On 10/24/2016 08:52 AM, Wang Nan wrote:
> Linux 4.7 (86e7972f690c1017fd086cdfe53d8524e68c661c) introduces
> PERF_EVENT_IOC_PAUSE_OUTPUT feature. Document it.

Just to confirm, I presume this patch has been superseded by the one
from Vince that I just applied.

Cheers,

Michael

> Signed-off-by: Wang Nan <wangnan0@huawei.com>
> Reviewed-by: Vince Weaver <vincent.weaver@maine.edu>
> Cc: Michael Kerrisk <mtk.manpages@gmail.com>
> ---
>   man2/perf_event_open.2 | 24 ++++++++++++++++++++++++
>   1 file changed, 24 insertions(+)
> 
> diff --git a/man2/perf_event_open.2 b/man2/perf_event_open.2
> index fade28c..561331c 100644
> --- a/man2/perf_event_open.2
> +++ b/man2/perf_event_open.2
> @@ -1687,6 +1687,15 @@ the
>   .I data_tail
>   value should be written by user space to reflect the last read data.
>   In this case, the kernel will not overwrite unread data.
> +
> +When the mapping is read only (without
> +.BR PROT_WRITE ),
> +setting .I data_tail is not allowed.
> +In this case, the kernel will overwrite data when sample coming, unless
> +the ring buffer is paused by a
> +.BR PERF_EVENT_IOC_PAUSE_OUTPUT
> +.BR ioctl (2)
> +system call before reading.
>   .TP
>   .IR data_offset " (since Linux 4.1)"
>   .\" commit e8c6deac69629c0cb97c3d3272f8631ef17f8f0f
> @@ -2865,6 +2874,21 @@ The argument is a BPF program file descriptor that was created by
>   a previous
>   .BR bpf (2)
>   system call.
> +.TP
> +.BR PERF_EVENT_IOC_PAUSE_OUTPUT " (since Linux 4.7)"
> +.\" commit 86e7972f690c1017fd086cdfe53d8524e68c661c
> +This allows pausing and resuming the event's ring-buffer. A
> +paused ring-buffer does not prevent generation of samples, but simply
> +discards the samples. The discarded samples are considered lost,
> +causing
> +.BR PERF_RECORD_LOST
> +to be generated when possible.
> +
> +The argument is an integer. A nonzero value pauses the ring-buffer,
> +zero resumes the ring-buffer.
> +
> +Pausing a read only ring buffer before reading from it without having
> +to worry about data being overwritten.
>   .SS Using prctl(2)
>   A process can enable or disable all the event groups that are
>   attached to it using the
>