Re: [PATCH 2/2] perf_event_open.2: Document write_backward
From: Vince Weaver
Date: Fri Oct 21 2016 - 17:25:49 EST
On Fri, 21 Oct 2016, Wang Nan wrote:
> context_switch : 1, /* context switch data */
> - __reserved_1 : 37;
> + write_backward : 1, /* Write ring buffer from end to beginning */
> + __reserved_1 : 36;
This removes a blank line, not sure if intentional or not.
> +.IR "write_backward" " (since Linux 4.6)"
It didn't committed until Linux 4.7 from what I can tell?
> +This makes the resuling event use a backward ring-buffer, which
> +writes samples from the end of the ring-buffer.
> +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 when the ring-buffer is overwritable
> +(created by readonly
> +.BR mmap (2)
A ring buffer is over-writable when it is mmapped readonly?
Is this a hard requirement?
Can you set the read-backwards bit if not mapped readonly?
Otherwise the documentation seems reasonable.
Reviewed-by: Vince Weaver <vincent.weaver@xxxxxxxxx>