Re: perfmon3 interface overview

["stephane eranian" <eranian@googlemail.com>]

David,

On Sat, Oct 4, 2008 at 8:05 AM, David Gibson
<david@gibson.dropbear.id.au> wrote:
>>
>> If we wanted to go even further, we could combine start/stop, attach/detach
>> into  a single syscall:
>
> Well, you could.  But the attach/detach take a parameter which
> start/stop don't, making it a less obvious merge to make.
>
Unless you make the 3rd argument optional and hide this in a
user library. This is how this is handled for pfm_create_session()
for instance. The library would define this as follows:

   int pfm_control_session(int fd, int flags, ...);

Based upon flags, it would use va_arg() to get to the 3rd argument and
pass it to the syscall which implements the version below. A dummy value
would be passed with the flags is not equal to PFM_CTFL_ATTACH.

>>    int pfm_control_session(int fd, int flags, int target);
>>    With flags:
>>        PFM_CTFL_START  : start monitoring
>>        PFM_CTFL_STOP    : stop monitoring
>>        PFM_CTFL_RESTART: resume after overflow notification
>>
>>        PFM_CTFL_ATTACH: attach to thread or cpu designated by 'target'
>>        PFM_CTFL_DETACH: detach session
>>