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SubjectRe: perfmon3 interface overview
>> [snip]
>> > III) attaching and detaching
>> >
>> > With v2.81:
>> > int pfm_load_context(int fd, pfarg_load_t *load);
>> > int pfm_unload_context(int fd);
>> >
>> > With v3.0:
>> > int pfm_attach_session(int fd, int flags, int target);
>> > int pfm_detach_session(int fd, int flags);
>> Couldn't you get rid of one more syscall here by making detach a
>> special case of attach with a special "null" value for target, or a
>> special flag?
> We could combine the two and use the flag field to indicate attach/detach.
> The target is not a pointer but an int. Some people suggested I use an
> unsigned long instead. In anycase, we could not use 0 to indicate "detach"
> because CPU0 is a valid choice for system-wide. Thus we would have to
> pick another value to mean "nothing", e.g, -1.
> > IV) starting and stopping
> >
> > With v2.81:
> > int pfm_start(int fd, pfarg_start_t *st);
> > int pfm_stop(int fd);
> > int pfm_restart(int fd);
> >
> > With v3.0:
> > int pfm_start_session(int fd, int flags);
> > int pfm_stop_session(int fd, int flags);
>> Likewise, couldn't you cut this down by one more syscall by making it
>> int pfm_set_session_state(int fd, int flags);
>> and having a 'RUNNING' flag, which selects start or stop behaviour?
> That one we can certainly do. That's a good idea.

Some more thoughts on this.

If we wanted to go even further, we could combine start/stop, attach/detach
into a single syscall:

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

But then, this is a form of ioctl() which people don't like....

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