In case we are building in a flatpak, we might want to rely on a system
installed sysprofd. This means we might want to pretend we have sysprofd
support (to be found on the system), but not actually build sysprofd.
The kernel says here http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/kernel/events/ring_buffer.c?id=7a1dcf6ad#n61 :
* Since the mmap() consumer (userspace) can run on a different CPU:
*
* kernel user
*
* if (LOAD ->data_tail) { LOAD ->data_head
* (A) smp_rmb() (C)
* STORE $data LOAD $data
* smp_wmb() (B) smp_mb() (D)
* STORE ->data_head STORE ->data_tail
* }
*
* Where A pairs with D, and B pairs with C.
*
* In our case (A) is a control dependency that separates the load of
* the ->data_tail and the stores of $data. In case ->data_tail
* indicates there is no room in the buffer to store $data we do not.
*
* D needs to be a full barrier since it separates the data READ
* from the tail WRITE.
*
* For B a WMB is sufficient since it separates two WRITEs, and for C
* an RMB is sufficient since it separates two READs.
*
* See perf_output_begin().
So I'm pretty sure we need a full barrier before writing out data_tail.
So changing this was a little backwards. The goal here is to allow using
libsysprof-2 without polkit, it just won't have the ability to elevate
privileges.
Without this, after logging in you are already multiple seconds into your
profiling session recording. Not ideal. So instead, we do the async polkit
auth upfront during SpSource::prepare(), and then toggle ready after we
have received notification.
This is a major redesign a modernization of Sysprof. The core data
structures and design are largely the same, but it has been ported to
Gtk3 and has lots of additions that should make your profiling experience
smoother. Especially for those that are new to profiling.
There are some very simple help docs added, but we really need the
experts to come in and write some documentation here.
