The .rtemsrwset section is used for the per-CPU data. This section has
loadable content. Place the ring buffers in the BSS section to avoid
large executable image sizes.
Not using the per-CPU data makes it possible to initialize the record
support earlier.
Update #3665.
Add low level event recording infrastructure for system and user
defined events. The infrastructure is able to record high frequency
events such as
* SMP lock acquire/release,
* interrupt entry/exit,
* thread switches,
* UMA zone allocate/free, and
* Ethernet packet input/output, etc.
It allows post-mortem analysis in fatal error handlers, e.g. the last
events are in the record buffer, the newest event overwrites the oldest
event. It is possible to detect record buffer overflows for consumers
that expect a continuous stream of events, e.g. to display the system
state in real-time.
The implementation supports high-end SMP machines (more than 1GHz
processor frequency, more than four processors).
Add a new API instead. The implementation uses per-processor data
structures and no atomic read-modify-write operations. It is uses
per-processor ring buffers to record the events.
The CPU counter is used to get the time of events. It is combined with
periodic uptime events to synchronize it with CLOCK_REALTIME.
The existing capture engine tries to solve this problem also, but its
performance is not good enough for high-end production systems. The
main issues are the variable-size buffers and the use of SMP locks for
synchronization. To fix this, the API would change significantly.
Update #3665.
