Refactor sample ring buffer ring handling to make it more robust to
long running callchain collection handling
r338112 introduced a (now fixed) regression that exposed a number of race
conditions within the management of the sample buffers. This
simplifies the handling and moves the decision to overwrite a
callchain sample that has taken too long out of the NMI in to the
hardlock handler. With this change the problem no longer shows up as a
ring corruption but as the code spending all of its time in callchain
collection.
- Makes the producer / consumer index incrementing monotonic, making it
easier (for me at least) to reason about.
- Moves the decision to overwrite a sample from NMI context to interrupt
context where we can enforce serialization.
- Puts a time limit on waiting to collect a user callchain - putting a
bound on head-of-line blocking causing samples to be dropped
- Removes the flush routine which was previously needed to purge
dangling references to the pmc from the sample buffers but now is only
a source of a race condition on unload.
Previously one could lock up or crash HEAD by running:
pmcstat -S inst_retired.any_p -T and then hitting ^C
After this change it is no longer possible.
PR: 231793
Reviewed by: markj@
Approved by: re (gjb@)
Differential Revision: https://reviews.freebsd.org/D17011
e651a0b9c0