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perf: Support deferred user unwind

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Add support for deferred userspace unwind to perf.

Where perf currently relies on in-place stack unwinding; from NMI
context and all that. This moves the userspace part of the unwind to
right before the return-to-userspace.

This has two distinct benefits, the biggest is that it moves the
unwind to a faultable context. It becomes possible to fault in debug
info (.eh_frame, SFrame etc.) that might not otherwise be readily
available. And secondly, it de-duplicates the user callchain where
multiple samples happen during the same kernel entry.

To facilitate this the perf interface is extended with a new record
type:

  PERF_RECORD_CALLCHAIN_DEFERRED

and two new attribute flags:

  perf_event_attr::defer_callchain - to request the user unwind be deferred
  perf_event_attr::defer_output    - to request PERF_RECORD_CALLCHAIN_DEFERRED records

The existing PERF_RECORD_SAMPLE callchain section gets a new
context type:

  PERF_CONTEXT_USER_DEFERRED

After which will come a single entry, denoting the 'cookie' of the
deferred callchain that should be attached here, matching the 'cookie'
field of the above mentioned PERF_RECORD_CALLCHAIN_DEFERRED.

The 'defer_callchain' flag is expected on all events with
PERF_SAMPLE_CALLCHAIN. The 'defer_output' flag is expect on the event
responsible for collecting side-band events (like mmap, comm etc.).
Setting 'defer_output' on multiple events will get you duplicated
PERF_RECORD_CALLCHAIN_DEFERRED records.

Based on earlier patches by Josh and Steven.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251023150002.GR4067720@noisy.programming.kicks-ass.net
This commit is contained in:
Peter Zijlstra 2025-10-23 15:17:05 +02:00
parent ae25884ad7
commit c69993ecdd
8 changed files with 145 additions and 20 deletions
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21
include/uapi/linux/perf_event.h
View file

@ -463,7 +463,9 @@ struct perf_event_attr {
inherit_thread : 1, /* children only inherit if cloned with CLONE_THREAD */
remove_on_exec : 1, /* event is removed from task on exec */
sigtrap : 1, /* send synchronous SIGTRAP on event */
__reserved_1 : 26;
defer_callchain: 1, /* request PERF_RECORD_CALLCHAIN_DEFERRED records */
defer_output : 1, /* output PERF_RECORD_CALLCHAIN_DEFERRED records */
__reserved_1 : 24;
union {
__u32 wakeup_events; /* wake up every n events */
@ -1239,6 +1241,22 @@ enum perf_event_type {
*/
PERF_RECORD_AUX_OUTPUT_HW_ID = 21,
/*
* This user callchain capture was deferred until shortly before
* returning to user space. Previous samples would have kernel
* callchains only and they need to be stitched with this to make full
* callchains.
*
* struct {
* struct perf_event_header header;
* u64 cookie;
* u64 nr;
* u64 ips[nr];
* struct sample_id sample_id;
* };
*/
PERF_RECORD_CALLCHAIN_DEFERRED = 22,
PERF_RECORD_MAX, /* non-ABI */
};
@ -1269,6 +1287,7 @@ enum perf_callchain_context {
PERF_CONTEXT_HV = (__u64)-32,
PERF_CONTEXT_KERNEL = (__u64)-128,
PERF_CONTEXT_USER = (__u64)-512,
PERF_CONTEXT_USER_DEFERRED = (__u64)-640,
PERF_CONTEXT_GUEST = (__u64)-2048,
PERF_CONTEXT_GUEST_KERNEL = (__u64)-2176,