From c69993ecdd4dfde2b7da08b022052a33b203da07 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Thu, 23 Oct 2025 15:17:05 +0200 Subject: perf: Support deferred user unwind 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) Link: https://patch.msgid.link/20251023150002.GR4067720@noisy.programming.kicks-ass.net --- kernel/bpf/stackmap.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'kernel/bpf/stackmap.c') diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c index 4d53cdd1374c..8f1dacaf01fe 100644 --- a/kernel/bpf/stackmap.c +++ b/kernel/bpf/stackmap.c @@ -315,7 +315,7 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map, max_depth = sysctl_perf_event_max_stack; trace = get_perf_callchain(regs, kernel, user, max_depth, - false, false); + false, false, 0); if (unlikely(!trace)) /* couldn't fetch the stack trace */ @@ -452,7 +452,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task, trace = get_callchain_entry_for_task(task, max_depth); else trace = get_perf_callchain(regs, kernel, user, max_depth, - crosstask, false); + crosstask, false, 0); if (unlikely(!trace) || trace->nr < skip) { if (may_fault) -- cgit v1.2.3