From 0592e57b24e7e05ec1f4c50b9666c013abff7017 Mon Sep 17 00:00:00 2001 From: Kan Liang Date: Tue, 5 Jun 2018 08:38:45 -0700 Subject: perf/x86/intel/lbr: Fix incomplete LBR call stack LBR has a limited stack size. If a task has a deeper call stack than LBR's stack size, only the overflowed part is reported. A complete call stack may not be reconstructed by perf tool. Current code doesn't access all LBR registers. It only read the ones below the TOS. The LBR registers above the TOS will be discarded unconditionally. When a CALL is captured, the TOS is incremented by 1 , modulo max LBR stack size. The LBR HW only records the call stack information to the register which the TOS points to. It will not touch other LBR registers. So the registers above the TOS probably still store the valid call stack information for an overflowed call stack, which need to be reported. To retrieve complete call stack information, we need to start from TOS, read all LBR registers until an invalid entry is detected. 0s can be used to detect the invalid entry, because: - When a RET is captured, the HW zeros the LBR register which TOS points to, then decreases the TOS. - The LBR registers are reset to 0 when adding a new LBR event or scheduling an existing LBR event. - A taken branch at IP 0 is not expected The context switch code is also modified to save/restore all valid LBR registers. Furthermore, the LBR registers, which don't have valid call stack information, need to be reset in restore, because they may be polluted while swapped out. Here is a small test program, tchain_deep. Its call stack is deeper than 32. noinline void f33(void) { int i; for (i = 0; i < 10000000;) { if (i%2) i++; else i++; } } noinline void f32(void) { f33(); } noinline void f31(void) { f32(); } ... ... noinline void f1(void) { f2(); } int main() { f1(); } Here is the test result on SKX. The max stack size of SKX is 32. Without the patch: $ perf record -e cycles --call-graph lbr -- ./tchain_deep $ perf report --stdio # # Children Self Command Shared Object Symbol # ........ ........ ........... ................ ................. # 100.00% 99.99% tchain_deep tchain_deep [.] f33 | --99.99%--f30 f31 f32 f33 With the patch: $ perf record -e cycles --call-graph lbr -- ./tchain_deep $ perf report --stdio # Children Self Command Shared Object Symbol # ........ ........ ........... ................ .................. # 99.99% 0.00% tchain_deep tchain_deep [.] f1 | ---f1 f2 f3 f4 f5 f6 f7 f8 f9 f10 f11 f12 f13 f14 f15 f16 f17 f18 f19 f20 f21 f22 f23 f24 f25 f26 f27 f28 f29 f30 f31 f32 f33 Signed-off-by: Kan Liang Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Peter Zijlstra Cc: Arnaldo Carvalho de Melo Cc: Jiri Olsa Cc: Stephane Eranian Cc: Vince Weaver Cc: Alexander Shishkin Cc: Thomas Gleixner Cc: acme@kernel.org Cc: eranian@google.com Link: https://lore.kernel.org/lkml/1528213126-4312-1-git-send-email-kan.liang@linux.intel.com Signed-off-by: Ingo Molnar --- arch/x86/events/perf_event.h | 1 + 1 file changed, 1 insertion(+) (limited to 'arch/x86/events/perf_event.h') diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h index 9f3711470ec1..6b72a92069fd 100644 --- a/arch/x86/events/perf_event.h +++ b/arch/x86/events/perf_event.h @@ -648,6 +648,7 @@ struct x86_perf_task_context { u64 lbr_to[MAX_LBR_ENTRIES]; u64 lbr_info[MAX_LBR_ENTRIES]; int tos; + int valid_lbrs; int lbr_callstack_users; int lbr_stack_state; }; -- cgit v1.2.3