From 565e60d3f37b77b2ace272cf5c854159df60cf3e Mon Sep 17 00:00:00 2001 From: Kim Phillips Date: Mon, 26 Aug 2019 14:57:30 -0500 Subject: [PATCH] perf/x86/amd/ibs: Fix sample bias for dispatched micro-ops When counting dispatched micro-ops with cnt_ctl=1, in order to prevent sample bias, IBS hardware preloads the least significant 7 bits of current count (IbsOpCurCnt) with random values, such that, after the interrupt is handled and counting resumes, the next sample taken will be slightly perturbed. The current count bitfield is in the IBS execution control h/w register, alongside the maximum count field. Currently, the IBS driver writes that register with the maximum count, leaving zeroes to fill the current count field, thereby overwriting the random bits the hardware preloaded for itself. Fix the driver to actually retain and carry those random bits from the read of the IBS control register, through to its write, instead of overwriting the lower current count bits with zeroes. Tested with: perf record -c 100001 -e ibs_op/cnt_ctl=1/pp -a -C 0 taskset -c 0 'perf annotate' output before: 15.70 65: addsd %xmm0,%xmm1 17.30 add $0x1,%rax 15.88 cmp %rdx,%rax je 82 17.32 72: test $0x1,%al jne 7c 7.52 movapd %xmm1,%xmm0 5.90 jmp 65 8.23 7c: sqrtsd %xmm1,%xmm0 12.15 jmp 65 'perf annotate' output after: 16.63 65: addsd %xmm0,%xmm1 16.82 add $0x1,%rax 16.81 cmp %rdx,%rax je 82 16.69 72: test $0x1,%al jne 7c 8.30 movapd %xmm1,%xmm0 8.13 jmp 65 8.24 7c: sqrtsd %xmm1,%xmm0 8.39 jmp 65 Tested on Family 15h and 17h machines. Machines prior to family 10h Rev. C don't have the RDWROPCNT capability, and have the IbsOpCurCnt bitfield reserved, so this patch shouldn't affect their operation. It is unknown why commit 975cf09be797 ("perf/x86: Implement 64-bit counter support for IBS") ignored the lower 4 bits of the IbsOpCurCnt field; the number of preloaded random bits has always been 7, AFAICT. Signed-off-by: Kim Phillips Signed-off-by: Peter Zijlstra (Intel) Cc: "Arnaldo Carvalho de Melo" Cc: Cc: Ingo Molnar Cc: Ingo Molnar Cc: Jiri Olsa Cc: Thomas Gleixner Cc: "Borislav Petkov" Cc: Stephane Eranian Cc: Alexander Shishkin Cc: "Namhyung Kim" Cc: "H. Peter Anvin" Link: https://lkml.kernel.org/r/20190826195730.30614-1-kim.phillips@amd.com --- arch/x86/events/amd/ibs.c | 13 ++++++++++--- arch/x86/include/asm/perf_event.h | 12 ++++++++---- 2 files changed, 18 insertions(+), 7 deletions(-) diff --git a/arch/x86/events/amd/ibs.c b/arch/x86/events/amd/ibs.c index 62f317c9113af..5b35b7ea5d728 100644 --- a/arch/x86/events/amd/ibs.c +++ b/arch/x86/events/amd/ibs.c @@ -661,10 +661,17 @@ fail: throttle = perf_event_overflow(event, &data, ®s); out: - if (throttle) + if (throttle) { perf_ibs_stop(event, 0); - else - perf_ibs_enable_event(perf_ibs, hwc, period >> 4); + } else { + period >>= 4; + + if ((ibs_caps & IBS_CAPS_RDWROPCNT) && + (*config & IBS_OP_CNT_CTL)) + period |= *config & IBS_OP_CUR_CNT_RAND; + + perf_ibs_enable_event(perf_ibs, hwc, period); + } perf_event_update_userpage(event); diff --git a/arch/x86/include/asm/perf_event.h b/arch/x86/include/asm/perf_event.h index 1392d5e6e8d67..ee26e9215f187 100644 --- a/arch/x86/include/asm/perf_event.h +++ b/arch/x86/include/asm/perf_event.h @@ -252,16 +252,20 @@ struct pebs_lbr { #define IBSCTL_LVT_OFFSET_VALID (1ULL<<8) #define IBSCTL_LVT_OFFSET_MASK 0x0F -/* ibs fetch bits/masks */ +/* IBS fetch bits/masks */ #define IBS_FETCH_RAND_EN (1ULL<<57) #define IBS_FETCH_VAL (1ULL<<49) #define IBS_FETCH_ENABLE (1ULL<<48) #define IBS_FETCH_CNT 0xFFFF0000ULL #define IBS_FETCH_MAX_CNT 0x0000FFFFULL -/* ibs op bits/masks */ -/* lower 4 bits of the current count are ignored: */ -#define IBS_OP_CUR_CNT (0xFFFF0ULL<<32) +/* + * IBS op bits/masks + * The lower 7 bits of the current count are random bits + * preloaded by hardware and ignored in software + */ +#define IBS_OP_CUR_CNT (0xFFF80ULL<<32) +#define IBS_OP_CUR_CNT_RAND (0x0007FULL<<32) #define IBS_OP_CNT_CTL (1ULL<<19) #define IBS_OP_VAL (1ULL<<18) #define IBS_OP_ENABLE (1ULL<<17) -- 2.39.5