Messages in this thread | | | Date | Mon, 27 Feb 2012 09:45:27 +0100 | Subject | Re: [PATCH v6 00/18] perf: add support for sampling taken branches | From | Stephane Eranian <> |
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On Mon, Feb 27, 2012 at 8:50 AM, Anshuman Khandual <khandual@linux.vnet.ibm.com> wrote: > On Friday 10 February 2012 03:50 AM, Stephane Eranian wrote: >> This patchset adds an important and useful new feature to >> perf_events: branch stack sampling. In other words, the >> ability to capture taken branches into each sample. >> >> Statistical sampling of taken branch should not be confused >> for branch tracing. Not all branches are necessarily captured >> >> Sampling taken branches is important for basic block profiling, >> statistical call graph, function call counts. Many of those >> measurements can help drive a compiler optimizer. >> >> The branch stack is a software abstraction which sits on top >> of the PMU hardware. As such, it is not available on all >> processors. For now, the patch provides the generic interface >> and the Intel X86 implementation where it leverages the Last >> Branch Record (LBR) feature (from Core2 to SandyBridge). >> >> Branch stack sampling is supported for both per-thread and >> system-wide modes. >> >> It is possible to filter the type and privilege level of branches >> to sample. The target of the branch is used to determine >> the privilege level. >> >> For each branch, the source and destination are captured. On >> some hardware platforms, it may be possible to also extract >> the target prediction and, in that case, it is also exposed >> to end users. >> >> The branch stack can record a variable number of taken >> branches per sample. Those branches are always consecutive >> in time. The number of branches captured depends on the >> filtering and the underlying hardware. On Intel Nehalem >> and later, up to 16 consecutive branches can be captured >> per sample. >> >> Branch sampling is always coupled with an event. It can >> be any PMU event but it can't be a SW or tracepoint event. >> >> Branch sampling is requested by setting a new sample_type >> flag called: PERF_SAMPLE_BRANCH_STACK. >> >> To support branch filtering, we introduce a new field >> to the perf_event_attr struct: branch_sample_type. We chose >> NOT to overload the config1, config2 field because those >> are related to the event encoding. Branch stack is a >> separate feature which is combined with the event. >> >> The branch_sample_type is a bitmask of possible filters. >> The following filters are defined (more can be added): >> - PERF_SAMPLE_BRANCH_ANY : any control flow change >> - PERF_SAMPLE_BRANCH_USER : branches when target is at user level >> - PERF_SAMPLE_BRANCH_KERNEL : branches when target is at kernel level >> - PERF_SAMPLE_BRANCH_HV : branches when target is at hypervisor level >> - PERF_SAMPLE_BRANCH_ANY_CALL: call branches (incl. syscalls) >> - PERF_SAMPLE_BRANCH_ANY_RET : return branches (incl. syscall returns) >> - PERF_SAMPLE_BRANCH_IND_CALL: indirect calls >> >> It is possible to combine filters, e.g., IND_CALL|USER|KERNEL. >> >> When the privilege level is not specified, the branch stack >> inherits that of the associated event. >> >> Some processors may not offer hardware branch filtering, e.g., Intel >> Atom. Some may have HW filtering bugs (e.g., Nehalem). The Intel >> X86 implementation in this patchset also provides a SW branch filter >> which works on a best effort basis. It can compensate for the lack >> of LBR filtering. But first and foremost, it helps work around LBR >> filtering errata. The goal is to only capture the type of branches >> requested by the user. >> >> It is possible to combine branch stack sampling with PEBS on Intel >> X86 processors. Depending on the precise_sampling mode, there are >> certain filterting restrictions. When precise_sampling=1, then >> there are no filtering restrictions. When precise_sampling > 1, >> then only ANY|USER|KERNEL filter can be used. This comes from >> the fact that the kernel uses LBR to compensate for the PEBS >> off-by-1 skid on the instruction pointer. >> >> To demonstrate how the perf_event branch stack sampling interface >> works, the patchset also modifies perf record to capture taken >> branches. Similarly perf report is enhanced to display a histogram >> of taken branches. >> >> I would like to thank Roberto Vitillo @ LBL for his work on the perf >> tool for this. >> >> Enough talking, let's take a simple example. Our trivial test program >> goes like this: >> >> void f2(void) >> {} >> void f3(void) >> {} >> void f1(unsigned long n) >> { >> if (n & 1UL) >> f2(); >> else >> f3(); >> } >> int main(void) >> { >> unsigned long i; >> >> for (i=0; i < N; i++) >> f1(i); >> return 0; >> } >> >> $ perf record -b any branchy >> $ perf report -b >> # Events: 23K cycles >> # >> # Overhead Source Symbol Target Symbol >> # ........ ................ ................ >> >> 18.13% [.] f1 [.] main >> 18.10% [.] main [.] main >> 18.01% [.] main [.] f1 >> 15.69% [.] f1 [.] f1 >> 9.11% [.] f3 [.] f1 >> 6.78% [.] f1 [.] f3 >> 6.74% [.] f1 [.] f2 >> 6.71% [.] f2 [.] f1 >> >> Of the total number of branches captured, 18.13% were from f1() -> main(). >> >> Let's make this clearer by filtering the user call branches only: >> >> $ perf record -b any_call -e cycles:u branchy >> $ perf report -b >> # Events: 19K cycles >> # >> # Overhead Source Symbol Target Symbol >> # ........ ......................... ......................... >> # >> 52.50% [.] main [.] f1 >> 23.99% [.] f1 [.] f3 >> 23.48% [.] f1 [.] f2 >> 0.03% [.] _IO_default_xsputn [.] _IO_new_file_overflow >> 0.01% [k] _start [k] __libc_start_main >> >> Now it is more obvious. %52 of all the captured branches where calls from main() -> f1(). >> The rest is split 50/50 between f1() -> f2() and f1() -> f3() which is expected given >> that f1() dispatches based on odd vs. even values of n which is constantly increasing. >> >> >> Here is a kernel example, where we want to sample indirect calls: >> $ perf record -a -C 1 -b ind_call -e r1c4:k sleep 10 >> $ perf report -b >> # >> # Overhead Source Symbol Target Symbol >> # ........ .......................... .......................... >> # >> 36.36% [k] __delay [k] delay_tsc >> 9.09% [k] ktime_get [k] read_tsc >> 9.09% [k] getnstimeofday [k] read_tsc >> 9.09% [k] notifier_call_chain [k] tick_notify >> 4.55% [k] cpuidle_idle_call [k] intel_idle >> 4.55% [k] cpuidle_idle_call [k] menu_reflect >> 2.27% [k] handle_irq [k] handle_edge_irq >> 2.27% [k] ack_apic_edge [k] native_apic_mem_write >> 2.27% [k] hpet_interrupt_handler [k] hrtimer_interrupt >> 2.27% [k] __run_hrtimer [k] watchdog_timer_fn >> 2.27% [k] enqueue_task [k] enqueue_task_rt >> 2.27% [k] try_to_wake_up [k] select_task_rq_rt >> 2.27% [k] do_timer [k] read_tsc >> >> Due to HW limitations, branch filtering may be approximate on >> Core, Atom processors. It is more accurate on Nehalem, Westmere >> and best on Sandy Bridge. >> >> In version 2, we've updated the patch to tip/master (commit 5734857) and >> we've incoporated the feedback from v1 concerning anynous bitfield >> struct for branch_stack_entry and the hanlding of i386 ABI binaries >> on 64-bit host in the instr decoder for the LBR SW filter. >> >> In version 3, we've updated to 3.2.0-tip. The Atom revision >> check has been put into its own patch. We fixed a browser >> issue with report report. We fixed all the style issues as well. >> >> In version 4, we've modified the branch stack API to add a missing >> priv level : hypervisor. There is a new PERF_SAMPLE_BRANCH_HV. It >> is not used on Intel X86. Thanks to khandual@linux.vnet.ibm.com >> for pointing this out. We also fix compilation error on ARM. >> >> In version 4, we also extend the patch to include the changes necessary >> to the perf tool to support reading perf.data files which were produced >> from older perf_event ABI revisions. This patch set extends the ABI >> with a new field in struct perf_event_attr. That struct is saved as >> is in the perf.data file. Therefore, older perf.data files contain >> smaller perf_event_attr struct, yet perf must process them transparently. >> That's not the case today. It dies with 'incompatible file format'. >> >> The patch solves this problem and, at the same time, decouples endianness >> detection from the size of perf_event_attr. Endianness is now detected via >> the signature (the first 8 bytes of the file). We introduce a new signature >> (PERFILE2). It is not laid out the same way in the file based on the endianness >> of the host where the file is written. Therefore, we can dynamically detect >> the endianness by simply reading the first 8 bytes. The size of the >> perf_event_attr struct can then be processed according to the endianness. >> The ambiguity between the size being at the same time, the endianness marker >> and the actual size is gone. We can now distinguish an older ABI by the size >> and not confuse it with an endianness mismatch. >> >> In version 5, we fix the PEBS+LBR vs. BRANCH_STACK check in x86_pmu_hw_config. >> We also changed the handling of PERF_SAMPLE_BRANCH_HV on X86. It is now ignored >> instead of triggering an error. That enables: perf record -b any -e cycles, >> without having to force a priv level on the branch type. We also fix an >> uninitialized variable bug in the perf tool reported by reviewers. Thanks >> to Anshuman Khandual for his comments. >> >> In version 6, we have fixed several issues in the perf tool code and >> especially in patch 11. We have fully implemented the --sort option >> on the branch source and target. We have fixed several column alignment >> issues. We have integrated feedback from David Ahern, concerning patch >> 16 and the ability to read perf.data files written from a different >> ABI. Perf will now reject any perf.data file that has a larger perf_event_attr >> size. Perf provides compatibility backward and not forward. > > > Hey Stephane, > > Could you please specify which tip tree I can apply and try out the > V6 patchset ? Thank you. > V6 was posted prior to the jump_label changes. I suspect any tip before:
77a73e5 static keys: Introduce 'struct static_key', very_[un]likely(), static_key_slow_[inc|dec]()
Should work, though I have not tried.
> >> >> Signed-off-by: Stephane Eranian <eranian@google.com> >> >> >> Roberto Agostino Vitillo (3): >> perf: add code to support PERF_SAMPLE_BRANCH_STACK >> perf: add support for sampling taken branch to perf record >> perf: add support for taken branch sampling to perf report >> >> Stephane Eranian (15): >> perf: add generic taken branch sampling support >> perf: add Intel LBR MSR definitions >> perf: add Intel X86 LBR sharing logic >> perf: sync branch stack sampling with X86 precise_sampling >> perf: add Intel X86 LBR mappings for PERF_SAMPLE_BRANCH filters >> perf: disable LBR support for older Intel Atom processors >> perf: implement PERF_SAMPLE_BRANCH for Intel X86 >> perf: add LBR software filter support for Intel X86 >> perf: disable PERF_SAMPLE_BRANCH_* when not supported >> perf: add hook to flush branch_stack on context switch >> perf: fix endianness detection in perf.data >> perf: add ABI reference sizes >> perf: enable reading of perf.data files from different ABI rev >> perf: fix bug print_event_desc() >> perf: make perf able to read file from older ABIs >> >> arch/alpha/kernel/perf_event.c | 4 + >> arch/arm/kernel/perf_event.c | 4 + >> arch/mips/kernel/perf_event_mipsxx.c | 4 + >> arch/powerpc/kernel/perf_event.c | 4 + >> arch/sh/kernel/perf_event.c | 4 + >> arch/sparc/kernel/perf_event.c | 4 + >> arch/x86/include/asm/msr-index.h | 7 + >> arch/x86/kernel/cpu/perf_event.c | 85 ++++- >> arch/x86/kernel/cpu/perf_event.h | 19 + >> arch/x86/kernel/cpu/perf_event_amd.c | 3 + >> arch/x86/kernel/cpu/perf_event_intel.c | 120 +++++-- >> arch/x86/kernel/cpu/perf_event_intel_ds.c | 22 +- >> arch/x86/kernel/cpu/perf_event_intel_lbr.c | 526 ++++++++++++++++++++++++++-- >> include/linux/perf_event.h | 82 ++++- >> kernel/events/core.c | 177 ++++++++++ >> kernel/events/hw_breakpoint.c | 6 + >> tools/perf/Documentation/perf-record.txt | 25 ++ >> tools/perf/Documentation/perf-report.txt | 7 + >> tools/perf/builtin-record.c | 74 ++++ >> tools/perf/builtin-report.c | 98 +++++- >> tools/perf/perf.h | 18 + >> tools/perf/util/annotate.c | 2 +- >> tools/perf/util/event.h | 1 + >> tools/perf/util/evsel.c | 14 + >> tools/perf/util/header.c | 230 +++++++++++-- >> tools/perf/util/hist.c | 93 ++++- >> tools/perf/util/hist.h | 7 + >> tools/perf/util/session.c | 72 ++++ >> tools/perf/util/session.h | 4 + >> tools/perf/util/sort.c | 362 ++++++++++++++----- >> tools/perf/util/sort.h | 5 + >> tools/perf/util/symbol.h | 13 + >> 32 files changed, 1866 insertions(+), 230 deletions(-) >> > > > -- > Anshuman Khandual > Linux Technology Centre > IBM Systems and Technology Group > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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