On Sat, May 11, 2013 at 09:50:08AM +0200, Ingo Molnar wrote: > That's really a red herring: there's absolutely no reason why the > kernel could not pass back the level of precision it provided.
All I've been saying is that doing random precision without feedback is confusing. We also don't really have a good feedback channel for this kind of thing. The best I can come up with is tagging each and every sample with the quality it represents. I think we can do with only one extra PERF_RECORD_MISC bit, but it looks like we're quickly running out of those things. But I think the biggest problem is PEBS's inability do deal with REP prefixes; see this email from Stephane: https://lkml.org/lkml/2011/2/1/177 It is really unfortunate for PEBS to have such a side-effect; but it makes all memset/memcpy/memmove things appear like they have no cost. I'm very sure that will surprise a number of people. --- arch/x86/include/asm/perf_event.h | 4 +++- arch/x86/kernel/cpu/perf_event.c | 2 ++ arch/x86/kernel/cpu/perf_event_intel_ds.c | 4 +++- include/uapi/linux/perf_event.h | 1 + 4 files changed, 9 insertions(+), 2 deletions(-) diff --git a/arch/x86/include/asm/perf_event.h b/arch/x86/include/asm/perf_event.h index 57cb634..6908838 100644 --- a/arch/x86/include/asm/perf_event.h +++ b/arch/x86/include/asm/perf_event.h @@ -205,14 +205,16 @@ static inline u32 get_ibs_caps(void) { return 0; } extern void perf_events_lapic_init(void); /* - * Abuse bits {3,5} of the cpu eflags register. These flags are otherwise + * Abuse bits {3,5,15} of the cpu eflags register. These flags are otherwise * unused and ABI specified to be 0, so nobody should care what we do with * them. * + * CONSTANT - the IP has constant skid. * EXACT - the IP points to the exact instruction that triggered the * event (HW bugs exempt). * VM - original X86_VM_MASK; see set_linear_ip(). */ +#define PERF_EFLAGS_CONSTANT (1UL << 15) #define PERF_EFLAGS_EXACT (1UL << 3) #define PERF_EFLAGS_VM (1UL << 5) diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c index 1025f3c..ca1f7dc 100644 --- a/arch/x86/kernel/cpu/perf_event.c +++ b/arch/x86/kernel/cpu/perf_event.c @@ -2078,6 +2078,8 @@ unsigned long perf_misc_flags(struct pt_regs *regs) if (regs->flags & PERF_EFLAGS_EXACT) misc |= PERF_RECORD_MISC_EXACT_IP; + else if (regs->flags & PERF_EFLAGS_CONSTANT) + misc |= PERF_RECORD_MISC_CONSTANT_SKID; return misc; } diff --git a/arch/x86/kernel/cpu/perf_event_intel_ds.c b/arch/x86/kernel/cpu/perf_event_intel_ds.c index 60250f6..757ecd4 100644 --- a/arch/x86/kernel/cpu/perf_event_intel_ds.c +++ b/arch/x86/kernel/cpu/perf_event_intel_ds.c @@ -753,10 +753,12 @@ static void __intel_pmu_pebs_event(struct perf_event *event, regs.bp = pebs->bp; regs.sp = pebs->sp; + regs.flags &= ~(PERF_EFLAGS_CONSTANT | PERF_EFLAGS_EXACT); + if (event->attr.precise_ip > 1 && intel_pmu_pebs_fixup_ip(®s)) regs.flags |= PERF_EFLAGS_EXACT; else - regs.flags &= ~PERF_EFLAGS_EXACT; + regs.flags |= PERF_EFLAGS_CONSTANT; if (has_branch_stack(event)) data.br_stack = &cpuc->lbr_stack; diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h index fb104e5..cb1f70f 100644 --- a/include/uapi/linux/perf_event.h +++ b/include/uapi/linux/perf_event.h @@ -451,6 +451,7 @@ struct perf_event_mmap_page { * the actual instruction that triggered the event. See also * perf_event_attr::precise_ip. */ +#define PERF_RECORD_MISC_CONSTANT_SKID (1 << 12) #define PERF_RECORD_MISC_EXACT_IP (1 << 14) /* * Reserve the last bit to indicate some extended misc field -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
