[patch V5 04/38] x86: Make hardware latency tracing explicit

Thomas Gleixner
2020-05-12T22:23:11Z

The hardware latency tracer calls into trace_sched_clock and ends up in
various instrumentable functions which is problemeatic vs. the kprobe
handling especially the text poke machinery. It's invoked from
nmi_enter/exit(), i.e. non-instrumentable code.

Use nmi_enter/exit_notrace() instead. These variants do not invoke the
hardware latency tracer which avoids chasing down complex callchains to
make them non-instrumentable.

The real interesting measurement is the actual NMI handler. Add an explicit
invocation for the hardware latency tracer to it.

#DB and #BP are uninteresting as they really should not be in use when
analzying hardware induced latencies.

If #DF hits, hardware latency is definitely not interesting anymore and in
case of a machine check the hardware latency is not the most troublesome
issue either.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
V5: New patch
---
arch/x86/kernel/cpu/mce/core.c | 4 ++--
arch/x86/kernel/nmi.c | 6 ++++--
arch/x86/kernel/traps.c | 10 +++++-----
3 files changed, 11 insertions(+), 9 deletions(-)

--- a/arch/x86/kernel/cpu/mce/core.c
+++ b/arch/x86/kernel/cpu/mce/core.c
@@ -1916,7 +1916,7 @@ static __always_inline void exc_machine_
mce_check_crashing_cpu())
return;

- nmi_enter();
+ nmi_enter_notrace();
/*
* The call targets are marked noinstr, but objtool can't figure
* that out because it's an indirect call. Annotate it.
@@ -1924,7 +1924,7 @@ static __always_inline void exc_machine_
instrumentation_begin();
machine_check_vector(regs);
instrumentation_end();
- nmi_exit();
+ nmi_exit_notrace();
}

static __always_inline void exc_machine_check_user(struct pt_regs *regs)
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -334,6 +334,7 @@ static noinstr void default_do_nmi(struc
__this_cpu_write(last_nmi_rip, regs->ip);

instrumentation_begin();
+ ftrace_nmi_enter();

handled = nmi_handle(NMI_LOCAL, regs);
__this_cpu_add(nmi_stats.normal, handled);
@@ -420,6 +421,7 @@ static noinstr void default_do_nmi(struc
unknown_nmi_error(reason, regs);

out:
+ ftrace_nmi_exit();
instrumentation_end();
}

@@ -536,14 +538,14 @@ DEFINE_IDTENTRY_NMI(exc_nmi)
}
#endif

- nmi_enter();
+ nmi_enter_notrace();

inc_irq_stat(__nmi_count);

if (!ignore_nmis)
default_do_nmi(regs);

- nmi_exit();
+ nmi_exit_notrace();

#ifdef CONFIG_X86_64
if (unlikely(this_cpu_read(update_debug_stack))) {
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -378,7 +378,7 @@ DEFINE_IDTENTRY_DF(exc_double_fault)
}
#endif

- nmi_enter();
+ nmi_enter_notrace();
instrumentation_begin();
notify_die(DIE_TRAP, str, regs, error_code, X86_TRAP_DF, SIGSEGV);

@@ -624,11 +624,11 @@ DEFINE_IDTENTRY_RAW(exc_int3)
instrumentation_end();
idtentry_exit(regs);
} else {
- nmi_enter();
+ nmi_enter_notrace();
instrumentation_begin();
do_int3(regs);
instrumentation_end();
- nmi_exit();
+ nmi_exit_notrace();
}
}

@@ -827,7 +827,7 @@ static void noinstr handle_debug(struct
static __always_inline void exc_debug_kernel(struct pt_regs *regs,
unsigned long dr6)
{
- nmi_enter();
+ nmi_enter_notrace();
/*
* The SDM says "The processor clears the BTF flag when it
* generates a debug exception." Clear TIF_BLOCKSTEP to keep
@@ -849,7 +849,7 @@ static __always_inline void exc_debug_ke
if (dr6)
handle_debug(regs, dr6, false);

- nmi_exit();
+ nmi_exit_notrace();
}

static __always_inline void exc_debug_user(struct pt_regs *regs,