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    SubjectRe: [PATCH] lockdep: Make lockstats counting per cpu
    I forgot to mention in the title of this patch, this is
    the "v2".


    On Fri, Mar 26, 2010 at 04:11:58AM +0100, Frederic Weisbecker wrote:
    > Locking statistics are implemented using global atomic variables.
    > This is usually fine unless some path write them very often.
    >
    > This is the case for the function and function graph tracers
    > that disable irqs for each entry saved (except if the function
    > tracer is in preempt disabled only mode).
    > And calls to local_irq_save/restore() increment hardirqs_on_events
    > and hardirqs_off_events stats (or similar stats for redundant
    > versions).
    >
    > Incrementing these global vars for each function ends up in too
    > much cache bouncing if lockstats are enabled.
    >
    > To solve this, implement the debug_atomic_*() operations using
    > per cpu vars. We can't use irqsafe per cpu counters for that as
    > these stats might be also written from NMI path, and irqsafe per
    > cpu counters are not NMI safe, but local_t oprations are.
    >
    > This version then uses local_t based per cpu counters.
    >
    > v2: Use per_cpu() instead of get_cpu_var() to fetch the desired
    > cpu vars on debug_atomic_read()
    >
    > Suggested-by: Steven Rostedt <rostedt@goodmis.org>
    > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    > Cc: Steven Rostedt <rostedt@goodmis.org>
    > ---
    > kernel/lockdep.c | 28 ++++++++--------
    > kernel/lockdep_internals.h | 71 +++++++++++++++++++++++++++++++-------------
    > 2 files changed, 64 insertions(+), 35 deletions(-)
    >
    > diff --git a/kernel/lockdep.c b/kernel/lockdep.c
    > index 65b5f5b..55e60a0 100644
    > --- a/kernel/lockdep.c
    > +++ b/kernel/lockdep.c
    > @@ -430,20 +430,20 @@ static struct stack_trace lockdep_init_trace = {
    > /*
    > * Various lockdep statistics:
    > */
    > -atomic_t chain_lookup_hits;
    > -atomic_t chain_lookup_misses;
    > -atomic_t hardirqs_on_events;
    > -atomic_t hardirqs_off_events;
    > -atomic_t redundant_hardirqs_on;
    > -atomic_t redundant_hardirqs_off;
    > -atomic_t softirqs_on_events;
    > -atomic_t softirqs_off_events;
    > -atomic_t redundant_softirqs_on;
    > -atomic_t redundant_softirqs_off;
    > -atomic_t nr_unused_locks;
    > -atomic_t nr_cyclic_checks;
    > -atomic_t nr_find_usage_forwards_checks;
    > -atomic_t nr_find_usage_backwards_checks;
    > +DEFINE_PER_CPU(local_t, chain_lookup_hits);
    > +DEFINE_PER_CPU(local_t, chain_lookup_misses);
    > +DEFINE_PER_CPU(local_t, hardirqs_on_events);
    > +DEFINE_PER_CPU(local_t, hardirqs_off_events);
    > +DEFINE_PER_CPU(local_t, redundant_hardirqs_on);
    > +DEFINE_PER_CPU(local_t, redundant_hardirqs_off);
    > +DEFINE_PER_CPU(local_t, softirqs_on_events);
    > +DEFINE_PER_CPU(local_t, softirqs_off_events);
    > +DEFINE_PER_CPU(local_t, redundant_softirqs_on);
    > +DEFINE_PER_CPU(local_t, redundant_softirqs_off);
    > +DEFINE_PER_CPU(local_t, nr_unused_locks);
    > +DEFINE_PER_CPU(local_t, nr_cyclic_checks);
    > +DEFINE_PER_CPU(local_t, nr_find_usage_forwards_checks);
    > +DEFINE_PER_CPU(local_t, nr_find_usage_backwards_checks);
    > #endif
    >
    > /*
    > diff --git a/kernel/lockdep_internals.h b/kernel/lockdep_internals.h
    > index a2ee95a..38c8ac7 100644
    > --- a/kernel/lockdep_internals.h
    > +++ b/kernel/lockdep_internals.h
    > @@ -110,29 +110,58 @@ lockdep_count_backward_deps(struct lock_class *class)
    > #endif
    >
    > #ifdef CONFIG_DEBUG_LOCKDEP
    > +
    > +#include <asm/local.h>
    > +/*
    > + * Various lockdep statistics.
    > + * We want them per cpu as they are often accessed in fast path
    > + * and we want to avoid too much cache bouncing.
    > + * We can't use irqsafe per cpu counters as those are not NMI safe, as
    > + * opposite to local_t.
    > + */
    > +DECLARE_PER_CPU(local_t, chain_lookup_hits);
    > +DECLARE_PER_CPU(local_t, chain_lookup_misses);
    > +DECLARE_PER_CPU(local_t, hardirqs_on_events);
    > +DECLARE_PER_CPU(local_t, hardirqs_off_events);
    > +DECLARE_PER_CPU(local_t, redundant_hardirqs_on);
    > +DECLARE_PER_CPU(local_t, redundant_hardirqs_off);
    > +DECLARE_PER_CPU(local_t, softirqs_on_events);
    > +DECLARE_PER_CPU(local_t, softirqs_off_events);
    > +DECLARE_PER_CPU(local_t, redundant_softirqs_on);
    > +DECLARE_PER_CPU(local_t, redundant_softirqs_off);
    > +DECLARE_PER_CPU(local_t, nr_unused_locks);
    > +DECLARE_PER_CPU(local_t, nr_cyclic_checks);
    > +DECLARE_PER_CPU(local_t, nr_cyclic_check_recursions);
    > +DECLARE_PER_CPU(local_t, nr_find_usage_forwards_checks);
    > +DECLARE_PER_CPU(local_t, nr_find_usage_forwards_recursions);
    > +DECLARE_PER_CPU(local_t, nr_find_usage_backwards_checks);
    > +DECLARE_PER_CPU(local_t, nr_find_usage_backwards_recursions);
    > +
    > +# define debug_atomic_inc(ptr) { \
    > + WARN_ON_ONCE(!irq_disabled()); \
    > + local_t *__ptr = &__get_cpu_var(ptr); \
    > + local_inc(__ptr); \
    > +}
    > +
    > +# define debug_atomic_dec(ptr) { \
    > + WARN_ON_ONCE(!irq_disabled()); \
    > + local_t *__ptr = &__get_cpu_var(ptr); \
    > + local_dec(__ptr); \
    > +}
    > +
    > /*
    > - * Various lockdep statistics:
    > + * It's fine to use local_read from other cpus. Read is racy anyway,
    > + * but it's guaranteed high and low parts are read atomically.
    > */
    > -extern atomic_t chain_lookup_hits;
    > -extern atomic_t chain_lookup_misses;
    > -extern atomic_t hardirqs_on_events;
    > -extern atomic_t hardirqs_off_events;
    > -extern atomic_t redundant_hardirqs_on;
    > -extern atomic_t redundant_hardirqs_off;
    > -extern atomic_t softirqs_on_events;
    > -extern atomic_t softirqs_off_events;
    > -extern atomic_t redundant_softirqs_on;
    > -extern atomic_t redundant_softirqs_off;
    > -extern atomic_t nr_unused_locks;
    > -extern atomic_t nr_cyclic_checks;
    > -extern atomic_t nr_cyclic_check_recursions;
    > -extern atomic_t nr_find_usage_forwards_checks;
    > -extern atomic_t nr_find_usage_forwards_recursions;
    > -extern atomic_t nr_find_usage_backwards_checks;
    > -extern atomic_t nr_find_usage_backwards_recursions;
    > -# define debug_atomic_inc(ptr) atomic_inc(ptr)
    > -# define debug_atomic_dec(ptr) atomic_dec(ptr)
    > -# define debug_atomic_read(ptr) atomic_read(ptr)
    > +# define debug_atomic_read(ptr) ({ \
    > + unsigned long long __total = 0; \
    > + int __cpu; \
    > + for_each_possible_cpu(__cpu) { \
    > + local_t *__ptr = &per_cpu(ptr, cpu); \
    > + __total += local_read(__ptr); \
    > + } \
    > + __total; \
    > +})
    > #else
    > # define debug_atomic_inc(ptr) do { } while (0)
    > # define debug_atomic_dec(ptr) do { } while (0)
    > --
    > 1.6.2.3
    >



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