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    SubjectRe: [PATCH RFC] tracepoint: Introduce tracepoint callbacks executing with preempt on
    On Thu, Apr 26, 2018 at 09:26:56PM -0700, Joel Fernandes wrote:
    > In recent tests with IRQ on/off tracepoints, a large performance
    > overhead ~10% is noticed when running hackbench. This is root caused to
    > calls to rcu_irq_enter_irqson and rcu_irq_exit_irqson from the
    > tracepoint code. Following a long discussion on the list [1] about this,
    > we concluded that srcu is a better alternative for use during rcu idle.
    > Although it does involve extra barriers, its lighter than the sched-rcu
    > version which has to do additional RCU calls to notify RCU idle about
    > entry into RCU sections.
    >
    > In this patch, we change the underlying implementation of the
    > trace_*_rcuidle API to use SRCU. This has shown to improve performance
    > alot for the high frequency irq enable/disable tracepoints.
    >
    > In the future, we can add a new may_sleep API which can use this
    > infrastructure for callbacks that actually can sleep which will support
    > Mathieu's usecase of blocking probes.
    >
    > Test: Tested idle and preempt/irq tracepoints.

    Looks good overall! One question and a few comments below.

    Thanx, Paul

    > [1] https://patchwork.kernel.org/patch/10344297/
    >
    > Cc: Steven Rostedt <rostedt@goodmis.org>
    > Cc: Peter Zilstra <peterz@infradead.org>
    > Cc: Ingo Molnar <mingo@redhat.com>
    > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
    > Cc: Tom Zanussi <tom.zanussi@linux.intel.com>
    > Cc: Namhyung Kim <namhyung@kernel.org>
    > Cc: Thomas Glexiner <tglx@linutronix.de>
    > Cc: Boqun Feng <boqun.feng@gmail.com>
    > Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
    > Cc: Frederic Weisbecker <fweisbec@gmail.com>
    > Cc: Randy Dunlap <rdunlap@infradead.org>
    > Cc: Masami Hiramatsu <mhiramat@kernel.org>
    > Cc: Fenguang Wu <fengguang.wu@intel.com>
    > Cc: Baohong Liu <baohong.liu@intel.com>
    > Cc: Vedang Patel <vedang.patel@intel.com>
    > Cc: kernel-team@android.com
    > Signed-off-by: Joel Fernandes <joelaf@google.com>
    > ---
    > include/linux/tracepoint.h | 37 +++++++++++++++++++++++++++++--------
    > kernel/tracepoint.c | 10 +++++++++-
    > 2 files changed, 38 insertions(+), 9 deletions(-)
    >
    > diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
    > index c94f466d57ef..a1c1987de423 100644
    > --- a/include/linux/tracepoint.h
    > +++ b/include/linux/tracepoint.h
    > @@ -15,6 +15,7 @@
    > */
    >
    > #include <linux/smp.h>
    > +#include <linux/srcu.h>
    > #include <linux/errno.h>
    > #include <linux/types.h>
    > #include <linux/cpumask.h>
    > @@ -33,6 +34,8 @@ struct trace_eval_map {
    >
    > #define TRACEPOINT_DEFAULT_PRIO 10
    >
    > +extern struct srcu_struct tracepoint_srcu;
    > +
    > extern int
    > tracepoint_probe_register(struct tracepoint *tp, void *probe, void *data);
    > extern int
    > @@ -77,6 +80,7 @@ int unregister_tracepoint_module_notifier(struct notifier_block *nb)
    > */
    > static inline void tracepoint_synchronize_unregister(void)
    > {
    > + synchronize_srcu(&tracepoint_srcu);
    > synchronize_sched();
    > }
    >
    > @@ -129,18 +133,26 @@ extern void syscall_unregfunc(void);
    > * as "(void *, void)". The DECLARE_TRACE_NOARGS() will pass in just
    > * "void *data", where as the DECLARE_TRACE() will pass in "void *data, proto".
    > */
    > -#define __DO_TRACE(tp, proto, args, cond, rcucheck) \
    > +#define __DO_TRACE(tp, proto, args, cond, preempt_on) \
    > do { \
    > struct tracepoint_func *it_func_ptr; \
    > void *it_func; \
    > void *__data; \
    > + int __maybe_unused idx = 0; \
    > \
    > if (!(cond)) \
    > return; \
    > - if (rcucheck) \
    > - rcu_irq_enter_irqson(); \
    > - rcu_read_lock_sched_notrace(); \
    > - it_func_ptr = rcu_dereference_sched((tp)->funcs); \
    > + if (preempt_on) { \
    > + WARN_ON_ONCE(in_nmi()); /* no srcu from nmi */ \

    Very good on this check, thank you!

    > + idx = srcu_read_lock(&tracepoint_srcu); \

    Hmmm... Do I need to create a _notrace variant of srcu_read_lock()
    and srcu_read_unlock()?

    > + it_func_ptr = srcu_dereference((tp)->funcs, \
    > + &tracepoint_srcu); \
    > + } else { \
    > + rcu_read_lock_sched_notrace(); \
    > + it_func_ptr = \
    > + rcu_dereference_sched((tp)->funcs); \
    > + } \
    > + \
    > if (it_func_ptr) { \
    > do { \
    > it_func = (it_func_ptr)->func; \
    > @@ -148,12 +160,21 @@ extern void syscall_unregfunc(void);
    > ((void(*)(proto))(it_func))(args); \
    > } while ((++it_func_ptr)->func); \
    > } \
    > - rcu_read_unlock_sched_notrace(); \
    > - if (rcucheck) \
    > - rcu_irq_exit_irqson(); \
    > + \
    > + if (preempt_on) \
    > + srcu_read_unlock(&tracepoint_srcu, idx); \
    > + else \
    > + rcu_read_unlock_sched_notrace(); \
    > } while (0)
    >
    > #ifndef MODULE
    > +/*
    > + * This is for tracepoints that may be called from an rcu idle path. To make
    > + * sure we its safe in the idle path, use the srcu instead of sched-rcu by
    > + * specifying the preempt_on flag below. This has the obvious effect that any
    > + * callback expecting preemption to be disabled should explicitly do so since
    > + * with srcu it'd run with preempt on.
    > + */
    > #define __DECLARE_TRACE_RCU(name, proto, args, cond, data_proto, data_args) \
    > static inline void trace_##name##_rcuidle(proto) \
    > { \
    > diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
    > index 671b13457387..b3b1d65a2460 100644
    > --- a/kernel/tracepoint.c
    > +++ b/kernel/tracepoint.c
    > @@ -31,6 +31,9 @@
    > extern struct tracepoint * const __start___tracepoints_ptrs[];
    > extern struct tracepoint * const __stop___tracepoints_ptrs[];
    >
    > +DEFINE_SRCU(tracepoint_srcu);
    > +EXPORT_SYMBOL_GPL(tracepoint_srcu);
    > +
    > /* Set to 1 to enable tracepoint debug output */
    > static const int tracepoint_debug;
    >
    > @@ -67,11 +70,16 @@ static inline void *allocate_probes(int count)
    > return p == NULL ? NULL : p->probes;
    > }
    >
    > -static void rcu_free_old_probes(struct rcu_head *head)
    > +static void srcu_free_old_probes(struct rcu_head *head)
    > {
    > kfree(container_of(head, struct tp_probes, rcu));
    > }
    >
    > +static void rcu_free_old_probes(struct rcu_head *head)
    > +{
    > + call_srcu(&tracepoint_srcu, head, srcu_free_old_probes);

    Chaining them through the two callbacks works, good!

    > +}
    > +
    > static inline void release_probes(struct tracepoint_func *old)
    > {
    > if (old) {
    > --
    > 2.17.0.441.gb46fe60e1d-goog
    >

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