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 <rost...@goodmis.org> > Cc: Peter Zilstra <pet...@infradead.org> > Cc: Ingo Molnar <mi...@redhat.com> > Cc: Mathieu Desnoyers <mathieu.desnoy...@efficios.com> > Cc: Tom Zanussi <tom.zanu...@linux.intel.com> > Cc: Namhyung Kim <namhy...@kernel.org> > Cc: Thomas Glexiner <t...@linutronix.de> > Cc: Boqun Feng <boqun.f...@gmail.com> > Cc: Paul McKenney <paul...@linux.vnet.ibm.com> > Cc: Frederic Weisbecker <fweis...@gmail.com> > Cc: Randy Dunlap <rdun...@infradead.org> > Cc: Masami Hiramatsu <mhira...@kernel.org> > Cc: Fenguang Wu <fengguang...@intel.com> > Cc: Baohong Liu <baohong....@intel.com> > Cc: Vedang Patel <vedang.pa...@intel.com> > Cc: kernel-t...@android.com > Signed-off-by: Joel Fernandes <joe...@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 >