On May 7, 2018 2:55:45 PM PDT, "Paul E. McKenney" <[email protected]> wrote: >On Mon, May 07, 2018 at 02:45:14PM -0700, Joel Fernandes wrote: >> On Mon, May 07, 2018 at 02:08:01PM -0700, Paul E. McKenney wrote: >> > On Mon, May 07, 2018 at 01:41:42PM -0700, Joel Fernandes wrote: >> > > From: "Joel Fernandes (Google)" <[email protected]> >> > > >> > > 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. >> [...] >> >> Signed-off-by: Joel Fernandes (Google) <[email protected]> >> > > --- >> > > include/linux/tracepoint.h | 46 >+++++++++++++++++++++++++++++++------- >> > > kernel/tracepoint.c | 15 ++++++++++++- >> > > 2 files changed, 52 insertions(+), 9 deletions(-) >> > > >> > > diff --git a/include/linux/tracepoint.h >b/include/linux/tracepoint.h >> > > index c94f466d57ef..f56f290cf8eb 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 +80,9 @@ int >unregister_tracepoint_module_notifier(struct notifier_block *nb) >> > > */ >> > > static inline void tracepoint_synchronize_unregister(void) >> > > { >> > > +#ifdef CONFIG_TRACEPOINTS >> > > + synchronize_srcu(&tracepoint_srcu); >> > > +#endif >> > > synchronize_sched(); >> > > } >> > > >> > > @@ -129,18 +135,38 @@ 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, rcuidle) >> > > \ >> > > 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); >> > > \ >> > > + >> > > \ >> > > + /* >> > > \ >> > > + * For rcuidle callers, use srcu since sched-rcu >> > > \ >> > > + * doesn't work from the idle path. >> > > \ >> > > + */ >> > > \ >> > > + if (rcuidle) { >> > > \ >> > > + if (in_nmi()) { >> > > \ >> > > + WARN_ON_ONCE(1); >> > > \ >> > > + return; /* no srcu from nmi */ >> > > \ >> > > + } >> > > \ >> > > + >> > > \ >> > > + idx = srcu_read_lock_notrace(&tracepoint_srcu); >> > > \ >> > > + it_func_ptr = >> > > \ >> > > + srcu_dereference_notrace((tp)->funcs, >> > > \ >> > > + &tracepoint_srcu); >> > > \ >> > > + /* To keep it consistent with !rcuidle path */ >> > > \ >> > > + preempt_disable_notrace(); >> > > \ >> > > + } 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; >> > > \ >> > > @@ -148,9 +174,13 @@ 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 (rcuidle) { >> > > \ >> > >> > Don't we also need an in_nmi() check here in order to avoid >unbalanced >> > srcu_read_unlock_notrace() calls? >> >> The in_nmi() in the lock path should take care of making sure its >balanced. >> >> The diff the way its formatted appears confusing as Mathieu pointed. > >Ah, right, I was for some reason thinking that the two hunks of the >diff were two separate macros. Apologies for my confusion! >
No worries! Thanks for the great idea to use in_nmi in the first place. - Joel > Thanx, Paul -- Sent from my Android device with K-9 Mail. Please excuse my brevity.
