----- On Jul 13, 2018, at 5:55 PM, Joel Fernandes, Google [email protected] 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. > > Test: Tested idle and preempt/irq tracepoints. > > Here are some performance numbers: > > With a run of the following 30 times on a single core x86 Qemu instance > with 1GB memory: > hackbench -g 4 -f 2 -l 3000 > > Completion times in seconds. CONFIG_PROVE_LOCKING=y. > > No patches (without this series) > Mean: 3.048 > Median: 3.025 > Std Dev: 0.064 > > With Lockdep using irq tracepoints with RCU implementation: > Mean: 3.451 (-11.66 %) > Median: 3.447 (-12.22%) > Std Dev: 0.049 > > With Lockdep using irq tracepoints with SRCU implementation (this series): > Mean: 3.020 (I would consider the improvement against the "without > this series" case as just noise). > Median: 3.013 > Std Dev: 0.033 > > [1] https://patchwork.kernel.org/patch/10344297/ > > Reviewed-by: Mathieu Desnoyers <[email protected]> I'm fine with the changes done since last iteration. Acked-by: Mathieu Desnoyers <[email protected]> Thanks! Mathieu > Cleaned-up-by: Peter Zijlstra <[email protected]> > Signed-off-by: Joel Fernandes (Google) <[email protected]> > --- > include/linux/tracepoint.h | 45 +++++++++++++++++++++++++++++++------- > kernel/tracepoint.c | 16 +++++++++++++- > 2 files changed, 52 insertions(+), 9 deletions(-) > > diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h > index 19a690b559ca..97e1d365a817 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 > @@ -75,10 +78,16 @@ int unregister_tracepoint_module_notifier(struct > notifier_block *nb) > * probe unregistration and the end of module exit to make sure there is no > * caller executing a probe when it is freed. > */ > +#ifdef CONFIG_TRACEPOINTS > static inline void tracepoint_synchronize_unregister(void) > { > + synchronize_srcu(&tracepoint_srcu); > synchronize_sched(); > } > +#else > +static inline void tracepoint_synchronize_unregister(void) > +{ } > +#endif > > #ifdef CONFIG_HAVE_SYSCALL_TRACEPOINTS > extern int syscall_regfunc(void); > @@ -129,18 +138,34 @@ 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); \ > + \ > + /* srcu can't be used from NMI */ \ > + if (rcuidle && in_nmi()) \ > + WARN_ON_ONCE(1); \ > + \ > + /* keep srcu and sched-rcu usage consistent */ \ > + preempt_disable_notrace(); \ > + \ > + /* \ > + * For rcuidle callers, use srcu since sched-rcu \ > + * doesn't work from the idle path. \ > + */ \ > + if (rcuidle) \ > + idx = srcu_read_lock_notrace(&tracepoint_srcu); \ > + else \ > + rcu_read_lock_sched_notrace(); \ > + \ > + it_func_ptr = rcu_dereference_raw((tp)->funcs); \ > + \ > if (it_func_ptr) { \ > do { \ > it_func = (it_func_ptr)->func; \ > @@ -148,9 +173,13 @@ 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) \ > + srcu_read_unlock_notrace(&tracepoint_srcu, idx);\ > + else \ > + rcu_read_unlock_sched_notrace(); \ > + \ > + preempt_enable_notrace(); \ > } while (0) > > #ifndef MODULE > diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c > index 6dc6356c3327..955148d91b74 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; > > @@ -67,16 +70,27 @@ 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); > +} > + > static inline void release_probes(struct tracepoint_func *old) > { > if (old) { > struct tp_probes *tp_probes = container_of(old, > struct tp_probes, probes[0]); > + /* > + * Tracepoint probes are protected by both sched RCU and SRCU, > + * by calling the SRCU callback in the sched RCU callback we > + * cover both cases. So let us chain the SRCU and sched RCU > + * callbacks to wait for both grace periods. > + */ > call_rcu_sched(&tp_probes->rcu, rcu_free_old_probes); > } > } > -- > 2.18.0.203.gfac676dfb9-goog -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
