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. [1] https://patchwork.kernel.org/patch/10344297/ Cc: Steven Rostedt <[email protected]> Cc: Peter Zilstra <[email protected]> Cc: Ingo Molnar <[email protected]> Cc: Mathieu Desnoyers <[email protected]> Cc: Tom Zanussi <[email protected]> Cc: Namhyung Kim <[email protected]> Cc: Thomas Glexiner <[email protected]> Cc: Boqun Feng <[email protected]> Cc: Paul McKenney <[email protected]> Cc: Frederic Weisbecker <[email protected]> Cc: Randy Dunlap <[email protected]> Cc: Masami Hiramatsu <[email protected]> Cc: Fenguang Wu <[email protected]> Cc: Baohong Liu <[email protected]> Cc: Vedang Patel <[email protected]> Cc: [email protected] Signed-off-by: Joel Fernandes <[email protected]> --- include/linux/tracepoint.h | 46 +++++++++++++++++++++++++++++++------- kernel/tracepoint.c | 10 ++++++++- 2 files changed, 47 insertions(+), 9 deletions(-) diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h index c94f466d57ef..4135e08fb5f1 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 */ \ + } \ + \ + /* To keep it consistent with !rcuidle path */ \ + preempt_disable_notrace(); \ + \ + idx = srcu_read_lock_notrace(&tracepoint_srcu); \ + 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; \ @@ -148,9 +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) { \ + srcu_read_unlock_notrace(&tracepoint_srcu, idx);\ + preempt_enable_notrace(); \ + } else { \ + rcu_read_unlock_sched_notrace(); \ + } \ } while (0) #ifndef MODULE 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); +} + static inline void release_probes(struct tracepoint_func *old) { if (old) { -- 2.17.0.441.gb46fe60e1d-goog
