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. [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 */ \ + idx = srcu_read_lock(&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; \ @@ -148,12 +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); +} + static inline void release_probes(struct tracepoint_func *old) { if (old) { -- 2.17.0.441.gb46fe60e1d-goog
