Re: [PATCH RFC v6 4/5] tracepoint: Make rcuidle tracepoint callers use SRCU

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On May 7, 2018, at 5:08 PM, Paul E. McKenney paulmck@linux.vnet.ibm.com wrote:

> On Mon, May 07, 2018 at 01:41:42PM -0700, Joel Fernandes wrote:
>> From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
>> 
>> 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/
>> 
>> Cc: Steven Rostedt <rostedt@goodmis.org>
>> Cc: Peter Zilstra <peterz@infradead.org>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>> Cc: Tom Zanussi <tom.zanussi@linux.intel.com>
>> Cc: Namhyung Kim <namhyung@kernel.org>
>> Cc: Thomas Glexiner <tglx@linutronix.de>
>> Cc: Boqun Feng <boqun.feng@gmail.com>
>> Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
>> Cc: Frederic Weisbecker <fweisbec@gmail.com>
>> Cc: Randy Dunlap <rdunlap@infradead.org>
>> Cc: Masami Hiramatsu <mhiramat@kernel.org>
>> Cc: Fenguang Wu <fengguang.wu@intel.com>
>> Cc: Baohong Liu <baohong.liu@intel.com>
>> Cc: Vedang Patel <vedang.patel@intel.com>
>> Cc: kernel-team@android.com
>> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
>> ---
>>  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?

AFAIU the "return;" in the if (in_nmi()) branch above takes care of never
executing the following code. diff appears to be a bit confused by the
preprocessor macros, but in reality this is all part of the same static
inline function.

Thanks,

Mathieu

> 
>							Thanx, Paul
> 
>> +			preempt_enable_notrace();			\
>> +			srcu_read_unlock_notrace(&tracepoint_srcu, idx);\
>> +		} else {						\
>> +			rcu_read_unlock_sched_notrace();		\
>> +		}							\
>>  	} while (0)
>> 
>>  #ifndef MODULE
>> diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
>> index 671b13457387..2089f579f790 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 +70,26 @@ 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 lets chain the SRCU and RCU callbacks.
>> +		 */
>>  		call_rcu_sched(&tp_probes->rcu, rcu_free_old_probes);
>>  	}
>>  }
>> --
>> 2.17.0.441.gb46fe60e1d-goog

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com