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Date: Mon, 7 May 2018 14:08:01 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Joel Fernandes <joelaf@...gle.com>
Cc: linux-kernel@...r.kernel.org,
"Joel Fernandes (Google)" <joel@...lfernandes.org>,
Steven Rostedt <rostedt@...dmis.org>,
Peter Zilstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Tom Zanussi <tom.zanussi@...ux.intel.com>,
Namhyung Kim <namhyung@...nel.org>,
Thomas Glexiner <tglx@...utronix.de>,
Boqun Feng <boqun.feng@...il.com>,
Frederic Weisbecker <fweisbec@...il.com>,
Randy Dunlap <rdunlap@...radead.org>,
Masami Hiramatsu <mhiramat@...nel.org>,
Fenguang Wu <fengguang.wu@...el.com>,
Baohong Liu <baohong.liu@...el.com>,
Vedang Patel <vedang.patel@...el.com>, kernel-team@...roid.com
Subject: Re: [PATCH RFC v6 4/5] tracepoint: Make rcuidle tracepoint callers
use SRCU
On Mon, May 07, 2018 at 01:41:42PM -0700, Joel Fernandes wrote:
> From: "Joel Fernandes (Google)" <joel@...lfernandes.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@...dmis.org>
> Cc: Peter Zilstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Tom Zanussi <tom.zanussi@...ux.intel.com>
> Cc: Namhyung Kim <namhyung@...nel.org>
> Cc: Thomas Glexiner <tglx@...utronix.de>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Paul McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Randy Dunlap <rdunlap@...radead.org>
> Cc: Masami Hiramatsu <mhiramat@...nel.org>
> Cc: Fenguang Wu <fengguang.wu@...el.com>
> Cc: Baohong Liu <baohong.liu@...el.com>
> Cc: Vedang Patel <vedang.patel@...el.com>
> Cc: kernel-team@...roid.com
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.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?
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
>
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