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Date: Tue, 01 May 2018 01:56:01 +0000
From: Joel Fernandes <joelaf@...gle.com>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Steven Rostedt <rostedt@...dmis.org>,
Peter Zijlstra <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 Gleixner <tglx@...utronix.de>,
Boqun Feng <boqun.feng@...il.com>,
Paul McKenney <paulmck@...ux.vnet.ibm.com>,
"Cc: 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>,
"Cc: Android Kernel" <kernel-team@...roid.com>
Subject: Re: [PATCH RFC v5 5/6] tracepoint: Make rcuidle tracepoint callers
use SRCU
On Mon, Apr 30, 2018 at 6:42 PM Joel Fernandes <joelaf@...gle.com> wrote:
> 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 <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 <joelaf@...gle.com>
> ---
> 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); \
This last bit is supposed to be srcu_dereference_notrace. The hunk to use
that is actually in patch 6/6 , sorry about that. I've fixed it in my tree
and it means patches 5/6 and 6/6 need an update. Steve, if you want me to
repost it right away I can do that, or can wait for additional comments
and then repost.
thanks,
- Joel
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