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Date: Tue, 17 Jul 2018 10:11:21 -0700
From: Joel Fernandes <joel@...lfernandes.org>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: linux-kernel <linux-kernel@...r.kernel.org>,
kernel-team <kernel-team@...roid.com>,
Boqun Feng <boqun.feng@...il.com>,
Byungchul Park <byungchul.park@....com>,
Erick Reyes <erickreyes@...gle.com>,
Ingo Molnar <mingo@...hat.com>,
Julia Cartwright <julia@...com>,
Masami Hiramatsu <mhiramat@...nel.org>,
Namhyung Kim <namhyung@...nel.org>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Peter Zijlstra <peterz@...radead.org>,
rostedt <rostedt@...dmis.org>,
Thomas Gleixner <tglx@...utronix.de>,
Todd Kjos <tkjos@...gle.com>,
Tom Zanussi <tom.zanussi@...ux.intel.com>,
Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v10 2/3] tracepoint: Make rcuidle tracepoint callers use
SRCU
On Sat, Jul 14, 2018 at 10:31:57AM -0400, Mathieu Desnoyers wrote:
> ----- On Jul 13, 2018, at 5:55 PM, Joel Fernandes, Google joel@...lfernandes.org 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/
> >
> > Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
>
> I'm fine with the changes done since last iteration.
>
> Acked-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Thanks a lot Mathieu!
Steve, Peter, looks good to you too now?
thanks,
- Joel
[...]
> > Cleaned-up-by: Peter Zijlstra <peterz@...radead.org>
> > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > ---
> > include/linux/tracepoint.h | 45 +++++++++++++++++++++++++++++++-------
> > kernel/tracepoint.c | 16 +++++++++++++-
> > 2 files changed, 52 insertions(+), 9 deletions(-)
> >
> > diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> > index 19a690b559ca..97e1d365a817 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
> > @@ -75,10 +78,16 @@ int unregister_tracepoint_module_notifier(struct
> > notifier_block *nb)
> > * probe unregistration and the end of module exit to make sure there is no
> > * caller executing a probe when it is freed.
> > */
> > +#ifdef CONFIG_TRACEPOINTS
> > static inline void tracepoint_synchronize_unregister(void)
> > {
> > + synchronize_srcu(&tracepoint_srcu);
> > synchronize_sched();
> > }
> > +#else
> > +static inline void tracepoint_synchronize_unregister(void)
> > +{ }
> > +#endif
> >
> > #ifdef CONFIG_HAVE_SYSCALL_TRACEPOINTS
> > extern int syscall_regfunc(void);
> > @@ -129,18 +138,34 @@ 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); \
> > + \
> > + /* srcu can't be used from NMI */ \
> > + if (rcuidle && in_nmi()) \
> > + WARN_ON_ONCE(1); \
> > + \
> > + /* keep srcu and sched-rcu usage consistent */ \
> > + preempt_disable_notrace(); \
> > + \
> > + /* \
> > + * For rcuidle callers, use srcu since sched-rcu \
> > + * doesn't work from the idle path. \
> > + */ \
> > + if (rcuidle) \
> > + idx = srcu_read_lock_notrace(&tracepoint_srcu); \
> > + else \
> > + rcu_read_lock_sched_notrace(); \
> > + \
> > + it_func_ptr = rcu_dereference_raw((tp)->funcs); \
> > + \
> > if (it_func_ptr) { \
> > do { \
> > it_func = (it_func_ptr)->func; \
> > @@ -148,9 +173,13 @@ 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);\
> > + else \
> > + rcu_read_unlock_sched_notrace(); \
> > + \
> > + preempt_enable_notrace(); \
> > } while (0)
> >
> > #ifndef MODULE
> > diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
> > index 6dc6356c3327..955148d91b74 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 +70,27 @@ 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 let us chain the SRCU and sched RCU
> > + * callbacks to wait for both grace periods.
> > + */
> > call_rcu_sched(&tp_probes->rcu, rcu_free_old_probes);
> > }
> > }
> > --
> > 2.18.0.203.gfac676dfb9-goog
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
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