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Message-ID: <CAJWu+orW_Ej8nGumYrB5vp-NBk0WrO75W7Te2yQ5MLyXXUFs4A@mail.gmail.com>
Date: Tue, 01 May 2018 15:53:53 +0000
From: Joel Fernandes <joelaf@...gle.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: LKML <linux-kernel@...r.kernel.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
Missed replying to some comments..
On Tue, May 1, 2018 at 7:24 AM Steven Rostedt <rostedt@...dmis.org> wrote:
> On Mon, 30 Apr 2018 18:42:03 -0700
> 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.
> Can you post some numbers?
Sure, I will post them in the next revision.
> > Test: Tested idle and preempt/irq tracepoints.
> >
> > [1] https://patchwork.kernel.org/patch/10344297/
> > [...]
> > 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
> Not related to your patch, but I find it interesting that we don't make
> this function a nop if CONFIG_TRACEPOINTS is not set. Is it because
> something might rely on our implementation that we call
> synchronize_sched here? I think that's a too tight of a coupling for
> others to rely on this, especially since it's not in the comments about
> this function.
If there's no CONFIG_TRACEPOINTS, then nothing should be replying on the
implementation?
Basically, if !TRACEPOINTS, then there shouldn't be any active rcu read
sections calling probes.
> Again, not related to this series, but something we should probably
> consider in the future. It would require auditing users of this too.
Yes, probably could be a noop in the future.
> > 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(); \
> Why not disable preemption after taking the srcu lock?
Sure. I don't have a strong preference for either way so I could disable it
after.
[...]
> > #ifndef MODULE
> > 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);
> Hmm, is it OK to call call_srcu() from a call_rcu() callback? I guess
> it would be.
> I think we should add a comment to why we are doing this. Something
> like:
> /*
> * Tracepoint probes are protected by both sched RCU and SRCU, by
> * calling the SRCU callback in the sched RCU callback we cover
> * both cases.
> */
> Or something along those lines.
Ok I'll add these. Thanks,
- Joel
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