From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Joel Fernandes <joelaf@google.com>,
linux-kernel@vger.kernel.org,
Peter Zilstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Tom Zanussi <tom.zanussi@linux.intel.com>,
Namhyung Kim <namhyung@kernel.org>,
Thomas Glexiner <tglx@linutronix.de>,
Boqun Feng <boqun.feng@gmail.com>,
Frederic Weisbecker <fweisbec@gmail.com>,
Randy Dunlap <rdunlap@infradead.org>,
Masami Hiramatsu <mhiramat@kernel.org>,
Fenguang Wu <fengguang.wu@intel.com>,
Baohong Liu <baohong.liu@intel.com>,
Vedang Patel <vedang.patel@intel.com>,
kernel-team@android.com
Subject: Re: [PATCH RFC v5 5/6] tracepoint: Make rcuidle tracepoint callers use SRCU
Date: Tue, 1 May 2018 07:36:01 -0700 [thread overview]
Message-ID: <20180501143601.GG26088@linux.vnet.ibm.com> (raw)
In-Reply-To: <20180501102401.2cac5781@gandalf.local.home>
On Tue, May 01, 2018 at 10:24:01AM -0400, Steven Rostedt wrote:
> On Mon, 30 Apr 2018 18:42:03 -0700
> Joel Fernandes <joelaf@google.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.
[ . . . ]
> > --- 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.
It is perfectly legal, and quite a bit simpler than setting something
up to wait for both to complete concurrently.
Of course, if you unconditionally call call_srcu() from that same
srcu_struct's callback, SRCU will be unable to safely delete the
srcu_struct, so cleanup_srcu_struct() will react by leaking memory. ;-)
Normal RCU deals with the analogous situation by leaving at least one
callback uninvoked when the system goes down.
Thanx, Paul
> 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.
>
> -- Steve
>
>
> > +}
> > +
> > static inline void release_probes(struct tracepoint_func *old)
> > {
> > if (old) {
>
next prev parent reply other threads:[~2018-05-01 14:34 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-01 1:41 [PATCH RFC v5 0/6] Centralize and unify usage of preempt/irq tracepoints Joel Fernandes
2018-05-01 1:41 ` [PATCH RFC v5 1/6] softirq: reorder trace_softirqs_on to prevent lockdep splat Joel Fernandes
2018-05-01 14:02 ` Steven Rostedt
2018-05-01 15:00 ` Joel Fernandes
2018-05-01 1:42 ` [PATCH RFC v5 2/6] srcu: Add notrace variants of srcu_read_{lock,unlock} Joel Fernandes
2018-05-01 14:04 ` Steven Rostedt
2018-05-01 1:42 ` [PATCH RFC v5 3/6] srcu: Add notrace variant of srcu_dereference Joel Fernandes
2018-05-01 14:06 ` Steven Rostedt
2018-05-01 15:07 ` Joel Fernandes
2018-05-01 14:18 ` Paul E. McKenney
2018-05-01 15:04 ` Steven Rostedt
2018-05-01 15:27 ` Paul E. McKenney
2018-05-01 1:42 ` [PATCH RFC v5 4/6] trace/irqsoff: Split reset into seperate functions Joel Fernandes
2018-05-01 3:45 ` Randy Dunlap
2018-05-01 3:46 ` [kernel-team] " Joel Fernandes
2018-05-01 1:42 ` [PATCH RFC v5 5/6] tracepoint: Make rcuidle tracepoint callers use SRCU Joel Fernandes
2018-05-01 1:56 ` Joel Fernandes
2018-05-01 14:24 ` Steven Rostedt
2018-05-01 14:24 ` Steven Rostedt
2018-05-01 14:36 ` Paul E. McKenney [this message]
2018-05-01 15:16 ` Joel Fernandes
2018-05-01 15:21 ` Paul E. McKenney
2018-05-01 15:23 ` Joel Fernandes
2018-05-01 15:42 ` Paul E. McKenney
2018-05-01 15:53 ` Joel Fernandes
2018-05-01 16:44 ` Joel Fernandes
2018-05-01 1:42 ` [PATCH RFC v5 6/6] tracing: Centralize preemptirq tracepoints and unify their usage Joel Fernandes
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