From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753113AbeEGVpR (ORCPT ); Mon, 7 May 2018 17:45:17 -0400 Received: from mail-pf0-f194.google.com ([209.85.192.194]:41152 "EHLO mail-pf0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753051AbeEGVpQ (ORCPT ); Mon, 7 May 2018 17:45:16 -0400 X-Google-Smtp-Source: AB8JxZpPqTAAQnlB6hIZFnaBaBd32P6yOpuMRqil4MCr6bS3FvCp3kDF7VaeAfrX9F0dbgWP099L9A== Date: Mon, 7 May 2018 14:45:14 -0700 From: Joel Fernandes To: "Paul E. McKenney" Cc: Joel Fernandes , linux-kernel@vger.kernel.org, Steven Rostedt , Peter Zilstra , Ingo Molnar , Mathieu Desnoyers , Tom Zanussi , Namhyung Kim , Thomas Glexiner , Boqun Feng , Frederic Weisbecker , Randy Dunlap , Masami Hiramatsu , Fenguang Wu , Baohong Liu , Vedang Patel , kernel-team@android.com Subject: Re: [PATCH RFC v6 4/5] tracepoint: Make rcuidle tracepoint callers use SRCU Message-ID: <20180507214514.GA13787@joelaf.mtv.corp.google.com> References: <20180507204143.13061-1-joelaf@google.com> <20180507204143.13061-5-joelaf@google.com> <20180507210801.GZ26088@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180507210801.GZ26088@linux.vnet.ibm.com> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 07, 2018 at 02:08:01PM -0700, Paul E. McKenney wrote: > On Mon, May 07, 2018 at 01:41:42PM -0700, Joel Fernandes wrote: > > From: "Joel Fernandes (Google)" > > > > 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. [...] >> Signed-off-by: Joel Fernandes (Google) > > --- > > 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 > > +#include > > #include > > #include > > #include > > @@ -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? The in_nmi() in the lock path should take care of making sure its balanced. The diff the way its formatted appears confusing as Mathieu pointed. thanks, - Joel