From mboxrd@z Thu Jan 1 00:00:00 1970 From: Joel Fernandes Subject: Re: [PATCH v4 16/27] tracing: Remove regular RCU context for _rcuidle tracepoints (again) Date: Fri, 6 Mar 2020 12:21:40 -0500 Message-ID: <20200306172140.GA237112@google.com> References: <20200221133416.777099322@infradead.org> <20200221134216.051596115@infradead.org> <20200306104335.GF3348@worktop.programming.kicks-ass.net> <20200306113135.GA8787@worktop.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from mail-qt1-f195.google.com ([209.85.160.195]:42003 "EHLO mail-qt1-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726231AbgCFRVm (ORCPT ); Fri, 6 Mar 2020 12:21:42 -0500 Received: by mail-qt1-f195.google.com with SMTP id r6so2240897qtt.9 for ; Fri, 06 Mar 2020 09:21:42 -0800 (PST) Content-Disposition: inline In-Reply-To: Sender: linux-arch-owner@vger.kernel.org List-ID: To: Alexei Starovoitov Cc: Peter Zijlstra , LKML , linux-arch , Steven Rostedt , Ingo Molnar , Greg Kroah-Hartman , "Gustavo A. R. Silva" , Thomas Gleixner , "Paul E. McKenney" , Josh Triplett , Mathieu Desnoyers , jiangshanlai@gmail.com, Andy Lutomirski , Tony Luck , Frederic Weisbecker , Dan Carpenter , Masami Hiramatsu On Fri, Mar 06, 2020 at 07:51:18AM -0800, Alexei Starovoitov wrote: > On Fri, Mar 6, 2020 at 3:31 AM Peter Zijlstra wrote: > > > > On Fri, Mar 06, 2020 at 11:43:35AM +0100, Peter Zijlstra wrote: > > > On Fri, Feb 21, 2020 at 02:34:32PM +0100, Peter Zijlstra wrote: > > > > Effectively revert commit 865e63b04e9b2 ("tracing: Add back in > > > > rcu_irq_enter/exit_irqson() for rcuidle tracepoints") now that we've > > > > taught perf how to deal with not having an RCU context provided. > > > > > > > > Signed-off-by: Peter Zijlstra (Intel) > > > > Reviewed-by: Steven Rostedt (VMware) > > > > --- > > > > include/linux/tracepoint.h | 8 ++------ > > > > 1 file changed, 2 insertions(+), 6 deletions(-) > > > > > > > > --- a/include/linux/tracepoint.h > > > > +++ b/include/linux/tracepoint.h > > > > @@ -179,10 +179,8 @@ static inline struct tracepoint *tracepo > > > > * For rcuidle callers, use srcu since sched-rcu \ > > > > * doesn't work from the idle path. \ > > > > */ \ > > > > - if (rcuidle) { \ > > > > + if (rcuidle) \ > > > > __idx = srcu_read_lock_notrace(&tracepoint_srcu);\ > > > > - rcu_irq_enter_irqsave(); \ > > > > - } \ > > > > \ > > > > it_func_ptr = rcu_dereference_raw((tp)->funcs); \ > > > > \ > > > > @@ -194,10 +192,8 @@ static inline struct tracepoint *tracepo > > > > } while ((++it_func_ptr)->func); \ > > > > } \ > > > > \ > > > > - if (rcuidle) { \ > > > > - rcu_irq_exit_irqsave(); \ > > > > + if (rcuidle) \ > > > > srcu_read_unlock_notrace(&tracepoint_srcu, __idx);\ > > > > - } \ > > > > \ > > > > preempt_enable_notrace(); \ > > > > } while (0) > > > > > > So what happens when BPF registers for these tracepoints? BPF very much > > > wants RCU on AFAIU. > > > > I suspect we needs something like this... > > > > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c > > index a2f15222f205..67a39dbce0ce 100644 > > --- a/kernel/trace/bpf_trace.c > > +++ b/kernel/trace/bpf_trace.c > > @@ -1475,11 +1475,13 @@ void bpf_put_raw_tracepoint(struct bpf_raw_event_map *btp) > > static __always_inline > > void __bpf_trace_run(struct bpf_prog *prog, u64 *args) > > { > > + int rcu_flags = trace_rcu_enter(); > > rcu_read_lock(); > > preempt_disable(); > > (void) BPF_PROG_RUN(prog, args); > > preempt_enable(); > > rcu_read_unlock(); > > + trace_rcu_exit(rcu_flags); > > One big NACK. > I will not slowdown 99% of cases because of one dumb user. > Absolutely no way. For the 99% usecases, they incur an additional atomic_read and a branch, with the above. Is that the concern? Just want to make sure we are talking about same thing. Speaking of slowdowns, you don't really need that rcu_read_lock/unlock() pair in __bpf_trace_run() AFAICS. The rcu_read_unlock() can run into the rcu_read_unlock_special() slowpath and if not, at least has branches. Most importantly, RCU is consolidated which means preempt_disable() implies rcu_read_lock(). thanks, - Joel