From: "Paul E. McKenney" <paulmck@kernel.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
linux-kernel <linux-kernel@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
rostedt <rostedt@goodmis.org>,
Masami Hiramatsu <mhiramat@kernel.org>,
Alexei Starovoitov <ast@kernel.org>,
"Joel Fernandes, Google" <joel@joelfernandes.org>,
Frederic Weisbecker <frederic@kernel.org>
Subject: Re: Instrumentation and RCU
Date: Tue, 10 Mar 2020 10:26:42 -0700 [thread overview]
Message-ID: <20200310172642.GG2935@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <1800881789.23773.1583860965967.JavaMail.zimbra@efficios.com>
On Tue, Mar 10, 2020 at 01:22:45PM -0400, Mathieu Desnoyers wrote:
> ----- On Mar 10, 2020, at 12:49 PM, paulmck paulmck@kernel.org wrote:
>
> > On Tue, Mar 10, 2020 at 11:13:27AM -0400, Mathieu Desnoyers wrote:
> >>
> >>
> >> ----- On Mar 9, 2020, at 4:47 PM, paulmck paulmck@kernel.org wrote:
> >> [...]
> >>
> >> >
> >> > Suppose that we had a variant of RCU that had about the same read-side
> >> > overhead as Preempt-RCU, but which could be used from idle as well as
> >> > from CPUs in the process of coming online or going offline? I have not
> >> > thought through the irq/NMI/exception entry/exit cases, but I don't see
> >> > why that would be problem.
> >> >
> >> > This would have explicit critical-section entry/exit code, so it would
> >> > not be any help for trampolines.
> >> >
> >> > Would such a variant of RCU help?
> >> >
> >> > Yeah, I know. Just what the kernel doesn't need, yet another variant
> >> > of RCU...
> >>
> >> Hi Paul,
> >>
> >> I think that before introducing yet another RCU flavor, it's important
> >> to take a step back and look at the tracer requirements first. If those
> >> end up being covered by currently available RCU flavors, then why add
> >> another ?
> >
> > Well, we have BPF requirements as well.
> >
> >> I can start with a few use-cases I have in mind. Others should feel free
> >> to pitch in:
> >>
> >> Tracing callsite context:
> >>
> >> 1) Thread context
> >>
> >> 1.1) Preemption enabled
> >>
> >> One tracepoint in this category is syscall enter/exit. We should introduce
> >> a variant of tracepoints relying on SRCU for this use-case so we can take
> >> page faults when fetching userspace data.
> >
> > Agreed, SRCU works fine for the page-fault case, as the read-side memory
> > barriers are in the noise compared to page-fault overhead. Back in
> > the day, there were light-weight system calls. Are all of these now
> > converted to VDSO or similar?
>
> There is a big difference between allowing page faults to happen, and expecting
> page faults to happen every time. I suspect many use-cases will end up having
> a fast-path which touches user-space data which is in the page cache, but
> may end up triggering page faults in rare occasions.
>
> Therefore, this might justify an SRCU which has low-overhead read-side.
OK, good to know, thank you!
Thanx, Paul
prev parent reply other threads:[~2020-03-10 17:26 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-09 17:02 Instrumentation and RCU Thomas Gleixner
2020-03-09 18:15 ` Steven Rostedt
2020-03-09 18:42 ` Joel Fernandes
2020-03-09 19:07 ` Steven Rostedt
2020-03-09 19:20 ` Mathieu Desnoyers
2020-03-16 15:02 ` Joel Fernandes
2020-03-09 18:59 ` Thomas Gleixner
2020-03-10 8:09 ` Masami Hiramatsu
2020-03-10 11:43 ` Thomas Gleixner
2020-03-10 15:31 ` Mathieu Desnoyers
2020-03-10 15:46 ` Steven Rostedt
2020-03-10 16:21 ` Mathieu Desnoyers
2020-03-11 0:18 ` Masami Hiramatsu
2020-03-11 0:37 ` Mathieu Desnoyers
2020-03-11 7:48 ` Masami Hiramatsu
2020-03-10 16:06 ` Masami Hiramatsu
2020-03-12 13:53 ` Peter Zijlstra
2020-03-10 15:24 ` Mathieu Desnoyers
2020-03-10 17:05 ` Daniel Thompson
2020-03-09 18:37 ` Mathieu Desnoyers
2020-03-09 18:44 ` Steven Rostedt
2020-03-09 18:52 ` Mathieu Desnoyers
2020-03-09 19:09 ` Steven Rostedt
2020-03-09 19:25 ` Mathieu Desnoyers
2020-03-09 19:52 ` Thomas Gleixner
2020-03-10 15:03 ` Mathieu Desnoyers
2020-03-10 16:48 ` Thomas Gleixner
2020-03-10 17:40 ` Mathieu Desnoyers
2020-03-10 18:31 ` Thomas Gleixner
2020-03-10 18:37 ` Mathieu Desnoyers
2020-03-10 1:40 ` Alexei Starovoitov
2020-03-10 8:02 ` Thomas Gleixner
2020-03-10 16:54 ` Paul E. McKenney
2020-03-17 17:56 ` Joel Fernandes
2020-03-09 20:18 ` Peter Zijlstra
2020-03-09 20:47 ` Paul E. McKenney
2020-03-09 20:58 ` Steven Rostedt
2020-03-09 21:25 ` Paul E. McKenney
2020-03-09 23:52 ` Frederic Weisbecker
2020-03-10 2:26 ` Paul E. McKenney
2020-03-10 15:13 ` Mathieu Desnoyers
2020-03-10 16:49 ` Paul E. McKenney
2020-03-10 17:22 ` Mathieu Desnoyers
2020-03-10 17:26 ` Paul E. McKenney [this message]
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