LKML Archive on lore.kernel.org
 help / color / Atom feed
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 09:49:27 -0700
Message-ID: <20200310164927.GD2935@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <379743142.23419.1583853207158.JavaMail.zimbra@efficios.com>

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?

>    1.2) Preemption disabled
> 
>    Tree-RCU works fine.
> 
>    1.3) IRQs disabled
> 
>    Tree-RCU works fine.
> 
> 2) IRQ handler context
> 
>    Tree-RCU works fine.
> 
> 3) NMI context
> 
>    Tree-RCU works fine.
> 
> 4) cpuidle context (!rcu_is_watching())
> 
>    - By all means, we should not have tracepoints requiring to temporarily enable
>      RCU in frequent code-paths. It appears that we should be able to remove the few
>      offenders we currently have (e.g. enter from usermode),
>    - For tracepoints which are infrequently called from !rcu_is_watching context, checking
>      whether RCU is watching and only enabling when needed should be fast enough.
> 
> Are there other use-cases am I missing that would justify adding another flavor of RCU ?

BPF programs that might sometimes sleep, but are usually lightweight.

I will be double-checking this, of course.

							Thanx, Paul

  reply index

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-09 17:02 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 [this message]
2020-03-10 17:22       ` Mathieu Desnoyers
2020-03-10 17:26         ` Paul E. McKenney

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200310164927.GD2935@paulmck-ThinkPad-P72 \
    --to=paulmck@kernel.org \
    --cc=ast@kernel.org \
    --cc=frederic@kernel.org \
    --cc=joel@joelfernandes.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mathieu.desnoyers@efficios.com \
    --cc=mhiramat@kernel.org \
    --cc=peterz@infradead.org \
    --cc=rostedt@goodmis.org \
    --cc=tglx@linutronix.de \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/lkml/0 lkml/git/0.git
	git clone --mirror https://lore.kernel.org/lkml/1 lkml/git/1.git
	git clone --mirror https://lore.kernel.org/lkml/2 lkml/git/2.git
	git clone --mirror https://lore.kernel.org/lkml/3 lkml/git/3.git
	git clone --mirror https://lore.kernel.org/lkml/4 lkml/git/4.git
	git clone --mirror https://lore.kernel.org/lkml/5 lkml/git/5.git
	git clone --mirror https://lore.kernel.org/lkml/6 lkml/git/6.git
	git clone --mirror https://lore.kernel.org/lkml/7 lkml/git/7.git
	git clone --mirror https://lore.kernel.org/lkml/8 lkml/git/8.git
	git clone --mirror https://lore.kernel.org/lkml/9 lkml/git/9.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ https://lore.kernel.org/lkml \
		linux-kernel@vger.kernel.org
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kernel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git