From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: paulmck <paulmck@kernel.org>
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 11:13:27 -0400 (EDT) [thread overview]
Message-ID: <379743142.23419.1583853207158.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <20200309204710.GU2935@paulmck-ThinkPad-P72>
----- 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 ?
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.
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 ?
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
next prev parent reply other threads:[~2020-03-10 15:13 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 [this message]
2020-03-10 16:49 ` Paul E. McKenney
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=379743142.23419.1583853207158.JavaMail.zimbra@efficios.com \
--to=mathieu.desnoyers@efficios.com \
--cc=ast@kernel.org \
--cc=frederic@kernel.org \
--cc=joel@joelfernandes.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mhiramat@kernel.org \
--cc=paulmck@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
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).