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 13:22:45 -0400 (EDT)
Message-ID: <1800881789.23773.1583860965967.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <20200310164927.GD2935@paulmck-ThinkPad-P72>
----- 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.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
next prev parent 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
2020-03-10 17:22 ` Mathieu Desnoyers [this message]
2020-03-10 17:26 ` Paul E. McKenney
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