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From: "Paul E. McKenney" <paulmck@kernel.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	"Joel Fernandes, Google" <joel@joelfernandes.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Gustavo A. R. Silva" <gustavo@embeddedor.com>,
	Ingo Molnar <mingo@redhat.com>,
	Richard Fontana <rfontana@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Josh Triplett <josh@joshtriplett.org>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
Subject: Re: [RFC 0/3] Revert SRCU from tracepoint infrastructure
Date: Mon, 10 Feb 2020 10:07:45 -0800	[thread overview]
Message-ID: <20200210180745.GA4947@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200210120552.1a06a7aa@gandalf.local.home>

On Mon, Feb 10, 2020 at 12:05:52PM -0500, Steven Rostedt wrote:
> On Mon, 10 Feb 2020 10:46:16 +0100
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > Furthermore, using srcu would be detrimental, because of how it has
> > smp_mb() in the read side primitives.
> 
> I didn't realize that there was a full memory barrier in the srcu read
> side. Seems to me that itself is rational for reverting it. And also a
> big NAK for any suggestion to have any of the function tracing to use
> it as well (which comes up here and there).

Yeah, that was added some years back when people were complaining about
three synchronize_sched()'s worth of latency for synchronize_srcu().

I did prepare a patch about a year ago that would allow an srcu_struct
to be set up to not need the read-side smp_mb() calls, but this means
longer-latency grace periods (though nowhere near as long as those of
synchronize_rcu_tasks()) and also that the "fast SRCU" readers cannot
be used from idle or offline CPUs.

But an easy change if it is useful.

							Thanx, Paul

  parent reply	other threads:[~2020-02-10 18:07 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-07 20:56 [RFC 0/3] Revert SRCU from tracepoint infrastructure Joel Fernandes (Google)
2020-02-07 20:56 ` [RFC 1/3] Revert "tracepoint: Use __idx instead of idx in DO_TRACE macro to make it unique" Joel Fernandes (Google)
2020-02-07 21:07   ` Steven Rostedt
2020-02-07 20:56 ` [RFC 2/3] Revert "tracing: Add back in rcu_irq_enter/exit_irqson() for rcuidle tracepoints" Joel Fernandes (Google)
2020-02-07 20:56 ` [RFC 3/3] Revert "tracepoint: Make rcuidle tracepoint callers use SRCU" Joel Fernandes (Google)
2020-02-07 21:24 ` [RFC 0/3] Revert SRCU from tracepoint infrastructure Paul E. McKenney
2020-02-07 21:43   ` Joel Fernandes
2020-02-08 16:39     ` Mathieu Desnoyers
2020-02-08 16:31 ` Mathieu Desnoyers
2020-02-10  9:46   ` Peter Zijlstra
2020-02-10 10:19     ` Peter Zijlstra
2020-02-10 13:36     ` Paul E. McKenney
2020-02-10 13:44       ` Peter Zijlstra
2020-02-10 13:57         ` Paul E. McKenney
2020-02-10 17:17       ` Joel Fernandes
2020-02-10 17:05     ` Steven Rostedt
2020-02-10 17:33       ` Mathieu Desnoyers
2020-02-10 18:30         ` Steven Rostedt
2020-02-10 19:05           ` Mathieu Desnoyers
2020-02-10 19:53           ` Joel Fernandes
2020-02-10 20:03             ` Steven Rostedt
2020-02-10 20:30               ` Joel Fernandes
2020-02-10 18:07       ` Paul E. McKenney [this message]
2020-02-10 16:59   ` Joel Fernandes

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