linux-kernel - Re: [RFC 0/3] Revert SRCU from tracepoint infrastructure

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Message-ID: <20200210180745.GA4947@paulmck-ThinkPad-P72>
Date:   Mon, 10 Feb 2020 10:07:45 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Ingo Molnar <mingo@...hat.com>,
        Richard Fontana <rfontana@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
Subject: Re: [RFC 0/3] Revert SRCU from tracepoint infrastructure

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@...radead.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