Re: [PATCH tip/core/rcu 0/4] Programmatic nestable expedited grace periods

From: Mathieu Desnoyers
Date: Sat Feb 21 2015 - 10:12:38 EST

----- Original Message -----
> From: "Josh Triplett" <josh@xxxxxxxxxxxxxxxx>
> To: "Peter Zijlstra" <peterz@xxxxxxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, mingo@xxxxxxxxxx,
> laijs@xxxxxxxxxxxxxx, dipankar@xxxxxxxxxx, akpm@xxxxxxxxxxxxxxxxxxxx, "mathieu desnoyers"
> <mathieu.desnoyers@xxxxxxxxxxxx>, tglx@xxxxxxxxxxxxx, rostedt@xxxxxxxxxxx, dhowells@xxxxxxxxxx, edumazet@xxxxxxxxxx,
> dvhart@xxxxxxxxxxxxxxx, fweisbec@xxxxxxxxx, oleg@xxxxxxxxxx, "bobby prani" <bobby.prani@xxxxxxxxx>
> Sent: Saturday, February 21, 2015 1:04:28 AM
> Subject: Re: [PATCH tip/core/rcu 0/4] Programmatic nestable expedited grace periods
> On Fri, Feb 20, 2015 at 05:54:09PM +0100, Peter Zijlstra wrote:
> > On Fri, Feb 20, 2015 at 08:37:37AM -0800, Paul E. McKenney wrote:
> > > On Fri, Feb 20, 2015 at 10:11:07AM +0100, Peter Zijlstra wrote:
> > > > Does it really make a machine boot much faster? Why are people using
> > > > synchronous gp primitives if they care about speed? Should we not fix
> > > > that instead?
> > >
> > > The report I heard was that it provided 10-15% faster boot times.
> >
> > That's not insignificant; got more details? I think we should really
> > look at why people are using the sync primitives.
> Paul, what do you think about adding a compile-time debug option to
> synchronize_rcu() that causes it to capture the time on entry and exit
> and print the duration together with the file:line of the caller?
> Similar to initcall_debug, but for blocking calls to synchronize_rcu().
> Put that together with initcall_debug, and you'd have a pretty good idea
> of where that holds up boot.
> We do want early boot to run as asynchronously as possible, and to avoid
> having later bits of boot waiting on a synchronize_rcu from earlier bits
> of boot. Switching a caller over to call_rcu() doesn't actually help if
> it still has to finish a grace period before it can allow later bits to
> run. Ideally, we ought to be able to work out the "depth" of boot in
> grace-periods.
> Has anyone wired initcall_debug up to a bootchart-like graph?

The information about begin/end of synchronize_rcu, as well as begin/end
of rcu_barrier() seems to be very relevant here. This should perhaps be
covered tracepoints ? Isn't it already ?



> - Josh Triplett

Mathieu Desnoyers
EfficiOS Inc.
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