linux-kernel - Re: [RFC 0/5] rcu: Introduce leaf_node_for_each_mask_possible

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Date:   Fri, 9 Dec 2016 15:49:45 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [RFC 0/5] rcu: Introduce leaf_node_for_each_mask_possible_cpu()
 and its friend

On Fri, Dec 09, 2016 at 04:48:22PM +0800, Boqun Feng wrote:
> Hi Paul,
> 
> While reading the discussion at:
> 
> https://marc.info/?l=linux-kernel&m=148044253400769

This discussion was for stalls specifically, rather than for routine
scans of the bitmasks.

But it does look to save some code, so worth looking into.

> I figured we might use this fact to save some extra checks in RCU core code,
> currently we iterate over all the possible CPUs on a leaf node, check whether
> they were masked in a certain mask and do something. However, given the fact
> that the masks on a leaf node should always be sparse than the corresponding
> part of cpu_possible_mask, we'd better iterate over all bits in a mask and
> check whether the corresponding CPU is possible or not.
> 
> So I made this RFC, I did a simple build/boot/rcutorture test on my box with
> SMP=4, nothing bad happens. Currently I'm waiting for the 0day and trying to
> test this one a bigger system, in the meanwhile, looking forwards to any
> comment and suggestion.
> 
> So thoughts?

By analogy with for_each_cpu() and for_each_possible_cpu(), the name
should instead be for_each_leaf_node_cpu(), the tradition of excessively
long names in RCU notwithstanding.  ;-)

							Thanx, Paul