linux-kernel - Re: [RFC v2 4/5] rcu: Use for_each_leaf_node_cpu() in force_qs

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Message-ID: <20161215144242.GN9728@tardis.cn.ibm.com>
Date:   Thu, 15 Dec 2016 22:42:42 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-kernel@...r.kernel.org,
        "Paul E . McKenney " <paulmck@...ux.vnet.ibm.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Colin King <colin.king@...onical.com>
Subject: Re: [RFC v2 4/5] rcu: Use for_each_leaf_node_cpu() in force_qs_rnp()

On Thu, Dec 15, 2016 at 12:04:59PM +0000, Mark Rutland wrote:
> On Thu, Dec 15, 2016 at 10:42:03AM +0800, Boqun Feng wrote:
> > ->qsmask of an RCU leaf node is usually more sparse than the
> > corresponding cpu_possible_mask. So replace the
> > for_each_leaf_node_possible_cpu() in force_qs_rnp() with
> > for_each_leaf_node_cpu() to save several checks.
> > 
> > [Note we need to use "1UL << bit" instead of "1 << bit" to generate the
> > corresponding mask for a bit because @mask is unsigned long, this was
> > spotted by Colin Ian King <colin.king@...onical.com> and CoverityScan in
> > a previous version of this patch.]
> 
> Nit: This note can go now that we use leaf_node_cpu_bit(). ;)
> 

I kinda keep this here for honoring the effort of finding out this bug
from Colin, but yes, it's no longer needed here for the current code.

Regards,
Boqun

> Thanks,
> Mark.
> 
> > Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> > ---
> >  kernel/rcu/tree.c | 12 +++++-------
> >  1 file changed, 5 insertions(+), 7 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 4e5b81c843de..1ef13e63bc95 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3046,13 +3046,11 @@ static void force_qs_rnp(struct rcu_state *rsp,
> >  				continue;
> >  			}
> >  		}
> > -		for_each_leaf_node_possible_cpu(rnp, cpu) {
> > -			unsigned long bit = leaf_node_cpu_bit(rnp, cpu);
> > -			if ((rnp->qsmask & bit) != 0) {
> > -				if (f(per_cpu_ptr(rsp->rda, cpu), isidle, maxj))
> > -					mask |= bit;
> > -			}
> > -		}
> > +
> > +		for_each_leaf_node_cpu(rnp, rnp->qsmask, cpu)
> > +			if (f(per_cpu_ptr(rsp->rda, cpu), isidle, maxj))
> > +				mask |= leaf_node_cpu_bit(rnp, cpu);
> > +
> >  		if (mask != 0) {
> >  			/* Idle/offline CPUs, report (releases rnp->lock. */
> >  			rcu_report_qs_rnp(mask, rsp, rnp, rnp->gpnum, flags);
> > -- 
> > 2.10.2
> > 

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