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

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Message-ID: <05a9953b-aaa4-6117-b120-85c12ad56ace@canonical.com>
Date:   Thu, 15 Dec 2016 14:51:36 +0000
From:   Colin Ian King <colin.king@...onical.com>
To:     Boqun Feng <boqun.feng@...il.com>,
        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>
Subject: Re: [RFC v2 4/5] rcu: Use for_each_leaf_node_cpu() in force_qs_rnp()

On 15/12/16 14:42, Boqun Feng wrote:
> 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.

Yep, remove it.

> 
> 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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