linux-kernel - Re: [PATCH] rcu: Clean up rcu_init_nohz() by removing unnecessary statements

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Date:   Fri, 2 Mar 2018 09:01:19 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     paulmck@...ux.vnet.ibm.com
Cc:     jiangshanlai@...il.com, josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, linux-kernel@...r.kernel.org,
        kernel-team@....com
Subject: Re: [PATCH] rcu: Clean up rcu_init_nohz() by removing unnecessary
 statements

On 3/1/2018 3:41 AM, Paul E. McKenney wrote:
> On Wed, Feb 28, 2018 at 06:04:55PM +0900, Byungchul Park wrote:
>> Since the commit 44c65ff2e3b0(rcu: Eliminate NOCBs CPU-state Kconfig
>> options) made nocb-cpus identified only through the rcu_nocbs= boot
>> parameter, we don't have to care NOCBs CPU-state Kconfig options
>> anymore, which means now we can just rely on rcu_nocb_mask to
>> decide whether going ahead in rcu_init_nohz().
>>
>> Remove the deprecated code.
>>
>> Signed-off-by: Byungchul Park <byungchul.park@....com>
> 
> Good catch!  However, you missed a (relatively harmless) bug in my commit
> 44c65ff2e3b0, namely that if neither the nohz_full= nor the rcu_nocbs=
> kernel boot parameters specify any CPUs, we don't need to allocate
> rcu_nocb_mask.  (That is, when both of those parameters are omitted.)
> 
> Now, if the rcu_nocbs= kernel boot parameter was specified, we know that
> rcu_nocb_mask was already allocated in rcu_nocb_setup().  So in
> rcu_init_nohz() we only need to do the allocation if NO_HZ_FULL needs
> some NOCBs CPUs, that is, when tick_nohz_full_running and when there
> is at least one CPU specified in tick_nohz_full_mask.

Why didn't I catch it in advance? :)

> So the change that is actually needed is to reverse the initialization
> of need_rcu_nocb_mask, that is, to initialize it to false rather than
> to true.  I annotated your patch with my reasoning and have a patch
> below with your Reported-by.  Please take a look and let me know what
> you think.

No doubt. I agree with you.

Acked-by: Byungchul Park <byungchul.park@....com>

> If I am not too confused, the only effect of this bug was to needlessly
> allocate rcu_nocb_mask and to initialize it to all zeros bits, but please
> let me know if I missed something.

I think so as you.

-- 
Thanks,
Byungchul