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

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Message-Id: <20180302010048.GM3777@linux.vnet.ibm.com>
Date:   Thu, 1 Mar 2018 17:00:48 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Byungchul Park <byungchul.park@....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 Fri, Mar 02, 2018 at 09:01:19AM +0900, Byungchul Park wrote:
> 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? :)

I guess that sentiment goes well with my "why didn't I avoid that bug
in the first place?"  ;-)

> >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>

Applied, thank you!

							Thanx, Paul

> >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
>