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

[Byungchul Park]

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@xxxxxxx>
>
> 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@xxxxxxx>

> 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