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 <[email protected]> > > > >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 <[email protected]> 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 >
