linux-kernel - Re: [PATCH RFC tip/core/rcu] Make SRCU be once again optional

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Message-Id: <20170512184155.GA9482@linux.vnet.ibm.com>
Date:   Fri, 12 May 2017 11:41:55 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC tip/core/rcu] Make SRCU be once again optional

On Fri, Apr 28, 2017 at 05:10:40PM -0700, Paul E. McKenney wrote:
> On Fri, Apr 28, 2017 at 05:51:15PM -0400, Nicolas Pitre wrote:
> > On Fri, 28 Apr 2017, Paul E. McKenney wrote:
> > 
> > > Hello, Nicolas!
> > > 
> > > Saw the TTY write up LWN and figured I should send this your way.
> > > It should be worth about 2K compared to current -next, which gave
> > > up the 2K compared to v4.10.  So really getting things back to where
> > > they were.
> > > 
> > > My current plan is to push this into v4.13.
> > 
> > Excellent!
> > 
> > If every maintainer finds a way to (optionally) reduce the size of the 
> > code they maintain by 2K then we'll get a much smaller kernel pretty 
> > soon.
> 
> I would feel better if it wasn't me who had added the 2K, but then
> again, I do look forward to seeing a negative-sized kernel!  ;-)

And I am getting a lot of offlist pressure to remove both Tiny RCU and
Tiny SRCU.  I am pushing back, but I might or might not prevail.  In case
my pushback gets pushed back, do you have a -tiny tree or some such where
the code could go?

							Thanx, Paul

> > > ------------------------------------------------------------------------
> > > 
> > > commit e01ef0529ed548c1b30206058c2b5eecbbc07998
> > > Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > > Date:   Fri Apr 28 13:53:04 2017 -0700
> > > 
> > >     srcu: Make SRCU be once again optional
> > >     
> > >     Commit d160a727c40e ("srcu: Make SRCU be built by default") in response
> > >     to build errors, which were caused by code that included srcu.h
> > >     despite !SRCU.  However, srcutiny.o is almost 2K of code, which is not
> > >     insignificant for those attempting to run the Linux kernel on IoT devices.
> > >     This commit therefore makes SRCU be once again optional, and adjusts
> > >     srcu.h to allow error-free inclusion in !SRCU kernel builds.
> > >     
> > >     Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > >     Cc: Nicolas Pitre <nicolas.pitre@...aro.org>
> > 
> > Acked-by: Nicolas Pitre <nico@...aro.org>
> 
> Applied, thank you!
> 
> 							Thanx, Paul
> 
> > > diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> > > index 167ad8831aaf..c0143fe2e39d 100644
> > > --- a/include/linux/srcu.h
> > > +++ b/include/linux/srcu.h
> > > @@ -62,7 +62,7 @@ int init_srcu_struct(struct srcu_struct *sp);
> > >  #include <linux/srcutree.h>
> > >  #elif defined(CONFIG_CLASSIC_SRCU)
> > >  #include <linux/srcuclassic.h>
> > > -#else
> > > +#elif defined(CONFIG_SRCU)
> > >  #error "Unknown SRCU implementation specified to kernel configuration"
> > >  #endif
> > >  
> > > diff --git a/init/Kconfig b/init/Kconfig
> > > index 42a346b0df43..fe72c12e06a5 100644
> > > --- a/init/Kconfig
> > > +++ b/init/Kconfig
> > > @@ -521,7 +521,6 @@ config RCU_EXPERT
> > >  
> > >  config SRCU
> > >  	bool
> > > -	default y
> > >  	help
> > >  	  This option selects the sleepable version of RCU. This version
> > >  	  permits arbitrary sleeping or blocking within RCU read-side critical
> > > 
> > > 
> >