Re: [PATCH] rcu: Provide a boot time parameter to enable lazy RCU

[Paul E. McKenney]

On Tue, Nov 21, 2023 at 10:15:56PM +0000, Qais Yousef wrote:
> On 11/22/23 15:26, Paul E. McKenney wrote:
> 
> > > Either way; I'll follow what the crowd wants too :-)
> > 
> > Usually a wise choice.  ;-)
> > 
> > But I must defer to the people using it.
> 
> >From Android PoV I'd like to be able to boot with default disabled and allow
> people to opt-in. At least for the time being until we have confidence no one
> is caught with surprise if this caused unexpected problems.
> 
> In the future it might default to on once it gets wider usage and testing.
> 
> So having a new Kconfig to DEFAULT_OFF sounds good to me to enable a compile
> time switch to pick the default with a boot time to further control.
> 
> Which would be my plan for v2 unless I hear another suggestion in the coming
> week (where I hope people would have had a chance to look and think about it).

Sounds good to me!  Silence will be interpreted as assent.  ;-)

							Thanx, Paul

> > > No need for the rcu_barrier() then? Only reason why we use the _cb flavour
> > 
> > The module_param() parameters are processed during early boot, before
> > the boot CPU has enabled interrupts.  In fact, before rcu_init()
> > is invoked, which is in turn long before the scheduler has started.
> > Calling rcu_barrier() that early is not so good for your kernel's
> > actuarial statistics.
> 
> Ah, I missed that it is done that early.
> 
> > 
> > I am guessing that the module_param_cb() processing happens somewhat
> > later in the kernel's lifetime.
> 
> 
> Thanks!
> 
> --
> Qais Yousef