Re: [PATCH RFC] rcu/nocb: Provide default all-CPUs mask for RCU_NOCB_CPU=y

[Uladzislau Rezki]

> On Fri, Apr 08, 2022 at 11:46:15PM +0200, Uladzislau Rezki wrote:
> > > 
> > > Here is what I believe you are proposing:
> > > 
> > > 
> > > 				---	rcu_nocbs	rcu_nocbs=???
> > > 
> > > CONFIG_RCU_NOCB_CPU_ALL=n	[1]	[2]		[3]
> > > 
> > > CONFIG_RCU_NOCB_CPU_ALL=y	[4]	[4]		[3]
> > > 
> > > 
> > > [1]	No CPUs are offloaded at boot.	CPUs cannot be offloaded at
> > > 	runtime.
> > > 
> > > [2]	No CPUs are offloaded at boot, but any CPU can be offloaded
> > > 	(and later de-offloaded) at runtime.
> > > 
> > > [3]	The set of CPUs that are offloaded at boot are specified by the
> > > 	mask, represented above with "???".  The CPUs that are offloaded
> > > 	at boot can be de-offloaded and offloaded at runtime.  The CPUs
> > > 	not offloaded at boot cannot be offloaded at runtime.
> > > 
> > > [4]	All CPUs are offloaded at boot, and any CPU can be de-offloaded
> > > 	and offloaded at runtime.  This is the same behavior that
> > > 	you would currently get with CONFIG_RCU_NOCB_CPU_ALL=n and
> > > 	rcu_nocbs=0-N.
> > > 
> > > 
> > > I am adding Frederic on CC, who will not be shy about correcting any
> > > confusion I be suffering from have with respect to the current code.
> > > 
> > > Either way, if this is not what you had in mind, what are you suggesting
> > > instead?
> > > 
> > > I believe that Steve Rostedt's review would carry weight for ChromeOS,
> > > however, I am suffering a senior moment on the right person for Android.
> > > 
> > We(in Sony) mark all CPUs as offloaded ones because of power reasons. The
> > energy aware scheduler has a better knowledge where to place an rcuop/x
> > task to invoke callbacks. The decision is taken based on many reason and
> > the most important is to drain less power as a result of task placement.
> > For example, power table, if OPP becomes higher or not, CPU is idle, etc.
> > 
> > What Joel does in this patch sounds natural to me at least from the first
> > glance. I mean converting the RCU_NOCB_CPU=y to make all CPUs to do offloading.
> 
> Just to be very clear, given appropriate acks/reviews, adding something
> like CONFIG_RCU_NOCB_CPU_ALL to get default rcu_nocbs=0-N is fine.
> However, Joel's original patch would not be good for the enterprise
> distros, which rely on the current default.
> 
Absolutely. It would be even easier in terms of changing the current concept
of RCU_NOCB_CPU config. Having an extra CONFIG_RCU_NOCB_CPU_ALL would simplify 
and get rid of a need of modifying the "rcu_nocbs=" boot parameter.

--
Uladzislau Rezki