linux-kernel - Re: [PATCH RFC] rcu/nocb: Provide default all-CPUs mask for RCU_NOCB

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Date:   Fri, 8 Apr 2022 23:46:15 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rcu <rcu@...r.kernel.org>, Steven Rostedt <rostedt@...dmis.org>,
        frederic@...nel.org
Subject: Re: [PATCH RFC] rcu/nocb: Provide default all-CPUs mask for
 RCU_NOCB_CPU=y

> 
> 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.

--
Uladzislau Rezki