linux-kernel - Re: Energy-efficiency options within RCU

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Message-ID: <20201210192358.GA2365@pc638.lan>
Date:   Thu, 10 Dec 2020 20:23:58 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     joel@...lfernandes.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: Energy-efficiency options within RCU

Hello, Paul.

[Dropping CC]

> Hello, Joel,
> 
> In case you are -seriously- interested...  ;-)
> 
> 						Thanx, Paul
> 
> rcu_nocbs=
> 
> 	Adding a CPU to this list offloads RCU callback invocation from
> 	that CPU's softirq handler to a kthread.  In big.LITTLE systems,
> 	this kthread can be placed on a LITTLE CPU, which has been
> 	demonstrated to save significant energy in benchmarks.
> 	http://www.rdrop.com/users/paulmck/realtime/paper/AMPenergy.2013.04.19a.pdf
> 
I have checked our config. We do use rcu_nocbs=0-7 as kthreads but what
i see those threads are not bound to 0-3 CPUs. In our case it is little
cluster. I think i should check and run some test cases regarding power 
savings if i pin all threads to little cluster.

> rcutree.rcu_idle_gp_delay=  (Only CONFIG_RCU_FAST_NO_HZ=y kernels.)
> 
> 	This defaults to four jiffies on the theory that grace periods
> 	tend to last about that long.  If grace periods tend to take
> 	longer, then it makes a lot of sense to increase this.	And maybe
> 	battery-powered devices would rather have it be about 2x or 3x
> 	the expected grace-period duration, who knows?
> 
> 	I would keep it to a power of two, but the code should work with
> 	other numbers.  Except that I don't know that this has ever been
> 	tested.  ;-)
> 
Same here. We do use it.

--
Vlad Rezki