linux-kernel - Re: [PATCH rcu 5/6] rcu: Make the rcu_nocb_poll boot parameter usable via boot config

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Message-ID: <3586af97-e9bc-341f-4a81-678b5e1e96f8@joelfernandes.org>
Date:   Tue, 18 Jul 2023 09:46:24 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...a.com,
        rostedt@...dmis.org
Subject: Re: [PATCH rcu 5/6] rcu: Make the rcu_nocb_poll boot parameter usable
 via boot config

On 7/17/23 14:03, Paul E. McKenney wrote:
> The rcu_nocb_poll kernel boot parameter is defined via early_param(),
> whose parsing functions are invoked from parse_early_param() which
> is in turn invoked by setup_arch(), which is very early indeed.  It
> is invoked so early that the console output timestamps read 0.000000,
> in other words, before time begins.
> 
> This use of early_param() means that the rcu_nocb_poll kernel boot
> parameter cannot usefully be embedded into the kernel image.  Yes, you
> can embed it, but setup_boot_config() is invoked from start_kernel()
> too late for it to be parsed.
> 
> But it makes no sense to parse this parameter so early.  After all,
> it cannot do anything until the rcuog kthreads are created, which is
> long after rcu_init() time, let alone setup_boot_config() time. >
> This commit therefore switches the rcu_nocb_poll kernel boot parameter
> from early_param() to __setup(), which allows boot-config parsing of
> this parameter, in turn allowing it to be embedded into the kernel image.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> ---
>   kernel/rcu/tree_nocb.h | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 43229d2b0c44..5598212d1f27 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -77,9 +77,9 @@ __setup("rcu_nocbs", rcu_nocb_setup);
>   static int __init parse_rcu_nocb_poll(char *arg)
>   {
>   	rcu_nocb_poll = true;
> -	return 0;
> +	return 1;
>   }
> -early_param("rcu_nocb_poll", parse_rcu_nocb_poll);
> +__setup("rcu_nocb_poll", parse_rcu_nocb_poll);

I was trying to see if core_param() could be used. But I am not sure if the RCU 
offload threads are spawned too early for that.

I am Ok with it:
Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>

thanks,

  -Joel


>   /*
>    * Don't bother bypassing ->cblist if the call_rcu() rate is low.