linux-kernel - Re: Thoughts on this RCU idle entry/exit patch?

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Message-ID: <20131008203427.GE8392@localhost.localdomain>
Date:	Tue, 8 Oct 2013 22:34:28 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Thoughts on this RCU idle entry/exit patch?

On Mon, Oct 07, 2013 at 08:39:55AM -0700, Paul E. McKenney wrote:
> Hello, Frederic!
> 
> The following patch seems to me to be a good idea to better handle
> task nesting.  Any reason why it would be a bad thing?
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> rcu: Allow task-level idle entry/exit nesting
> 
> The current task-level idle entry/exit code forces an entry/exit on
> each call, regardless of the nesting level.  This commit therefore
> properly accounts for nesting.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

Looks good. In fact, the current code is even buggy because two nesting rcu_user_eqs()
as in:

        rcu_eqs_enter()
            rcu_eqs_enter()
            rcu_eqs_exit()
        rcu_eqs_exit()

would result in rdtp->dynticks wrong increment, right?

So that's even a bug fix. I wonder if it's a regression. That said rcu_eqs_enter_common()
should warn on such miscount, so may be these functions actually don't nest in practice
or you would have received such warnings.

So I wonder, do we want to continue to allow this nesting? I remember that DYNTICK_TASK_NEST_*
stuff is there to protects against non finishing interrupts on some archs (I also remember that
this, or at least a practical scenario for this, was hard to really define though :o)
But then wouldn't it involve other kind of scenario like this?

       rcu_irq_enter()
           rcu_eqs_enter()
           rcu_eqs_exit()
           ...

Anyway, that's just random thougths on further simplifications, in any case, this
patch looks good.

Thanks.

> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 106f7f5cdd1d..f0be20886617 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -411,11 +411,12 @@ static void rcu_eqs_enter(bool user)
>  	rdtp = this_cpu_ptr(&rcu_dynticks);
>  	oldval = rdtp->dynticks_nesting;
>  	WARN_ON_ONCE((oldval & DYNTICK_TASK_NEST_MASK) == 0);
> -	if ((oldval & DYNTICK_TASK_NEST_MASK) == DYNTICK_TASK_NEST_VALUE)
> +	if ((oldval & DYNTICK_TASK_NEST_MASK) == DYNTICK_TASK_NEST_VALUE) {
>  		rdtp->dynticks_nesting = 0;
> -	else
> +		rcu_eqs_enter_common(rdtp, oldval, user);
> +	} else {
>  		rdtp->dynticks_nesting -= DYNTICK_TASK_NEST_VALUE;
> -	rcu_eqs_enter_common(rdtp, oldval, user);
> +	}
>  }
>  
>  /**
> @@ -533,11 +534,12 @@ static void rcu_eqs_exit(bool user)
>  	rdtp = this_cpu_ptr(&rcu_dynticks);
>  	oldval = rdtp->dynticks_nesting;
>  	WARN_ON_ONCE(oldval < 0);
> -	if (oldval & DYNTICK_TASK_NEST_MASK)
> +	if (oldval & DYNTICK_TASK_NEST_MASK) {
>  		rdtp->dynticks_nesting += DYNTICK_TASK_NEST_VALUE;
> -	else
> +	} else {
>  		rdtp->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
> -	rcu_eqs_exit_common(rdtp, oldval, user);
> +		rcu_eqs_exit_common(rdtp, oldval, user);
> +	}
>  }
>  
>  /**
> 
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