linux-kernel - Re: [PATCH 3/3] rcu: Call trace_rcu_batch

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Message-ID: <20150211161330.GH4166@linux.vnet.ibm.com>
Date:	Wed, 11 Feb 2015 08:13:30 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Alexander Gordeev <agordeev@...hat.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] rcu: Call trace_rcu_batch_start() with enabled
 interrupts

On Wed, Feb 11, 2015 at 03:42:39PM +0100, Alexander Gordeev wrote:
> Currently trace_rcu_batch_start() is called with local
> interrupts disabled. Yet, there is no reason to do so.
> 
> Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> Signed-off-by: Alexander Gordeev <agordeev@...hat.com>

Hmmm...  I am not seeing this one.  As you noted in the commit log for
your earlier patch, the purpose of Tiny RCU is to be tiny, not to be
all that fast.  This commit increases the size a bit (admittedly only
when CONFIG_RCU_TRACE=y), and also increases complexity a bit.

So it does not look to me to be something we want for Tiny RCU.

So what am I missing here?

							Thanx, Paul

> ---
>  kernel/rcu/tiny.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
> index 069742d..01e80ac 100644
> --- a/kernel/rcu/tiny.c
> +++ b/kernel/rcu/tiny.c
> @@ -166,11 +166,12 @@ static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp)
>  	const char *rn = NULL;
>  	struct rcu_head *next, *list;
>  	unsigned long flags;
> +	RCU_TRACE(long qlen);
>  	RCU_TRACE(int cb_count = 0);
> 
>  	/* Move the ready-to-invoke callbacks to a local list. */
>  	local_irq_save(flags);
> -	RCU_TRACE(trace_rcu_batch_start(rcp->name, 0, rcp->qlen, -1));
> +	RCU_TRACE(qlen = rcp->qlen);
>  	list = rcp->rcucblist;
>  	rcp->rcucblist = *rcp->donetail;
>  	*rcp->donetail = NULL;
> @@ -180,6 +181,7 @@ static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp)
>  	local_irq_restore(flags);
> 
>  	/* Invoke the callbacks on the local list. */
> +	RCU_TRACE(trace_rcu_batch_start(rcp->name, 0, qlen, -1));
>  	RCU_TRACE(rn = rcp->name);
>  	while (list) {
>  		next = list->next;
> -- 
> 1.8.3.1
> 

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