linux-kernel - Re: [PATCH] rcu: restore correct batch limiting

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Date:	Thu, 18 Oct 2012 04:58:59 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: restore correct batch limiting

On Wed, Oct 17, 2012 at 09:14:12AM +0200, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> 
> Commit 29c00b4a1d9e27 (rcu: Add event-tracing for RCU callback
> invocation) added a regression in rcu_do_batch()
> 
> Under stress, RCU is supposed to allow to process all items in queue,
> instead of a batch of 10 items (blimit), but an integer overflow makes
> the effective limit being 1.
> 
> So RCU cannot recover and machine eventually crash because of OOM.
> 
> Using long instead of int is not really needed, convert everything
> to integers.

<facepalm>

Good catch!!!

The reason for favoring long over int is that there are a few systems out
there with terabytes of main memory.  In addition, there have been a few
bugs over the past few years that could result in RCU CPU stalls of more
than a minute.  This makes it impossible to rule out the possibility of
a billion callbacks appearing on one CPU.

So, does the following patch fix things, or a I still confused?

							Thanx, Paul

------------------------------------------------------------------------

rcu: Fix batch-limit size problem

Commit 29c00b4a1d9e27 (rcu: Add event-tracing for RCU callback
invocation) added a regression in rcu_do_batch()

Under stress, RCU is supposed to allow to process all items in queue,
instead of a batch of 10 items (blimit), but an integer overflow makes
the effective limit being 1.  So, unless there is frequent idle periods
(during which RCU ignores batch limits), RCU can be forced into a
state where it cannot keep up with the callback-generation rate,
eventually resulting in OOM.

This commit therefore converts a few variables in rcu_do_batch() from
int to long to fix this problem.

Reported-by: Eric Dumazet <edumazet@...gle.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index e36d085..e056e1e 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1823,7 +1823,8 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
 {
 	unsigned long flags;
 	struct rcu_head *next, *list, **tail;
-	int bl, count, count_lazy, i;
+	long bl, count, count_lazy;
+	int i;

 	/* If no callbacks are ready, just return.*/
 	if (!cpu_has_callbacks_ready_to_invoke(rdp)) {

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