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Date: Sun, 21 Dec 2014 09:46:17 -0800 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Miroslav Benes <mbenes@...e.cz> Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: rcu_sched stall detected, but no state dump On Fri, Dec 19, 2014 at 07:32:20AM -0800, Paul E. McKenney wrote: > On Fri, Dec 19, 2014 at 12:09:48PM +0100, Miroslav Benes wrote: > > On Mon, 15 Dec 2014, Miroslav Benes wrote: [ . . . ] > > But I'm not sure whether it is possible that the list of pending callback > > is empty. Maybe the bug is elsewhere. My understanding of RCU is limited > > but I'll try to look around. Also the commit log might be imprecise :/ > > You found the problem, good show! However, we do need a slightly > different fix, please see below. And, as I should have said the first time, please update your patch as mentioned below so that I can add it to the -rcu tree. Thanx, Paul > > Best regards, > > Miroslav > > > > -- >8 -- > > >From be3495519513356daa366517bb458bc4f05ecf9b Mon Sep 17 00:00:00 2001 > > From: Miroslav Benes <mbenes@...e.cz> > > Date: Fri, 19 Dec 2014 11:11:04 +0100 > > Subject: [PATCH] rcu: fix rcu stall detection in tiny implementation > > The first word following a ":" needs to be capitalized. > > > The current tiny RCU stall detection depends on presence of pending callbacks in > > the list (*rcp->curtail et al.). But they are none even in case of real CPU > > stalls prohibiting the detection. When the check for callback presence is > > removed, INFO about the stall is correctly printed. > > As you say, this needs to be reworked, but first please see below. > > > Signed-off-by: Miroslav Benes <mbenes@...e.cz> > > --- > > kernel/rcu/tiny_plugin.h | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/kernel/rcu/tiny_plugin.h b/kernel/rcu/tiny_plugin.h > > index 858c565..3f743fa 100644 > > --- a/kernel/rcu/tiny_plugin.h > > +++ b/kernel/rcu/tiny_plugin.h > > @@ -145,7 +145,7 @@ static void check_cpu_stall(struct rcu_ctrlblk *rcp) > > rcp->ticks_this_gp++; > > j = jiffies; > > js = ACCESS_ONCE(rcp->jiffies_stall); > > - if (*rcp->curtail && ULONG_CMP_GE(j, js)) { > > + if (ULONG_CMP_GE(j, js)) { > > The problem is that ->curtail is a tail pointer, and thus always points > to a NULL pointer, so the expression is always false. However, we don't > want to give RCU CPU stall warnings if there are no callbacks, because > this can result in spurious stall warnings after long idle periods. > > So we need to check the header pointer as follows: > > + if (rcp->rcucblist && ULONG_CMP_GE(j, js)) { > > But what do you do if your test module happens to start spinning when > there is no callback? > > One approach is to do a call_rcu() just after starting your loop. > Alternatively, do several just before starting your loop. > > Thanx, Paul > > > pr_err("INFO: %s stall on CPU (%lu ticks this GP) idle=%llx (t=%lu jiffies q=%ld)\n", > > rcp->name, rcp->ticks_this_gp, rcu_dynticks_nesting, > > jiffies - rcp->gp_start, rcp->qlen); > > -- > > 2.1.2 > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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