linux-kernel - Re: [PATCH tip/core/rcu 1/5] rcu: Fix soft lockup for rcu_nocb

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Message-Id: <20160822164429.GE3482@linux.vnet.ibm.com>
Date:   Mon, 22 Aug 2016 09:44:29 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Nikolay Borisov <n.borisov.lkml@...il.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
        bobby.prani@...il.com, Ding Tianhong <dingtianhong@...wei.com>
Subject: Re: [PATCH tip/core/rcu 1/5] rcu: Fix soft lockup for
 rcu_nocb_kthread

On Mon, Aug 22, 2016 at 07:19:53PM +0300, Nikolay Borisov wrote:
> 
> [SNIP]
> > 
> > Signed-off-by: Ding Tianhong <dingtianhong@...wei.com>
> > [ paulmck: Substituted cond_resched_rcu_qs for cond_resched. ]
> 
> This contradicts...
> 
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > ---
> >  kernel/rcu/tree_plugin.h | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 0082fce402a0..85c5a883c6e3 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -2173,6 +2173,7 @@ static int rcu_nocb_kthread(void *arg)
> >  				cl++;
> >  			c++;
> >  			local_bh_enable();
> > +			cond_resched_rcu_qs();
> 
> with what's here?

Ding Tianhong's original patch:

http://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1167918.html

had cond_resched() here, which works for his workload, but can result
in stall warnings in other cases.  I therfore changed his cond_resched()
to the cond_resched_rcu_qs() that you see above, and documented this
change in the "paulmck" note after Ding Tianhong's Signed-off-by.

So all is as it should be.

							Thanx, Paul

> >  			list = next;
> >  		}
> >  		trace_rcu_batch_end(rdp->rsp->name, c, !!list, 0, 0, 1);
> > 
>