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

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Date:   Mon, 22 Aug 2016 19:47:14 +0300
From:   Nikolay Borisov <n.borisov.lkml@...il.com>
To:     paulmck@...ux.vnet.ibm.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 22.08.2016 19:44, Paul E. McKenney wrote:
> 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.

I think my english escaped me since I took your paulmck note as "we had
cond_resched_rcu_qs initially and I replaced it with cond_resched". But
apparently it was the opposite.

Cheers,
Nik

> 
> So all is as it should be.
> 
> 							Thanx, Paul
> 
>>>  			list = next;
>>>  		}
>>>  		trace_rcu_batch_end(rdp->rsp->name, c, !!list, 0, 0, 1);
>>>
>>
>