linux-kernel - Re: [RFC] rcu: Prevent expedite reporting within RCU read-side section

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Date:   Fri, 9 Mar 2018 17:53:32 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Boqun Feng <boqun.feng@...il.com>, jiangshanlai@...il.com,
        josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, linux-kernel@...r.kernel.org,
        kernel-team@....com
Subject: Re: [RFC] rcu: Prevent expedite reporting within RCU read-side
 section

On 3/9/2018 5:41 PM, Byungchul Park wrote:
> On Thu, Mar 08, 2018 at 10:01:56AM -0800, Paul E. McKenney wrote:
>> On Thu, Mar 08, 2018 at 07:08:25PM +0900, Byungchul Park wrote:
> 
> [...]
> 
>>>     2. Clear its bit of ->expmask *only* when it's out of RCU read
>>>        sections and keep others unchanged. So it will be cleared at the
>>>        end of the RCU read section in that case.
>>>
>>>        This option would also work because we anyway check both
>>>        ->exp_tasks and ->expmask to finish the expedite-gp.
>>
>> This could be made to work, but one shortcoming is that the grace
>> period would end up waiting on later read-side critical sections
>> that it does not really need to wait on.  Also, eventually all the
> 
> I don't think it waits on any later ones since ->expmask would be
> cleared at the end of the previous RCU read section.

Now having simplified my patch, with the simplified version, I can
see what you were saying. Right, the expedite-gp might be extended
as you said. That optimization cannot be achieved this way without
reggression.

You're awesome. Thank you for explaning the reason why not.

-- 
Thanks,
Byungchul