linux-kernel - Re: [PATCH v5 04/18] rcu: Fix late wakeup when flush of bypass cblist happens

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Message-ID: <c96c36f2-c324-39f7-1d66-7634eb26d909@joelfernandes.org>
Date:   Sat, 3 Sep 2022 10:05:24 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     paulmck@...nel.org
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        rushikesh.s.kadam@...el.com, urezki@...il.com,
        neeraj.iitr10@...il.com, frederic@...nel.org, rostedt@...dmis.org,
        vineeth@...byteword.org, boqun.feng@...il.com
Subject: Re: [PATCH v5 04/18] rcu: Fix late wakeup when flush of bypass cblist
 happens



On 9/3/2022 10:04 AM, Paul E. McKenney wrote:
> On Thu, Sep 01, 2022 at 10:17:06PM +0000, Joel Fernandes (Google) wrote:
>> When the bypass cblist gets too big or its timeout has occurred, it is
>> flushed into the main cblist. However, the bypass timer is still running
>> and the behavior is that it would eventually expire and wake the GP
>> thread.
>>
>> Since we are going to use the bypass cblist for lazy CBs, do the wakeup
>> soon as the flush happens. Otherwise, the lazy-timer will go off much
>> later and the now-non-lazy cblist CBs can get stranded for the duration
>> of the timer.
>>
>> This is a good thing to do anyway (regardless of this series), since it
>> makes the behavior consistent with behavior of other code paths where queueing
>> something into the ->cblist makes the GP kthread in a non-sleeping state
>> quickly.
>>
>> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
>> ---
>>  kernel/rcu/tree_nocb.h | 8 +++++++-
>>  1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
>> index 0a5f0ef41484..31068dd31315 100644
>> --- a/kernel/rcu/tree_nocb.h
>> +++ b/kernel/rcu/tree_nocb.h
>> @@ -447,7 +447,13 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
>>  			rcu_advance_cbs_nowake(rdp->mynode, rdp);
>>  			rdp->nocb_gp_adv_time = j;
>>  		}
>> -		rcu_nocb_unlock_irqrestore(rdp, flags);
>> +
>> +		// The flush succeeded and we moved CBs into the ->cblist.
>> +		// However, the bypass timer might still be running. Wakeup the
>> +		// GP thread by calling a helper with was_all_done set so that
>> +		// wake up happens (needed if main CB list was empty before).
>> +		__call_rcu_nocb_wake(rdp, true, flags)
> 
> TREE01 and TREE04 gripe about the missing ";".  I added it.

Sorry about that, I must have messed lost the semicolon during re-basing.

 - Joel