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Date:   Sat, 28 Oct 2017 09:19:52 +0530
From:   Neeraj Upadhyay <neeraju@...eaurora.org>
To:     paulmck@...ux.vnet.ibm.com
Cc:     josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Query regarding srcu_funnel_exp_start()

On 10/28/2017 03:50 AM, Paul E. McKenney wrote:
> On Fri, Oct 27, 2017 at 10:15:04PM +0530, Neeraj Upadhyay wrote:
>> On 10/27/2017 05:56 PM, Paul E. McKenney wrote:
>>> On Fri, Oct 27, 2017 at 02:23:07PM +0530, Neeraj Upadhyay wrote:
>>>> Hi,
>>>>
>>>> One query regarding srcu_funnel_exp_start() function in
>>>> kernel/rcu/srcutree.c.
>>>>
>>>> static void srcu_funnel_exp_start(struct srcu_struct *sp, struct
>>>> srcu_node *snp,
>>>> 				  unsigned long s)
>>>> {
>>>> 	<snip>
>>>> 	if (!ULONG_CMP_LT(sp->srcu_gp_seq_needed_exp, s))
>>>> 		sp->srcu_gp_seq_needed_exp = s;
>>>> 	<snip>
>>>> }
>>>>
>>>> Why is sp->srcu_gp_seq_needed_exp set to 's' if srcu_gp_seq_needed_exp is >=
>>>> 's'. Shouldn't srcu_gp_seq_needed_exp be equal to the greater of both?
>>>
>>> Let's suppose that it is incorrect as currently written.  Can you
>>> construct a test case demonstrating a failure of some sort, then provide
>>> a fix?
>>
>> Will check this. Might take some time to build a test case.
> 
> Fair enough!
> 
> I suggest checking to see if kernel/rcu/rcuperf.c can do what you need for
> this test.  (Might not with a single test, but perhaps a before-and-after
> comparison.  Or maybe you really do need to add some test code somewhere.)
> 

Thanks for the suggestion, will try that out.

>>> To start with, if it is currently incorrect, what would be the nature
>>> of the failure?
>>>
>>> 							Thanx, Paul
>>>
>>
>> Hi Paul,
>>
>> I see below scenario, where new gp won't be expedited. Please correct
>> me if I am missing something here.
>>
>> 1. CPU0 calls synchronize_srcu_expedited()
>>
>> synchronize_srcu_expedited()
>>    __synchronize_srcu()
>>      __call_srcu()
>>              s = rcu_seq_snap(&sp->srcu_gp_seq); // lets say
>> srcu_gp_seq  = 0;
>>                                                  // s = 0x100
> 
> Looks like you have one hex digit and then two binary digits, but why not?
> (RCU_SEQ_STATE_MASK is 3 rather than 0xff >

Yeah, sorry I confused myself while representing the values. 0x100 need 
to be replaced with b'100' and 0x200 with b'1000'.

>>              sdp->srcu_gp_seq_needed = s // 0x100
>>              needgp = true
>>              sdp->srcu_gp_seq_needed_exp = s // 0x100
>>          srcu_funnel_gp_start()
>>                  sp->srcu_gp_seq_needed_exp = s;
>>              srcu_gp_start(sp);
>>                  rcu_seq_start(&sp->srcu_gp_seq);
>>
>> 2. CPU1 calls normal synchronize_srcu()
>>
>> synchronize_srcu()
>>      __synchronize_srcu(sp, true)
>>          __call_srcu()
>>                  s = rcu_seq_snap(&sp->srcu_gp_seq); // srcu_gp_seq = 1
>>                                                      // s= 0x200
>>                  sdp->srcu_gp_seq_needed = s; // 0x200
>>              srcu_funnel_gp_start()
>>                  smp_store_release(&sp->srcu_gp_seq_needed, s); // 0x200
>>
>> 3. CPU3 calls synchronize_srcu_expedited()
>>
>> synchronize_srcu_expedited()
>>    __synchronize_srcu()
>>      __call_srcu()
>>              s = rcu_seq_snap(&sp->srcu_gp_seq); // srcu_gp_seq = 1
>>                                                  // s = 0x200
>>              sdp->srcu_gp_seq_needed_exp = s // 0x200
>>          srcu_funnel_exp_start(sp, sdp->mynode, s);
>>              // sp->srcu_gp_seq_needed_exp = 0x100
>>              // s = 0x200 ; sp->srcu_gp_seq_needed_exp is not updated
>>              if (!ULONG_CMP_LT(sp->srcu_gp_seq_needed_exp, s))
>>                  sp->srcu_gp_seq_needed_exp = s;
> 
> Seems plausible, but you should be able to show the difference in
> grace-period duration with a test.
> 

Ok sure, will attempt that.

> While you are in srcu_funnel_exp_start(), should it be rechecking
> rcu_seq_done(&sp->srcu_gp_seq, s) as well as the current
> ULONG_CMP_GE(snp->srcu_gp_seq_needed_exp, s) under the lock?
> Why or why not?
> 
> 							Thanx, Paul
> 

Hi Paul,

I don't see how it will impact. I have put markers in code snippet
below to explain my points. My understanding is

* rcu_seq_done check @a is a fastpath return, and avoid contention
for snp lock, if the gp has already elapsed.

* Checking it @b, inside srcu_node  lock might not make any
difference, as sp->srcu_gp_seq counter portion is updated
under srcu_struct lock. Also, we cannot lock srcu_struct at this
point, as it will cause lock contention among multiple CPUs.

* Checking rcu_seq_done @c also does not impact, as we have already
done all the work of traversing the entire parent chain and if
rcu_seq_done() is true srcu_gp_seq_needed_exp will be greater
than or equal to 's'.

   srcu_gp_end()
     raw_spin_lock_irq_rcu_node(sp);
     rcu_seq_end(&sp->srcu_gp_seq);
     gpseq = rcu_seq_current(&sp->srcu_gp_seq);
     if (ULONG_CMP_LT(sp->srcu_gp_seq_needed_exp, gpseq))
         sp->srcu_gp_seq_needed_exp = gpseq;
     raw_spin_unlock_irq_rcu_node(sp);

static void srcu_funnel_exp_start(...)
{
     <snip>

     for (; snp != NULL; snp = snp->srcu_parent) {
         if (rcu_seq_done(&sp->srcu_gp_seq, s) ||  /* a */
             ULONG_CMP_GE(READ_ONCE(snp->srcu_gp_seq_needed_exp), s))
             return;
         raw_spin_lock_irqsave_rcu_node(snp, flags);
         /* b */
         if (ULONG_CMP_GE(snp->srcu_gp_seq_needed_exp, s)) {
             raw_spin_unlock_irqrestore_rcu_node(snp, flags);
             return;
         }
         <snip>
         raw_spin_unlock_irqrestore_rcu_node(snp, flags);
     }
     raw_spin_lock_irqsave_rcu_node(sp, flags);
     /* c */
     if (!ULONG_CMP_LT(sp->srcu_gp_seq_needed_exp, s))
         sp->srcu_gp_seq_needed_exp = s;
     raw_spin_unlock_irqrestore_rcu_node(sp, flags);
}

Thanks
Neeraj

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