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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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