Re: Question about usage of RCU in the input layer

[Eric Dumazet <dada1@cosmosbay.com>]

Paul E. McKenney a écrit :
> On Thu, Mar 19, 2009 at 08:20:32PM -0700, Arjan van de Ven wrote:
>> On Thu, 19 Mar 2009 19:07:50 -0700
>> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
>>
>>> On Thu, Mar 19, 2009 at 07:18:41AM -0700, Arjan van de Ven wrote:
>>>> On Thu, 19 Mar 2009 14:26:28 +0530
>>>> Dipankar Sarma <dipankar@in.ibm.com> wrote:
>>>>
>>>>> On Wed, Mar 18, 2009 at 09:58:12PM -0700, Arjan van de Ven wrote:
>>>>>> Hi,
>>>>>>
>>>>>> the input layer does a "synchronize_rcu()" after a
>>>>>> list_add_tail_rcu(), which is costing me 1 second of boot
>>>>>> time..... And based on my understanding of the RCU concept, you
>>>>>> only need to synchronize on delete, not on addition... so I
>>>>>> think the synchronize is entirely redundant here...
>>>>> The more appropriate question is - why is synchronize_rcu() taking
>>>>> 1 second ? Any idea what the other CPUs are doing at the time
>>>>> of calling synchronize_rcu() ?
>>>> one cpu is doing a lot of i2c traffic which is a bunch of udelay()s
>>>> in loops.. then it does quite a bit of uncached memory access, and
>>>> the lot takes quite while.
>>>>
>>>>> What driver is this ? How early
>>>>> in the boot is this happening ? 
>>>> during kernel boot.
>>>>
>>>> I suppose my question is also more generic.. why synchronize when
>>>> it's not needed? At least based on my understanding of RCU (but
>>>> you're the expert), you don't need to synchronize for an add, only
>>>> between a delete and a (k)free.....
>>> I don't claim to understand the code in question, so it is entirely
>>> possible that the following is irrelevant.  But one other reason for
>>> synchronize_rcu() is:
>>>
>>> 1.	Make change.
>>>
>>> 2.	synchronize_rcu()
>>>
>>> 3.	Now you are guaranteed that all CPUs/tasks/whatever
>>> currently running either are not messing with you on the one hand, or
>>> 	have seen the change on the other.
>> ok so this is for the case where someone is already iterating the list.
>>
>> I don't see anything in the code that assumes this..
> 
> I must let the networking guys sort this out.
> 
>>> It sounds like you are seeing these delays later in boot, however,
>> yeah it's during driver init/
>>
>>> Alternatively, again assuming a single-CPU system
>> single CPU is soooo last decade ;-)
>> But seriously I no longer have systems that aren't dual core or SMT in
>> some form... 
> 
> OK, I will ask the stupid question...
> 
> Why not delay bringing up the non-boot CPUs until later in boot?
> The first patch in my earlier email (which is in mainline) will shortcut
> synchronize_rcu() whenever there is only one CPU is online, at least
> for Classic RCU and Hierarchical RCU.
> 

Hmm... point is to make linux boot as fast as possible, so ...

Use a special variant of udelay() in offending drivers that make appropriate
RCU call to increment quiescent state ?