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    SubjectRe: Question about usage of RCU in the input layer
    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 ?



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