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

    Thanx, Paul


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