lkml.org 
[lkml]   [2012]   [Feb]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH RFC tip/core/rcu 05/41] rcu: Avoid waking up CPUs having only kfree_rcu() callbacks
    On Wed, Feb 01, 2012 at 05:15:52PM -0800, Josh Triplett wrote:
    > On Wed, Feb 01, 2012 at 11:41:23AM -0800, Paul E. McKenney wrote:
    > > From: "Paul E. McKenney" <paul.mckenney@linaro.org>
    > >
    > > When CONFIG_RCU_FAST_NO_HZ is enabled, RCU will allow a given CPU to
    > > enter dyntick-idle mode even if it still has RCU callbacks queued.
    > > RCU avoids system hangs in this case by scheduling a timer for several
    > > jiffies in the future. However, if all of the callbacks on that CPU
    > > are from kfree_rcu(), there is no reason to wake the CPU up, as it is
    > > not a problem to defer freeing of memory.
    > >
    > > This commit therefore tracks the number of callbacks on a given CPU
    > > that are from kfree_rcu(), and avoids scheduling the timer if all of
    > > a given CPU's callbacks are from kfree_rcu().
    >
    > Minor nit: I think it would make much more sense to track the number of
    > "strict" callbacks *not* from kfree_rcu, and check for that number != 0,
    > rather than tracking the number of "lazy" callbacks from kfree_rcu and
    > checking for all != lazy. You can always compute one number from the
    > other, but since you only ever need to know the strict count, not the
    > lazy count, why not directly track the thing you care about?

    I will think about this. My guess is that when I add call_rcu_lazy(),
    the naming will outweigh the slowpath subtraction, but will see.

    > Also, any way this could hide the new kfree_call_rcu internally rather
    > than adding it as a new export? In particular, why introduce a new
    > exported API only suitable for internal use or foot-shooting?

    It is called from __kfree_rcu, which is a static inline in
    include/linux/rcupdate.h, so needs to be exported so that modules can
    use kfree_rcu().

    Of course, if I take the earlier patch that removes the first
    BUILD_BUG_ON() from __kfree_rcu(), then I could move __kfree_rcu()
    to rcupdate.c, and then I could move the export from kfree_call_rcu()
    to __kfree_rcu(). But either way I would be exporting a function that
    should not be invoked directly.

    My concern with the earlier patch is that someone might have something
    silly like a structure with an array with an rcu_head in each element.
    Calling kfree_rcu() on such a beast would be ill-advised. This might
    just be me being overly paranoid, though, hence my lack of additional
    response to that patch.

    Thanx, Paul



    \
     
     \ /
      Last update: 2012-02-02 17:43    [W:3.429 / U:0.028 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site