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    SubjectRe: [PATCH tip/core/rcu 01/40] rcu: Maintain special bits at bottom of ->dynticks counter
    On Thu, Apr 13, 2017 at 11:20:44AM +0200, Peter Zijlstra wrote:
    > On Wed, Apr 12, 2017 at 10:39:46AM -0700, Paul E. McKenney wrote:
    > > Currently, IPIs are used to force other CPUs to invalidate their TLBs
    > > in response to a kernel virtual-memory mapping change. This works, but
    > > degrades both battery lifetime (for idle CPUs) and real-time response
    > > (for nohz_full CPUs), and in addition results in unnecessary IPIs due to
    > > the fact that CPUs executing in usermode are unaffected by stale kernel
    > > mappings. It would be better to cause a CPU executing in usermode to
    > > wait until it is entering kernel mode to do the flush, first to avoid
    > > interrupting usemode tasks and second to handle multiple flush requests
    > > with a single flush in the case of a long-running user task.
    > >
    > > This commit therefore reserves a bit at the bottom of the ->dynticks
    > > counter, which is checked upon exit from extended quiescent states.
    > > If it is set, it is cleared and then a new rcu_eqs_special_exit() macro is
    > > invoked, which, if not supplied, is an empty single-pass do-while loop.
    > > If this bottom bit is set on -entry- to an extended quiescent state,
    > > then a WARN_ON_ONCE() triggers.
    > >
    > > This bottom bit may be set using a new rcu_eqs_special_set() function,
    > > which returns true if the bit was set, or false if the CPU turned
    > > out to not be in an extended quiescent state. Please note that this
    > > function refuses to set the bit for a non-nohz_full CPU when that CPU
    > > is executing in usermode because usermode execution is tracked by RCU
    > > as a dyntick-idle extended quiescent state only for nohz_full CPUs.
    > >
    > > Reported-by: Andy Lutomirski <luto@amacapital.net>
    >
    > Isn't that more a: Requested-by ?

    I am not too worried about the distinction. Request a feature, report
    the lack of a needed feature, or report a bug, but either way I had to
    write the code. ;-)

    Thanx, Paul

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