[lkml]   [2012]   [Mar]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
    SubjectRe: [ANNOUNCE] 3.2.9-rt17
    On Thu, 2012-03-08 at 16:44 -0500, Steven Rostedt wrote:
    > On Thu, 2012-03-08 at 22:37 +0100, Peter Zijlstra wrote:
    > > > Now when the original task releases the lock again, the other task can
    > > > take it just like it does on mainline.
    > >
    > > Now interleave it with a third task of even higher priority that puts
    > > the spinner to sleep.
    > So? It will eventually have to allow the task to run. Adding a "third
    > higher priority" task can cause problems in any other part of the -rt
    > kernel.
    > We don't need to worry about priority inversion. If the higher task
    > blocks on the original task, it will boost its priority (even if it does
    > the adaptive spin) which will again boost the task that it preempted.
    > Now we may need to add a sched_yield() in the adaptive spin to let the
    > other task run.

    That's not what I mean,..

    task-A (cpu0) task-B (cpu1) task-C (cpu1)

    lock ->d_lock
    lock ->i_lock
    lock ->d_lock
    <-------------- preempts B
    trylock ->i_lock

    While is is perfectly normal, the result is that A stops spinning and
    goes to sleep. Now B continues and loops ad infinitum because it keeps
    getting ->d_lock before A because its cache hot on cpu1 and waking A
    takes a while etc..

    No progress guarantee -> fail.

    Test-and-set spinlocks have unbounded latency and we've hit pure
    starvation cases in mainline. In fact it was so bad mainline had to grow
    ticket locks to cope -- we don't want to rely on anything like this in

     \ /
      Last update: 2012-03-08 22:57    [W:0.046 / U:9.828 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site