lkml.org 
[lkml]   [2014]   [Apr]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [RFC][PATCH 0/8] sched,idle: need resched polling rework
    On Fri, Apr 11, 2014 at 08:00:23AM -0700, Andy Lutomirski wrote:
    > That being said, I think that this addresses once one of the two major
    > issues. While the race you're fixing is more interesting, I think its
    > impact is dwarfed by the fact that ttwu_queue_remote completely
    > ignores polling. (NB: I haven't actually tested this patch set, but I
    > did try to instrument this stuff awhile ago.)
    >
    > To fix this, presumably the wake-from-idle path needs a
    > sched_ttwu_pending call, and ttwu_queue_remote could use resched_task.
    > sched_ttwu_pending could benefit from a straightforward optimization:
    > it doesn't need rq->lock if llist is empty.
    >
    > If you're not planning on trying to fix that, I can try to write up a
    > patch in the next day or two.

    Right; I forgot to write about that; I was going to look at both ttwu
    and arch_send_call_function_single_ipi() after this got sorted.

    While you didn't complain about the remote function call IPI, Venki
    (while @google) did and this was their reason to look at this.

    > Even with all of this fixed, what happens when ttwu_queue_remote is
    > called with a task that has lower priority than whatever is currently
    > running on the targeted cpu? I think the result is an IPI that serves
    > very little purpose other than avoiding taking a spinlock in the
    > waking thread. This may be a bad tradeoff. I doubt that this matters
    > for my particular workload, though.

    Today Mike also noted that on very high freq the IPI is actually a lot
    slower than doing the remote accesses for some weird reason --
    previously I've seen the remote wakeups queue a lot of wakeups and have
    the IPI take too long.

    So there's definitely something to prod at there.


    \
     
     \ /
      Last update: 2014-04-11 17:41    [W:4.925 / U:0.004 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site