lkml.org 
[lkml]   [2013]   [Apr]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [tip:sched/core] sched: Lower chances of cputime scaling overflow
    From
    Date
    On Thu, 2013-04-11 at 08:38 -0700, Linus Torvalds wrote:
    > So *now*, once we are in the uncommon case, let's start counting bits.
    > Like this:
    >
    > /* We know one of the values has a bit set in the high 32 bits */
    > for (;;) {
    > /* Make sure "stime" is the bigger of stime/rtime */
    > if (rtime > stime) {
    > u64 tmp = stime; stime = rtime; rtime = tmp;
    > }
    >
    > /* Do we need to balance stime/rtime bits? */
    > if (stime >> 32) {
    > if (rtime >> 31)
    > goto drop_precision;
    >
    > /* We can grow rtime and shrink stime and try to make them
    > both fit */
    > rtime <<= 1;
    > stime >>= 1;
    > continue;
    > }
    >
    > /* stime/rtime fits in 32 bits, how about total? */
    > if (!(total >> 32))
    > break;
    >
    > drop_precision:
    > /* We drop from stime, it has more bits than rtime */
    > stime >>= 1;
    > total >>= 1;
    > }
    >
    > The above is totally untested, but each step is pretty damn simple and
    > fairly cheap. Sure, it's a loop, but it's bounded to 32 (cheap)
    > iterations, and the normal case is that it's not done at all, or done
    > only a few times.

    Right it gets gradually heavier the bigger the numbers get; which is
    more and more unlikely.

    > And the advantage is that the end result is always that simple
    > 32x32/32 case that we started out with as the common case.
    >
    > I dunno. Maybe I'm overlooking something, and the above is horrible,
    > but the above seems reasonably efficient if not optimal, and
    > *understandable*.

    I suppose that entirely matters on what one is used to ;-) I had to
    stare rather hard at it for a little while.

    But yes, you take it one step further and are willing to ditch rtime
    bits too and I suppose that's fine.

    Should work,.. Stanislaw could you stick this into your userspace
    thingy and verify the numbers are sane enough?




    \
     
     \ /
      Last update: 2013-04-12 10:21    [W:5.650 / U:0.004 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site