lkml.org 
[lkml]   [2005]   [Nov]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [RT] read_tsc: ACK! TSC went backward! Unsynced TSCs?
    On Mon, Nov 28, 2005 at 07:05:54AM -0500, Steven Rostedt wrote:
    > With -rt20 on the AMD64 x2, I'm getting a crap load of these:
    >
    > read_tsc: ACK! TSC went backward! Unsynced TSCs?
    >
    > So bad that the system wont even boot (at least I won't wait long enough
    > to let it finish).

    The kernel's use of TSC is wholly incorrect. TSCs can ramp up and down
    and *do* vary between nodes as well as between cores within a node. You
    really can not compare TSCs between cpu cores at all, as is (and the
    kernel assumes 1 global TSC in at least a few places).

    If you have any sort of power-management enabled on a k8 (including 'hlt'
    C1 state), you *will* get hosed.

    We got into a situation where 1 CPU had somehow lagged behind the other
    because it was idle for a while. Suddenly gettimeofday() was only giving
    me HZ granularity. Successive reads would get the exact same timeval, as
    much as 1 ms later.

    What happened was the last_tsc was set to the higher-TSC CPU. The
    gettimeofday code for TSC was running on the lower-TSC CPU. The code
    recognized that current tsc < last tsc and set current = last. As long as
    I was running on the laggy CPU, time stood still for bursts. Then if I
    bounced CPUs it would shoot forward.

    Switching to HPET for timing made it all go away, but (at least as of
    2.6.11) it was horribly broken.

    Tim
    -
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

    \
     
     \ /
      Last update: 2005-11-28 18:28    [W:0.036 / U:90.884 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site