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    SubjectRe: Regression in gdm-2.18 since 2.6.24
    On Sat, Apr 05, 2008 at 10:03:47PM +0100, Ken Moffat wrote:
    > On Sat, Apr 05, 2008 at 08:10:43PM +0530, Srivatsa Vaddagiri wrote:
    > >
    > > Given that you seem to be seeing the problem even without
    > > CONFIG_GROUP_SCHED, only the second hunk of the patch seems to be making
    > > a difference for your problem i.e just the hunk below applied on
    > > 2.6.25-rc8 (to kernel/sched_fair.c) should fix your problem too:
    > >
    > > @@ -1145,7 +1145,7 @@ static void check_preempt_wakeup(struct
    > > * More easily preempt - nice tasks, while not making
    > > * it harder for + nice tasks.
    > > */
    > > - if (unlikely(se->load.weight > NICE_0_LOAD))
    > > + if (unlikely(se->load.weight != NICE_0_LOAD))
    > > gran = calc_delta_fair(gran, &se->load);
    > >
    > > if (pse->vruntime + gran < se->vruntime)
    > >
    > > [The first hunk is a no-op under !CONFIG_GROUP_SCHED, since
    > > entity_is_task() is always 1 for !CONFIG_GROUP_SCHED]
    > >
    > > This second hunk changes how fast + or - niced tasks get preempted.
    > >
    > > 2.6.25-rc8 (Bad case):
    > > Sets preempt granularity for + niced tasks at 5ms (1 CPU)
    > >
    > > 2.6.25-rc8 + the hunk above (Good case):
    > > Sets preempt granularity for + niced tasks at >5ms
    > >
    > Well, I'm no longer sure exactly what was in the config, but after
    > I had confirmed the reversion would fix 2.6.24.4 I _did_ try just
    > the second part of the patch applied to 2.6.25-rc8 and it gave a 60%
    > success rate across 10 tests.
    > >
    > > So bumping up preempt granularity for + niced tasks seems to make things
    > > work for you. IMO the deeper problem lies somewhere else (perhaps is
    > > some race issue in gdm itself), which is easily exposed with 2.6.25-rc8
    > > which lets + niced tasks be preempted quickly.
    > >
    >
    > I agree this is probably exposing a problem somewhere else.
    >
    > > To help validate this, can you let us know the result of tuning preempt
    > > granularity on native 2.6.25-rc8 (without any patches applied and
    > > CONFIG_GROUP_SCHED disabled)?
    > >
    > > # echo 100000000 > /proc/sys/kernel/sched_wakeup_granularity_ns
    > >
    > > To check if echo command worked, do:
    > >
    > > # cat /proc/sys/kernel/sched_wakeup_granularity_ns
    > >
    > > It should return 100000000.
    > >
    > > Now try shutting down thr' gdm and pls let me know if it makes a
    > > difference.
    > >
    > > --
    > > Regards,
    > > vatsa
    >
    > Will do, but it might be a day or so before I can get to this.
    >
    > Thanks.
    >
    > Ken

    Well, I found your analysis convincing. Unfortunately, my hardware
    disagreed. Testing -rc8 with CONFIG_GROUP_SCHED disabled (a test is
    a mixture of 5 attempts to restart and 5 to shutdown):

    1. the base version success is 4/10

    2. increasing the granularity by a factor of 10 as you requested,
    success is 8/10

    3. applying the second part of the patch (and not altering the
    granularity) success is 3/10

    4. applying both parts of the patch (and not altering the
    granularity), success is 5/10.

    Clearly, 3/10 and 5/10 may not be meaningfully different on such a
    small sample size (but, 10 attempts is probably as much as my mind
    and blood-pressure can stand!). Whether 8/10 is meaningfully better
    I don't know, the point is that it still failed some of the time.

    At this point, I started to doubt my previous results, so I
    retested rc8 with CONFIG_GROUP_SCHED=y and both parts of the patch,
    and again success is 10/10. So, that combination has run through at
    least 20 shutdowns or restarts without a problem.

    Summary: if I apply the patch to revert both hunks, AND use
    CONFIG_GROUP_SCHED, everything is good. All other variations fail
    sooner or later within 10 tests (for the little it's worth, the
    longest string of successful runs between failures is 6, so a
    minimum of 10 tests is probably necessary before saying a version
    seems ok).

    If I was confused earlier, I guess I must be dazed and confused
    now!

    Ken
    --

    das eine Mal als Tragödie, das andere Mal als Farce
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