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    Subject[ 023/102] sched: Avoid prev->stime underflow
    Date
    3.9-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Stanislaw Gruszka <sgruszka@redhat.com>

    commit 68aa8efcd1ab961e4684ef5af32f72a6ec1911de upstream.

    Dave Hansen reported strange utime/stime values on his system:
    https://lkml.org/lkml/2013/4/4/435

    This happens because prev->stime value is bigger than rtime
    value. Root of the problem are non-monotonic rtime values (i.e.
    current rtime is smaller than previous rtime) and that should be
    debugged and fixed.

    But since problem did not manifest itself before commit
    62188451f0d63add7ad0cd2a1ae269d600c1663d "cputime: Avoid
    multiplication overflow on utime scaling", it should be threated
    as regression, which we can easily fixed on cputime_adjust()
    function.

    For now, let's apply this fix, but further work is needed to fix
    root of the problem.

    Reported-and-tested-by: Dave Hansen <dave@sr71.net>
    Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
    Cc: Frederic Weisbecker <fweisbec@gmail.com>
    Cc: rostedt@goodmis.org
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Dave Hansen <dave@sr71.net>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Link: http://lkml.kernel.org/r/1367314507-9728-3-git-send-email-sgruszka@redhat.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    kernel/sched/cputime.c | 14 +++++++-------
    1 file changed, 7 insertions(+), 7 deletions(-)

    --- a/kernel/sched/cputime.c
    +++ b/kernel/sched/cputime.c
    @@ -574,7 +574,7 @@ static void cputime_adjust(struct task_c
    struct cputime *prev,
    cputime_t *ut, cputime_t *st)
    {
    - cputime_t rtime, stime, total;
    + cputime_t rtime, stime, utime, total;

    stime = curr->stime;
    total = stime + curr->utime;
    @@ -599,13 +599,13 @@ static void cputime_adjust(struct task_c
    if (prev->stime + prev->utime >= rtime)
    goto out;

    - if (!rtime) {
    - stime = 0;
    - } else if (!total) {
    - stime = rtime;
    - } else {
    + if (total) {
    stime = scale_stime((__force u64)stime,
    (__force u64)rtime, (__force u64)total);
    + utime = rtime - stime;
    + } else {
    + stime = rtime;
    + utime = 0;
    }

    /*
    @@ -614,7 +614,7 @@ static void cputime_adjust(struct task_c
    * Let's enforce monotonicity.
    */
    prev->stime = max(prev->stime, stime);
    - prev->utime = max(prev->utime, rtime - prev->stime);
    + prev->utime = max(prev->utime, utime);

    out:
    *ut = prev->utime;



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