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    Subject[PATCH AUTOSEL 5.13 182/189] sched/fair: Ensure _sum and _avg values stay consistent
    Date
    From: Odin Ugedal <odin@uged.al>

    [ Upstream commit 1c35b07e6d3986474e5635be566e7bc79d97c64d ]

    The _sum and _avg values are in general sync together with the PELT
    divider. They are however not always completely in perfect sync,
    resulting in situations where _sum gets to zero while _avg stays
    positive. Such situations are undesirable.

    This comes from the fact that PELT will increase period_contrib, also
    increasing the PELT divider, without updating _sum and _avg values to
    stay in perfect sync where (_sum == _avg * divider). However, such PELT
    change will never lower _sum, making it impossible to end up in a
    situation where _sum is zero and _avg is not.

    Therefore, we need to ensure that when subtracting load outside PELT,
    that when _sum is zero, _avg is also set to zero. This occurs when
    (_sum < _avg * divider), and the subtracted (_avg * divider) is bigger
    or equal to the current _sum, while the subtracted _avg is smaller than
    the current _avg.

    Reported-by: Sachin Sant <sachinp@linux.vnet.ibm.com>
    Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
    Signed-off-by: Odin Ugedal <odin@uged.al>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
    Tested-by: Sachin Sant <sachinp@linux.vnet.ibm.com>
    Link: https://lore.kernel.org/r/20210624111815.57937-1-odin@uged.al
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/sched/fair.c | 6 +++---
    1 file changed, 3 insertions(+), 3 deletions(-)

    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index 23663318fb81..1dd1ed98171b 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -3716,15 +3716,15 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq)

    r = removed_load;
    sub_positive(&sa->load_avg, r);
    - sub_positive(&sa->load_sum, r * divider);
    + sa->load_sum = sa->load_avg * divider;

    r = removed_util;
    sub_positive(&sa->util_avg, r);
    - sub_positive(&sa->util_sum, r * divider);
    + sa->util_sum = sa->util_avg * divider;

    r = removed_runnable;
    sub_positive(&sa->runnable_avg, r);
    - sub_positive(&sa->runnable_sum, r * divider);
    + sa->runnable_sum = sa->runnable_avg * divider;

    /*
    * removed_runnable is the unweighted version of removed_load so we
    --
    2.30.2
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