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    SubjectRe: [RFC PATCH v2 11/17] sched: Basic tracking of matching tasks
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    On 5/1/19 4:27 PM, Tim Chen wrote:
    > On 4/28/19 11:15 PM, Aaron Lu wrote:
    >> On Tue, Apr 23, 2019 at 04:18:16PM +0000, Vineeth Remanan Pillai wrote:
    >>> +/*
    >>> + * Find left-most (aka, highest priority) task matching @cookie.
    >>> + */
    >>> +struct task_struct *sched_core_find(struct rq *rq, unsigned long cookie)
    >>> +{
    >>> + struct rb_node *node = rq->core_tree.rb_node;
    >>> + struct task_struct *node_task, *match;
    >>> +
    >>> + /*
    >>> + * The idle task always matches any cookie!
    >>> + */
    >>> + match = idle_sched_class.pick_task(rq);
    >>> +
    >>> + while (node) {
    >>> + node_task = container_of(node, struct task_struct, core_node);
    >>> +
    >>> + if (node_task->core_cookie < cookie) {
    >>> + node = node->rb_left;
    >>
    >> Should go right here?
    >>
    >
    > I think Aaron is correct. We order the rb tree where tasks with smaller core cookies
    > go to the left part of the tree.
    >
    > In this case, the cookie we are looking for is larger than the current node's cookie.
    > It seems like we should move to the right to look for a node with matching cookie.
    >
    > At least making the following change still allow us to run the system stably for sysbench.
    > Need to gather more data to see how performance changes.

    Pawan ran an experiment setting up 2 VMs, with one VM doing a parallel kernel build and one VM doing sysbench,
    limiting both VMs to run on 16 cpu threads (8 physical cores), with 8 vcpu for each VM.
    Making the fix did improve kernel build time by 7%.

    Tim


    >
    > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    > index 25638a47c408..ed4cfa49e3f2 100644
    > --- a/kernel/sched/core.c
    > +++ b/kernel/sched/core.c
    > @@ -208,9 +208,9 @@ static struct task_struct *sched_core_find(struct rq *rq, unsigned long cookie)
    > while (node) {
    > node_task = container_of(node, struct task_struct, core_node);
    >
    > - if (node_task->core_cookie < cookie) {
    > + if (cookie < node_task->core_cookie) {
    > node = node->rb_left;
    > - } else if (node_task->core_cookie > cookie) {
    > + } else if (cookie > node_task->core_cookie) {
    > node = node->rb_right;
    > } else {
    > match = node_task;
    >
    >

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