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    SubjectRe: CFS: some bad numbers with Java/database threading

    * Antoine Martin <antoine@nagafix.co.uk> wrote:

    > Basically, all the previous kernels are pretty close (2.6.16 through
    > to 2.6.20 performed almost identically to 2.6.22 and are not shown
    > here to avoid cluttering the graphs)
    >
    > All the 2.6.23-rc kernels performed poorly (except -rc3!): much more
    > erratically and with a sharp performance drop above 800 threads. The
    > load starts to go up and the performance takes a nosedive.

    hm, could you try the patch below ontop of 2.6.23-rc6 and do:

    echo 1 > /proc/sys/kernel/sched_yield_bug_workaround

    does this improve the numbers?

    Ingo

    -------------->
    Subject: sched: yield debugging
    From: Ingo Molnar <mingo@elte.hu>

    introduce various sched_yield implementations:

    # default one:
    echo 0 > /proc/sys/kernel/sched_yield_bug_workaround

    # always queues the current task next to the next task:
    echo 1 > /proc/sys/kernel/sched_yield_bug_workaround

    # NOP:
    echo 2 > /proc/sys/kernel/sched_yield_bug_workaround

    tunability depends on CONFIG_SCHED_DEBUG=y.

    Not-yet-signed-off-by: Ingo Molnar <mingo@elte.hu>
    ---
    include/linux/sched.h | 2 +
    kernel/sched_fair.c | 73 +++++++++++++++++++++++++++++++++++++++++++++-----
    kernel/sysctl.c | 19 +++++++++++++
    3 files changed, 88 insertions(+), 6 deletions(-)

    Index: linux/include/linux/sched.h
    ===================================================================
    --- linux.orig/include/linux/sched.h
    +++ linux/include/linux/sched.h
    @@ -1392,10 +1392,12 @@ extern void sched_idle_next(void);
    #ifdef CONFIG_SCHED_DEBUG
    extern unsigned int sysctl_sched_latency;
    extern unsigned int sysctl_sched_min_granularity;
    +extern unsigned int sysctl_sched_yield_granularity;
    extern unsigned int sysctl_sched_wakeup_granularity;
    extern unsigned int sysctl_sched_batch_wakeup_granularity;
    extern unsigned int sysctl_sched_stat_granularity;
    extern unsigned int sysctl_sched_runtime_limit;
    +extern unsigned int sysctl_sched_yield_bug_workaround;
    extern unsigned int sysctl_sched_child_runs_first;
    extern unsigned int sysctl_sched_features;
    #endif
    Index: linux/kernel/sched_fair.c
    ===================================================================
    --- linux.orig/kernel/sched_fair.c
    +++ linux/kernel/sched_fair.c
    @@ -42,6 +42,16 @@ const_debug unsigned int sysctl_sched_la
    */
    const_debug unsigned int sysctl_sched_child_runs_first = 1;

    +const_debug unsigned int sysctl_sched_yield_granularity = 10000000ULL;
    +
    +/*
    + * sys_sched_yield workaround switch.
    + *
    + * This option switches the yield implementation of the
    + * old scheduler back on.
    + */
    +const_debug unsigned int sysctl_sched_yield_bug_workaround;
    +
    /*
    * Minimal preemption granularity for CPU-bound tasks:
    * (default: 2 msec, units: nanoseconds)
    @@ -675,15 +685,66 @@ static void dequeue_task_fair(struct rq
    */
    static void yield_task_fair(struct rq *rq, struct task_struct *p)
    {
    - struct cfs_rq *cfs_rq = task_cfs_rq(p);
    + if (!sysctl_sched_yield_bug_workaround) {
    + struct cfs_rq *cfs_rq = task_cfs_rq(p);
    + __update_rq_clock(rq);
    +
    + /*
    + * Dequeue and enqueue the task to update its
    + * position within the tree:
    + */
    + dequeue_entity(cfs_rq, &p->se, 0);
    + enqueue_entity(cfs_rq, &p->se, 0);
    + return;
    + }
    +
    + if (sysctl_sched_yield_bug_workaround == 1) {
    + struct cfs_rq *cfs_rq = task_cfs_rq(p);
    + struct rb_node *curr, *next, *first;
    + struct task_struct *p_next;
    + s64 yield_key;
    +
    + __update_rq_clock(rq);
    + curr = &p->se.run_node;
    + first = first_fair(cfs_rq);
    + /*
    + * Move this task to the second place in the tree:
    + */
    + if (unlikely(curr != first)) {
    + next = first;
    + } else {
    + next = rb_next(curr);
    + /*
    + * We were the last one already - nothing to do, return
    + * and reschedule:
    + */
    + if (unlikely(!next))
    + return;
    + }
    +
    + p_next = rb_entry(next, struct task_struct, se.run_node);
    + /*
    + * Minimally necessary key value to be the second in the tree:
    + */
    + yield_key = p_next->se.fair_key + (int)sysctl_sched_yield_granularity;
    +
    + dequeue_entity(cfs_rq, &p->se, 0);
    +
    + /*
    + * Only update the key if we need to move more backwards
    + * than the minimally necessary position to be the second:
    + */
    + if (p->se.fair_key < yield_key)
    + p->se.fair_key = yield_key;
    +
    + __enqueue_entity(cfs_rq, &p->se);
    + return;
    + }

    - __update_rq_clock(rq);
    /*
    - * Dequeue and enqueue the task to update its
    - * position within the tree:
    + * Just reschedule, do nothing else:
    */
    - dequeue_entity(cfs_rq, &p->se, 0);
    - enqueue_entity(cfs_rq, &p->se, 0);
    + resched_task(p);
    }

    /*
    Index: linux/kernel/sysctl.c
    ===================================================================
    --- linux.orig/kernel/sysctl.c
    +++ linux/kernel/sysctl.c
    @@ -244,6 +244,17 @@ static ctl_table kern_table[] = {
    },
    {
    .ctl_name = CTL_UNNUMBERED,
    + .procname = "sched_yield_granularity_ns",
    + .data = &sysctl_sched_yield_granularity,
    + .maxlen = sizeof(unsigned int),
    + .mode = 0644,
    + .proc_handler = &proc_dointvec_minmax,
    + .strategy = &sysctl_intvec,
    + .extra1 = &min_sched_granularity_ns,
    + .extra2 = &max_sched_granularity_ns,
    + },
    + {
    + .ctl_name = CTL_UNNUMBERED,
    .procname = "sched_wakeup_granularity_ns",
    .data = &sysctl_sched_wakeup_granularity,
    .maxlen = sizeof(unsigned int),
    @@ -266,6 +277,14 @@ static ctl_table kern_table[] = {
    },
    {
    .ctl_name = CTL_UNNUMBERED,
    + .procname = "sched_yield_bug_workaround",
    + .data = &sysctl_sched_yield_bug_workaround,
    + .maxlen = sizeof(unsigned int),
    + .mode = 0644,
    + .proc_handler = &proc_dointvec,
    + },
    + {
    + .ctl_name = CTL_UNNUMBERED,
    .procname = "sched_child_runs_first",
    .data = &sysctl_sched_child_runs_first,
    .maxlen = sizeof(unsigned int),
    -
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