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    Subject[PATCH rfc 4/6] sched: cfs: add bpf hooks to control wakeup and tick preemption
    Date
    This patch adds 3 hooks to control wakeup and tick preemption:
    cfs_check_preempt_tick
    cfs_check_preempt_wakeup
    cfs_wakeup_preempt_entity

    The first one allows to force or suppress a preemption from a tick
    context. An obvious usage example is to minimize the number of
    non-voluntary context switches and decrease an associated latency
    penalty by (conditionally) providing tasks or task groups an extended
    execution slice. It can be used instead of tweaking
    sysctl_sched_min_granularity.

    The second one is called from the wakeup preemption code and allows
    to redefine whether a newly woken task should preempt the execution
    of the current task. This is useful to minimize a number of
    preemptions of latency sensitive tasks. To some extent it's a more
    flexible analog of a sysctl_sched_wakeup_granularity.

    The third one is similar, but it tweaks the wakeup_preempt_entity()
    function, which is called not only from a wakeup context, but also
    from pick_next_task(), which allows to influence the decision on which
    task will be running next.

    It's a place for a discussion whether we need both these hooks or only
    one of them: the second is more powerful, but depends more on the
    current implementation. In any case, bpf hooks are not an ABI, so it's
    not a deal breaker.

    The idea of the wakeup_preempt_entity hook belongs to Rik van Riel. He
    also contributed a lot to the whole patchset by proving his ideas,
    recommendations and a feedback for earlier (non-public) versions.

    Signed-off-by: Roman Gushchin <guro@fb.com>
    ---
    include/linux/bpf_sched.h | 1 +
    include/linux/sched_hook_defs.h | 4 +++-
    kernel/sched/fair.c | 27 +++++++++++++++++++++++++++
    3 files changed, 31 insertions(+), 1 deletion(-)

    diff --git a/include/linux/bpf_sched.h b/include/linux/bpf_sched.h
    index 6e773aecdff7..5c238aeb853c 100644
    --- a/include/linux/bpf_sched.h
    +++ b/include/linux/bpf_sched.h
    @@ -40,6 +40,7 @@ static inline RET bpf_sched_##NAME(__VA_ARGS__) \
    { \
    return DEFAULT; \
    }
    +#include <linux/sched_hook_defs.h>
    #undef BPF_SCHED_HOOK

    static inline bool bpf_sched_enabled(void)
    diff --git a/include/linux/sched_hook_defs.h b/include/linux/sched_hook_defs.h
    index 14344004e335..f075b32698cd 100644
    --- a/include/linux/sched_hook_defs.h
    +++ b/include/linux/sched_hook_defs.h
    @@ -1,2 +1,4 @@
    /* SPDX-License-Identifier: GPL-2.0 */
    -BPF_SCHED_HOOK(int, 0, dummy, void)
    +BPF_SCHED_HOOK(int, 0, cfs_check_preempt_tick, struct sched_entity *curr, unsigned long delta_exec)
    +BPF_SCHED_HOOK(int, 0, cfs_check_preempt_wakeup, struct task_struct *curr, struct task_struct *p)
    +BPF_SCHED_HOOK(int, 0, cfs_wakeup_preempt_entity, struct sched_entity *curr, struct sched_entity *se)
    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index ff69f245b939..35ea8911b25c 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -21,6 +21,7 @@
    * Copyright (C) 2007 Red Hat, Inc., Peter Zijlstra
    */
    #include "sched.h"
    +#include <linux/bpf_sched.h>

    /*
    * Targeted preemption latency for CPU-bound tasks:
    @@ -4447,6 +4448,16 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)

    ideal_runtime = sched_slice(cfs_rq, curr);
    delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
    +
    + if (bpf_sched_enabled()) {
    + int ret = bpf_sched_cfs_check_preempt_tick(curr, delta_exec);
    +
    + if (ret < 0)
    + return;
    + else if (ret > 0)
    + resched_curr(rq_of(cfs_rq));
    + }
    +
    if (delta_exec > ideal_runtime) {
    resched_curr(rq_of(cfs_rq));
    /*
    @@ -7083,6 +7094,13 @@ wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
    {
    s64 gran, vdiff = curr->vruntime - se->vruntime;

    + if (bpf_sched_enabled()) {
    + int ret = bpf_sched_cfs_wakeup_preempt_entity(curr, se);
    +
    + if (ret)
    + return ret;
    + }
    +
    if (vdiff <= 0)
    return -1;

    @@ -7168,6 +7186,15 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
    likely(!task_has_idle_policy(p)))
    goto preempt;

    + if (bpf_sched_enabled()) {
    + int ret = bpf_sched_cfs_check_preempt_wakeup(current, p);
    +
    + if (ret < 0)
    + return;
    + else if (ret > 0)
    + goto preempt;
    + }
    +
    /*
    * Batch and idle tasks do not preempt non-idle tasks (their preemption
    * is driven by the tick):
    --
    2.31.1
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