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    Subject[PATCH -tip 23/32] sched: Add a per-thread core scheduling interface
    Date
    Add a per-thread core scheduling interface which allows a thread to share a
    core with another thread, or have a core exclusively for itself.

    ChromeOS uses core-scheduling to securely enable hyperthreading. This cuts
    down the keypress latency in Google docs from 150ms to 50ms while improving
    the camera streaming frame rate by ~3%.

    Tested-by: Julien Desfossez <jdesfossez@digitalocean.com>
    Reviewed-by: Aubrey Li <aubrey.intel@gmail.com>
    Co-developed-by: Chris Hyser <chris.hyser@oracle.com>
    Signed-off-by: Chris Hyser <chris.hyser@oracle.com>
    Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
    ---
    include/linux/sched.h | 1 +
    include/uapi/linux/prctl.h | 3 ++
    kernel/sched/core.c | 51 +++++++++++++++++++++++++++++---
    kernel/sys.c | 3 ++
    tools/include/uapi/linux/prctl.h | 3 ++
    5 files changed, 57 insertions(+), 4 deletions(-)

    diff --git a/include/linux/sched.h b/include/linux/sched.h
    index c6a3b0fa952b..79d76c78cc8e 100644
    --- a/include/linux/sched.h
    +++ b/include/linux/sched.h
    @@ -2083,6 +2083,7 @@ void sched_core_unsafe_enter(void);
    void sched_core_unsafe_exit(void);
    bool sched_core_wait_till_safe(unsigned long ti_check);
    bool sched_core_kernel_protected(void);
    +int sched_core_share_pid(pid_t pid);
    #else
    #define sched_core_unsafe_enter(ignore) do { } while (0)
    #define sched_core_unsafe_exit(ignore) do { } while (0)
    diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
    index c334e6a02e5f..217b0482aea1 100644
    --- a/include/uapi/linux/prctl.h
    +++ b/include/uapi/linux/prctl.h
    @@ -248,4 +248,7 @@ struct prctl_mm_map {
    #define PR_SET_IO_FLUSHER 57
    #define PR_GET_IO_FLUSHER 58

    +/* Request the scheduler to share a core */
    +#define PR_SCHED_CORE_SHARE 59
    +
    #endif /* _LINUX_PRCTL_H */
    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index 7ccca355623a..a95898c75bdf 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -310,6 +310,7 @@ static int __sched_core_stopper(void *data)
    }

    static DEFINE_MUTEX(sched_core_mutex);
    +static DEFINE_MUTEX(sched_core_tasks_mutex);
    static int sched_core_count;

    static void __sched_core_enable(void)
    @@ -4037,8 +4038,9 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
    RB_CLEAR_NODE(&p->core_node);

    /*
    - * Tag child via per-task cookie only if parent is tagged via per-task
    - * cookie. This is independent of, but can be additive to the CGroup tagging.
    + * If parent is tagged via per-task cookie, tag the child (either with
    + * the parent's cookie, or a new one). The final cookie is calculated
    + * by concatenating the per-task cookie with that of the CGroup's.
    */
    if (current->core_task_cookie) {

    @@ -9855,7 +9857,7 @@ static int sched_core_share_tasks(struct task_struct *t1, struct task_struct *t2
    unsigned long cookie;
    int ret = -ENOMEM;

    - mutex_lock(&sched_core_mutex);
    + mutex_lock(&sched_core_tasks_mutex);

    /*
    * NOTE: sched_core_get() is done by sched_core_alloc_task_cookie() or
    @@ -9954,10 +9956,51 @@ static int sched_core_share_tasks(struct task_struct *t1, struct task_struct *t2

    ret = 0;
    out_unlock:
    - mutex_unlock(&sched_core_mutex);
    + mutex_unlock(&sched_core_tasks_mutex);
    return ret;
    }

    +/* Called from prctl interface: PR_SCHED_CORE_SHARE */
    +int sched_core_share_pid(pid_t pid)
    +{
    + struct task_struct *task;
    + int err;
    +
    + if (pid == 0) { /* Recent current task's cookie. */
    + /* Resetting a cookie requires privileges. */
    + if (current->core_task_cookie)
    + if (!capable(CAP_SYS_ADMIN))
    + return -EPERM;
    + task = NULL;
    + } else {
    + rcu_read_lock();
    + task = pid ? find_task_by_vpid(pid) : current;
    + if (!task) {
    + rcu_read_unlock();
    + return -ESRCH;
    + }
    +
    + get_task_struct(task);
    +
    + /*
    + * Check if this process has the right to modify the specified
    + * process. Use the regular "ptrace_may_access()" checks.
    + */
    + if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
    + rcu_read_unlock();
    + err = -EPERM;
    + goto out_put;
    + }
    + rcu_read_unlock();
    + }
    +
    + err = sched_core_share_tasks(current, task);
    +out_put:
    + if (task)
    + put_task_struct(task);
    + return err;
    +}
    +
    /* CGroup interface */
    static u64 cpu_core_tag_read_u64(struct cgroup_subsys_state *css, struct cftype *cft)
    {
    diff --git a/kernel/sys.c b/kernel/sys.c
    index a730c03ee607..61a3c98e36de 100644
    --- a/kernel/sys.c
    +++ b/kernel/sys.c
    @@ -2530,6 +2530,9 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,

    error = (current->flags & PR_IO_FLUSHER) == PR_IO_FLUSHER;
    break;
    + case PR_SCHED_CORE_SHARE:
    + error = sched_core_share_pid(arg2);
    + break;
    default:
    error = -EINVAL;
    break;
    diff --git a/tools/include/uapi/linux/prctl.h b/tools/include/uapi/linux/prctl.h
    index 7f0827705c9a..4c45b7dcd92d 100644
    --- a/tools/include/uapi/linux/prctl.h
    +++ b/tools/include/uapi/linux/prctl.h
    @@ -247,4 +247,7 @@ struct prctl_mm_map {
    #define PR_SET_IO_FLUSHER 57
    #define PR_GET_IO_FLUSHER 58

    +/* Request the scheduler to share a core */
    +#define PR_SCHED_CORE_SHARE 59
    +
    #endif /* _LINUX_PRCTL_H */
    --
    2.29.2.299.gdc1121823c-goog
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