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SubjectRe: [RFC][PATCH 0/3] sched: User Managed Concurrency Groups
On Wed, Dec 15, 2021 at 11:06:20AM +0100, Peter Zijlstra wrote:
> On Tue, Dec 14, 2021 at 07:46:25PM -0800, Peter Oskolkov wrote:
> > On Tue, Dec 14, 2021 at 12:55 PM Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > Hi,
> > >
> > > This is actually tested code; but still missing the SMP wake-to-idle machinery.
> > > I still need to think about that.
> >
> > Thanks, Peter!
> >
> > At a first glance, your main patch does not look much smaller than
> > mine, and I thought the whole point of re-doing it was to throw away
> > extra features and make things smaller/simpler...
>
> Well, simpler was the goal. I didn't really focus on size much. It isn't
> really big to begin with.
>
> But yes, it has 5 hooks now, 3 syscalls and lots of comments and all
> that under 900 lines, not bad I'd say.
>
> Also I think you wanted something like this? I'm not sure of the LAZY
> name, but I can't seem to come up with anything saner atm.
>
>
> ---
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1297,6 +1297,7 @@ struct task_struct {
>
> #ifdef CONFIG_UMCG
> /* setup by sys_umcg_ctrl() */
> + u32 umcg_flags;
> clockid_t umcg_clock;
> struct umcg_task __user *umcg_task;
>
> --- a/include/uapi/linux/umcg.h
> +++ b/include/uapi/linux/umcg.h
> @@ -133,11 +133,13 @@ struct umcg_task {
> * @UMCG_CTL_REGISTER: register the current task as a UMCG task
> * @UMCG_CTL_UNREGISTER: unregister the current task as a UMCG task
> * @UMCG_CTL_WORKER: register the current task as a UMCG worker
> + * @UMCG_CTL_LAZY: don't wake server on runnable enqueue
> */
> enum umcg_ctl_flag {
> UMCG_CTL_REGISTER = 0x00001,
> UMCG_CTL_UNREGISTER = 0x00002,
> UMCG_CTL_WORKER = 0x10000,
> + UMCG_CTL_LAZY = 0x20000,
> };
>
> #endif /* _UAPI_LINUX_UMCG_H */
> --- a/kernel/sched/umcg.c
> +++ b/kernel/sched/umcg.c
> @@ -416,6 +416,27 @@ static int umcg_enqueue_runnable(struct
> }
>
> /*
> + * Enqueue tsk to it's server's runnable list and wake the server for pickup if
> + * so desired. Notable LAZY workers will not wake the server and rely on the
> + * server to do pickup whenever it naturally runs next.
> + *
> + * Returns:
> + * 0: success
> + * -EFAULT
> + */
> +static int umcg_enqueue_and_wake(struct task_struct *tsk, bool force)
> +{
> + int ret = umcg_enqueue_runnable(tsk);
> + if (ret)
> + return ret;
> +
> + if (force || !(tsk->umcg_flags & UMCG_CTL_LAZY))
> + ret = umcg_wake_server(tsk);
> +
> + return ret;
> +}

Aah, this has a problem when the server is otherwise idle. I think we
need that TF_IDLE thing for this too. Let me go write a test-case for
all this.

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