linux-kernel - Re: [RFC][PATCH v2 5/5] sched: User Mode Concurency Groups

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Date:   Mon, 24 Jan 2022 15:29:56 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     mingo@...hat.com, tglx@...utronix.de, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-api@...r.kernel.org, x86@...nel.org, pjt@...gle.com,
        posk@...gle.com, avagin@...gle.com, jannh@...gle.com,
        tdelisle@...terloo.ca, mark.rutland@....com, posk@...k.io
Subject: Re: [RFC][PATCH v2 5/5] sched: User Mode Concurency Groups

On Fri, Jan 21, 2022 at 04:18:46PM +0100, Peter Zijlstra wrote:
> Something like this, still yuck though. Also still need to write me a
> test for this.
> 
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1300,12 +1300,14 @@ struct task_struct {
>  	clockid_t		umcg_clock;
>  	struct umcg_task __user	*umcg_task;
>  
> -	/* setup by umcg_pin_enter() */
> +	/* setup by umcg_pin_pages() */
>  	struct page		*umcg_page;
>  
>  	struct task_struct	*umcg_server;
>  	struct umcg_task __user *umcg_server_task;
>  	struct page		*umcg_server_page;
> +
> +	u64			umcg_timeout;
>  #endif
>  
>  	struct tlbflush_unmap_batch	tlb_ubc;
> --- a/kernel/sched/umcg.c
> +++ b/kernel/sched/umcg.c
> @@ -232,6 +232,8 @@ static int umcg_update_state(struct task
>  /* Called from syscall enter path and exceptions that can schedule */
>  void umcg_sys_enter(struct pt_regs *regs, long syscall)
>  {
> +	current->umcg_timeout = 0;
> +
>  	/* avoid recursion vs our own syscalls */
>  	if (syscall == __NR_umcg_wait ||
>  	    syscall == __NR_umcg_ctl)
> @@ -519,6 +521,7 @@ void umcg_notify_resume(struct pt_regs *
>  	struct umcg_task __user *self = tsk->umcg_task;
>  	bool worker = tsk->flags & PF_UMCG_WORKER;
>  	u32 state;
> +	int ret;
>  
>  	/* avoid recursion vs schedule() */
>  	if (worker)
> @@ -554,12 +557,17 @@ void umcg_notify_resume(struct pt_regs *
>  		umcg_unpin_pages();
>  	}
>  
> -	switch (umcg_wait(0)) {
> +	ret = umcg_wait(tsk->umcg_timeout);

Oh how I hate signals... this can get scribbled by a syscall/fault from
sigcontext :/

Maybe I can recover the timo argument from the original syscall
pt_regs.. let me try.

> +	switch (ret) {
>  	case 0:
>  	case -EINTR:
>  		/* we will resume the wait after the signal */
>  		break;
>  
> +	case -ETIMEDOUT:
> +		regs_set_return_value(regs, ret);
> +		break;
> +
>  	default:
>  		UMCG_DIE("wait");
>  	}
> @@ -759,6 +767,7 @@ SYSCALL_DEFINE2(umcg_wait, u32, flags, u
>  	switch (ret) {
>  	case 0:		/* all done */
>  	case -EINTR:	/* umcg_notify_resume() will continue the wait */
> +		tsk->umcg_timeout = timo;
>  		ret = 0;
>  		break;
>