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

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Date:   Thu, 27 Jan 2022 13:25:36 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tao Zhou <tao.zhou@...ux.dev>
Cc:     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, 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 Mon, Jan 24, 2022 at 10:46:17PM +0800, Tao Zhou wrote:

> > +/* Called from syscall exit path and exceptions that can schedule */
> > +void umcg_sys_exit(struct pt_regs *regs)
> > +{
> > +	struct task_struct *tsk = current;
> > +	long syscall = syscall_get_nr(tsk, regs);
> > +
> > +	if (syscall == __NR_umcg_wait ||
> > +	    syscall == __NR_umcg_ctl)
> > +		return;
> > +
> > +	if (tsk->umcg_server) {
> > +		/*
> > +		 * Didn't block, we done.
> > +		 */
> > +		umcg_unpin_pages();
> > +		return;
> > +	}
> > +
> > +	umcg_unblock_and_wait();
> 
> umcg_unblock_and_wait() -> umcg_enqueue_and_wake() ->
> umcg_wake_server() -> umcg_wake_task(tsk->umcg_server, ...)
> 
> tsk->umcg_server is NULL here and umcg_wake_task() use it to update
> state in umcg_update_state(NULL, ...), that means tsk->umcg_clock
> will happen something i do not know.

I think umcg_unblock_and_wait() will repin, at which point we should
have tsk->umcg_server again.

> There are two places to call umcg_unblock_and_wait(). One is in 
> umcg_register() where the server is set. Another one is in
> umcg_sys_exit() where the server is not set. May use a bool to
> indicate if the server is set.

I'm not sure what you're on about, but I absolutely hate redundant
state, that only leads to problems.