linux-kernel - Re: [RFC PATCH for 4.21 06/16] cpu_opv: Provide cpu

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Date:   Wed, 17 Oct 2018 12:49:31 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>,
        Ben Maurer <bmaurer@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Joel Fernandes <joelaf@...gle.com>
Subject: Re: [RFC PATCH for 4.21 06/16] cpu_opv: Provide cpu_opv system call
 (v8)


Hi Mathieu,

> +static int do_cpu_opv(struct cpu_op *cpuop, int cpuopcnt,
> +		      struct cpu_opv_vaddr *vaddr_ptrs, int cpu)
> +{
> +	struct mm_struct *mm = current->mm;
> +	int ret;
> +
> +retry:
> +	if (cpu != raw_smp_processor_id()) {
> +		ret = push_task_to_cpu(current, cpu);
> +		if (ret)
> +			goto check_online;
> +	}
> +	down_read(&mm->mmap_sem);
> +	ret = vaddr_ptrs_check(vaddr_ptrs);
> +	if (ret)
> +		goto end;
> +	preempt_disable();
> +	if (cpu != smp_processor_id()) {
> +		preempt_enable();
> +		up_read(&mm->mmap_sem);
> +		goto retry;
> +	}

If we have a higher priority task/s either pinned to the cpu, dont we end up
in busy-looping till the task exits/sleeps?

> +	ret = __do_cpu_opv(cpuop, cpuopcnt);
> +	preempt_enable();
> +end:
> +	up_read(&mm->mmap_sem);
> +	return ret;
> +
> +check_online:
> +	/*
> +	 * push_task_to_cpu() returns -EINVAL if the requested cpu is not part
> +	 * of the current thread's cpus_allowed mask.
> +	 */
> +	if (ret == -EINVAL)
> +		return ret;
> +	get_online_cpus();
> +	if (cpu_online(cpu)) {
> +		put_online_cpus();
> +		goto retry;
> +	}