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

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Message-ID: <1006597066.825.1539789100381.JavaMail.zimbra@efficios.com>
Date:   Wed, 17 Oct 2018 11:11:40 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <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>,
        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)

----- On Oct 17, 2018, at 3:19 AM, Srikar Dronamraju srikar@...ux.vnet.ibm.com wrote:

> 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?

You're right!

How about we ditch the thread migration altogether, and simply perform
the cpu_opv operations in a IPI handler ?

This is possible now that cpu_opv uses a temporary vmap() rather than
try to touch the user-space page through the current thread's page table.

Thoughts ?

Thanks,

Mathieu

> 
>> +	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;
> > +	}

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com