linux-kernel - Re: [PATCH v15 04/13] task_isolation: add initial support

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Date:   Mon, 29 Aug 2016 12:53:30 -0400
From:   Chris Metcalf <cmetcalf@...lanox.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Gilad Ben Yossef <giladb@...lanox.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Rik van Riel <riel@...hat.com>, Tejun Heo <tj@...nel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Christoph Lameter <cl@...ux.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Andy Lutomirski <luto@...capital.net>,
        Michal Hocko <mhocko@...e.com>, <linux-mm@...ck.org>,
        <linux-doc@...r.kernel.org>, <linux-api@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v15 04/13] task_isolation: add initial support

On 8/29/2016 12:48 PM, Peter Zijlstra wrote:
> On Mon, Aug 29, 2016 at 12:40:32PM -0400, Chris Metcalf wrote:
>> On 8/29/2016 12:33 PM, Peter Zijlstra wrote:
>>> On Tue, Aug 16, 2016 at 05:19:27PM -0400, Chris Metcalf wrote:
>>>> +	/*
>>>> +	 * Request rescheduling unless we are in full dynticks mode.
>>>> +	 * We would eventually get pre-empted without this, and if
>>>> +	 * there's another task waiting, it would run; but by
>>>> +	 * explicitly requesting the reschedule, we may reduce the
>>>> +	 * latency.  We could directly call schedule() here as well,
>>>> +	 * but since our caller is the standard place where schedule()
>>>> +	 * is called, we defer to the caller.
>>>> +	 *
>>>> +	 * A more substantive approach here would be to use a struct
>>>> +	 * completion here explicitly, and complete it when we shut
>>>> +	 * down dynticks, but since we presumably have nothing better
>>>> +	 * to do on this core anyway, just spinning seems plausible.
>>>> +	 */
>>>> +	if (!tick_nohz_tick_stopped())
>>>> +		set_tsk_need_resched(current);
>>> This is broken.. and it would be really good if you don't actually need
>>> to do this.
>> Can you elaborate?
> Naked use of TIF_NEED_RESCHED like this is busted. There is more state
> that needs to be poked to keep things consistent / working.

Would it be cleaner to just replace the set_tsk_need_resched() call
with something like:

     set_current_state(TASK_INTERRUPTIBLE);
     schedule();
     __set_current_state(TASK_RUNNING);

or what would you recommend?

Or, as I said, just doing a busy loop here while testing to see
if need_resched or signal had been set?

-- 
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com