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

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Message-ID: <20160829163352.GV10153@twins.programming.kicks-ass.net>
Date:   Mon, 29 Aug 2016 18:33:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Chris Metcalf <cmetcalf@...lanox.com>
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 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.