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Date: Mon, 29 Aug 2016 18:48:09 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Chris Metcalf <cmetcalf@mellanox.com>
Cc: Gilad Ben Yossef <giladb@mellanox.com>,
        Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@kernel.org>,
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Subject: Re: [PATCH v15 04/13] task_isolation: add initial support
Message-ID: <20160829164809.GW10153@twins.programming.kicks-ass.net>
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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.