linux-kernel - Re: [PATCH 1/3] sched: add sched_task

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Date:	Thu, 19 Feb 2015 17:11:41 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	Vojtech Pavlik <vojtech@...e.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Seth Jennings <sjenning@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] sched: add sched_task_call()

On Thu, Feb 19, 2015 at 10:26:09PM +0100, Jiri Kosina wrote:
> On Thu, 19 Feb 2015, Josh Poimboeuf wrote:
> 
> > How about with a TIF_IN_USERSPACE thread flag?  It could be cleared/set
> > right at the border.  Then for running tasks it's as simple as:
> > 
> > if (test_tsk_thread_flag(task, TIF_IN_USERSPACE))
> > 	klp_switch_task_universe(task);
> 
> That's in principle what CONTEXT_TRACKING is doing, i.e. the condition 
> we'd be interested in would be
> 
> 	__this_cpu_read(context_tracking.state) == IN_USER
> 
> But it has overhead.

Yeah, that does seem to pretty much do what we want.  Unfortunately it
has a much higher overhead than just setting a thread flag.

And from the Kconfig description for CONTEXT_TRACKING_FORCE, which would
enable it on all CPUs during boot, "this option brings an overhead that
you don't want in production."

Maybe that code needs a rewrite to rely on a thread flag instead.  Then
we could use it too.

-- 
Josh
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