Date: Thu, 19 Feb 2015 11:32:55 -0600
From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Vojtech Pavlik <vojtech@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@redhat.com>, Jiri Kosina <jkosina@suse.cz>,
        Seth Jennings <sjenning@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/3] sched: add sched_task_call()
Message-ID: <20150219173255.GC15980@treble.redhat.com>
References: <20150217212532.GJ11861@treble.redhat.com>
 <20150218152100.GZ5029@twins.programming.kicks-ass.net>
 <20150218171256.GA28553@treble.hsd1.ky.comcast.net>
 <20150219002058.GD5029@twins.programming.kicks-ass.net>
 <20150219041753.GA13423@treble.redhat.com>
 <20150219101607.GG5029@twins.programming.kicks-ass.net>
 <20150219162429.GA15980@treble.redhat.com>
 <20150219163359.GA25438@suse.cz>
 <20150219170353.GB15980@treble.redhat.com>
 <20150219171929.GA13178@suse.cz>
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On Thu, Feb 19, 2015 at 06:19:29PM +0100, Vojtech Pavlik wrote:
> On Thu, Feb 19, 2015 at 11:03:53AM -0600, Josh Poimboeuf wrote:
> > On Thu, Feb 19, 2015 at 05:33:59PM +0100, Vojtech Pavlik wrote:
> > > On Thu, Feb 19, 2015 at 10:24:29AM -0600, Josh Poimboeuf wrote:
> > > 
> > > > > No, these tasks will _never_ make syscalls. So you need to guarantee
> > > > > they don't accidentally enter the kernel while you flip them. Something
> > > > > like so should do.
> > > > > 
> > > > > You set TIF_ENTER_WAIT on them, check they're still in userspace, flip
> > > > > them then clear TIF_ENTER_WAIT.
> > > > 
> > > > Ah, that's a good idea.  But how do we check if they're in user space?
> > > 
> > > I don't see the benefit in holding them in a loop - you can just as well
> > > flip them from the syscall code as kGraft does.
> > 
> > But we were talking specifically about HPC tasks which never make
> > syscalls.
> 
> Yes. I'm saying that rather than guaranteeing they don't enter the
> kernel (by having them spin) you can flip them in case they try to do
> that instead. That solves the race condition just as well.

Ok, gotcha.

We'd still need a safe way to check if they're in user space though.

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);

-- 
Josh
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