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

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Message-ID: <20150219171929.GA13178@suse.cz>
Date:	Thu, 19 Feb 2015 18:19:29 +0100
From:	Vojtech Pavlik <vojtech@...e.com>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>, Jiri Kosina <jkosina@...e.cz>,
	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 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.

-- 
Vojtech Pavlik
Director SUSE Labs
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