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

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Date:	Thu, 19 Feb 2015 21:40:36 +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 11:32:55AM -0600, Josh Poimboeuf wrote:
> 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.

Having a safe way would be very nice and actually quite useful in other
cases, too.

For this specific purpose, however, we don't need a very safe way,
though. We don't require atomicity in any way, we don't mind even if it
creates false negatives, only false positives would be bad.

kGraft looks at the stacktrace of CPU hogs and if it finds no kernel
addresses there, it assumes userspace. Not very nice, but does the job.

> 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);

-- 
Vojtech Pavlik
Director SUSE Labs
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