linux-kernel - Re: [patch] don't preempt not TASK

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Date:	Fri, 20 Mar 2009 12:39:43 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	peterz@...radead.org, roland@...hat.com, efault@....de,
	rjw@...k.pl, jdike@...toit.com,
	user-mode-linux-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org
Subject: Re: [patch] don't preempt not TASK_RUNNING tasks


* Miklos Szeredi <miklos@...redi.hu> wrote:

> On Fri, 20 Mar 2009, Ingo Molnar wrote:
> > > > The first line of attack for this problem is making 
> > > > wait_task_inactive() sucks less, which shouldn't be too hard, 
> > > > that unconditional 1 jiffy sleep is simply retarded.
> > > 
> > > I completely agree.  However, I'd like to have a non-invasive 
> > > solution that can go into current and stable kernels so UML users 
> > > don't need to suffer any more.
> > 
> > Agreed. task_unlock_no_resched() should do that i think.
> 
> I don't see how that would help.

it more clearly expresses the need there, and we already have 
_no_resched API variants (we add them on an as-needed basis).

Doing:

 preempt_disable();
 read_lock();
 ...
 read_unlock();
 preempt_enable_no_resched();

Really just open-codes read_unlock_no_resched() and uglifies the 
code.

> ptrace_stop() specifically would need read_unlock_no_resched().  
> But I'm reluctant to add more spinlock functions with all their 
> variants...

if you worry about backportability, we can certainly add the easy 
fix too, if it's followed by the more involved fix(es).

	Ingo
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