linux-kernel - Re: [RFC] [PATCH] Pre-emption control for userspace

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Date:	Thu, 6 Mar 2014 06:25:50 -0800 (PST)
From:	David Lang <david@...g.hm>
To:	Kevin Easton <kevin@...rana.org>
cc:	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Khalid Aziz <khalid.aziz@...cle.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	peterz@...radead.org, akpm@...ux-foundation.org,
	viro@...iv.linux.org.uk, oleg@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] [PATCH] Pre-emption control for userspace

On Thu, 6 Mar 2014, Kevin Easton wrote:

> On Tue, Mar 04, 2014 at 04:51:15PM -0800, Andi Kleen wrote:
>> Anything else?
>
> If it was possible to make the time remaining in the current timeslice
> available to userspace through the vdso, the thread could do something like:
>
> if (sys_timeleft() < CRITICAL_SECTION_SIZE)
>    yield();
> lock();
>
> to avoid running out of timeslice in the middle of the critical section.

but won't the system call result in context switches? According to Kevin, even a 
context switch to another thread and back immediatly is bad enough to need to be 
avoided, so replacing that with the context switch to the kernel and back isn't 
a subtantial win.

David Lang
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