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

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Date:	Thu, 06 Mar 2014 09:12:42 -0700
From:	Khalid Aziz <khalid.aziz@...cle.com>
To:	David Lang <david@...g.hm>, Kevin Easton <kevin@...rana.org>
CC:	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	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 03/06/2014 07:25 AM, David Lang wrote:
> 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

Using vdso reduces the cost of system call significantly, but as Peter 
pointed out a thread can not really rely upon the number it will get back.

--
Khalid
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