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

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Message-ID: <5317BE94.20703@oracle.com>
Date:	Wed, 05 Mar 2014 17:17:24 -0700
From:	Khalid Aziz <khalid.aziz@...cle.com>
To:	David Lang <david@...g.hm>
CC:	Oleg Nesterov <oleg@...hat.com>, 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, linux-kernel@...r.kernel.org
Subject: Re: [RFC] [PATCH] Pre-emption control for userspace

On 03/05/2014 04:59 PM, David Lang wrote:
> what's the cost to setup mmap of this file in /proc. this is sounding
> like a lot of work.

That is a one time cost paid when a thread initializes itself.

>
>>> is this gain from not giving up the CPU at all? or is it from avoiding
>>> all the delays due to the contending thread trying in turn? the
>>> yield_to() approach avoids all those other threads trying in turn so it
>>> should get fairly close to the same benefits.
>>>
>>
>> The gain is from avoiding contention by giving locking thread a chance
>> to complete its critical section which is expected to be very short
>> (certainly shorter than timeslice). Pre-emption immunity gives it one
>> and only one additional timeslice.
>
> but the yield_to() does almost the same thing, there is a small bump,
> but you don't have to wait for thread B to spin, thread C..ZZZ etc to
> spin before thread A can finish it's work. As soon as the second thread
> hits the critical section, thread A is going to be able to do more work
> (and hopefully finish)
>
>> Hope this helps clear things up.
>
> It doesn't sound like you and I are understanding how the yield_to()
> approach would work. I hope my comments have helped get us on the same
> page.
>

I apologize if I am being dense. My understanding of yield_to() is what 
Oleg had said in his reply earlier, so I will quote the example he gave:

	my_lock()
	{
		if (!TRY_LOCK()) {
			yield_to(owner);
			LOCK();
		}

		owner = gettid();
	}

If thread A had already lost the processor by the time thread B executes 
above code, wouldn't we have paid the price of two context switches for 
thread A?

Thanks,
Khalid
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