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

From: Khalid Aziz <khalid.aziz@oracle.com>
To: "H. Peter Anvin" <hpa@zytor.com>, David Lang <david@lang.hm>
Cc: Oleg Nesterov <oleg@redhat.com>, Andi Kleen <andi@firstfloor.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>,
	Ingo Molnar <mingo@kernel.org>,
	peterz@infradead.org, akpm@linux-foundation.org,
	viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org
Subject: Re: [RFC] [PATCH] Pre-emption control for userspace
Date: Wed, 05 Mar 2014 17:02:50 -0700	[thread overview]
Message-ID: <5317BB2A.8000703@oracle.com> (raw)
In-Reply-To: <5317B9B2.8070205@zytor.com>

On 03/05/2014 04:56 PM, H. Peter Anvin wrote:
> On 03/05/2014 03:48 PM, Khalid Aziz wrote:
>>
>> Cost is writing to a memory location since thread is using mmap, not
>> insignificant but hardly expensive. Thread does not need to know how
>> much time it has left in current timeslice. It always sets the flag to
>> request pre-emption immunity before entering the critical section and
>> clears the flag when it exits its critical section. If the thread comes
>> up for pre-emption while the flag is set, it gets immunity. If it does
>> not, flag will be cleared at the end of critical section any way.
>>
>
> A little more than that.  The scheduler needs to set *another* flag
> telling the process to yield upon leaving the critical section; if the
> process doesn't, the scheduler needs to keep enough accounting to know
> to penalize the process, or this method will not be usable for
> unprivileged processes.

Yes, you had made that suggestion earlier and I like it. It will be in 
v2 patch. I am thinking of making the penalty be denial of next 
preemption immunity request if a process fails to yield when it should 
have. Sounds good?

Thanks,
Khalid