linux-kernel - Re: question about RCU dynticks

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Message-ID: <55493183.2070201@redhat.com>
Date:	Tue, 05 May 2015 17:09:23 -0400
From:	Rik van Riel <riel@...hat.com>
To:	paulmck@...ux.vnet.ibm.com, Peter Zijlstra <peterz@...radead.org>
CC:	Paolo Bonzini <pbonzini@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, williams@...hat.com,
	Andrew Lutomirski <luto@...nel.org>, fweisbec@...hat.com,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: question about RCU dynticks_nesting

On 05/05/2015 02:35 PM, Paul E. McKenney wrote:
> On Tue, May 05, 2015 at 03:00:26PM +0200, Peter Zijlstra wrote:
>> On Tue, May 05, 2015 at 05:34:46AM -0700, Paul E. McKenney wrote:
>>> On Tue, May 05, 2015 at 12:53:46PM +0200, Peter Zijlstra wrote:
>>>> On Mon, May 04, 2015 at 12:39:23PM -0700, Paul E. McKenney wrote:
>>>>> But in non-preemptible RCU, we have PREEMPT=n, so there is no preempt
>>>>> counter in production kernels.  Even if there was, we have to sample this
>>>>> on other CPUs, so the overhead of preempt_disable() and preempt_enable()
>>>>> would be where kernel entry/exit is, so I expect that this would be a
>>>>> net loss in overall performance.
>>>>
>>>> We unconditionally have the preempt_count, its just not used much for
>>>> PREEMPT_COUNT=n kernels.
>>>
>>> We have the field, you mean?  I might be missing something, but it still
>>> appears to me thta preempt_disable() does nothing for PREEMPT=n kernels.
>>> So what am I missing?
>>
>> There's another layer of accessors that can in fact manipulate the
>> preempt_count even for !PREEMPT_COUNT kernels. They are currently used
>> by things like pagefault_disable().
> 
> OK, fair enough.
> 
> I am going to focus first on getting rid of (or at least greatly reducing)
> RCU's interrupt disabling on the user-kernel entry/exit paths, since
> that seems to be the biggest cost.

Interrupts are already disabled on kernel-user and kernel-guest
switches.  Paolo and I have patches to move a bunch of the calls
to user_enter, user_exit, guest_enter, and guest_exit to places
where interrupts are already disabled, so we do not need to
disable them again.

With those in place, the vtime calculations are the largest
CPU user. I am working on those.

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