linux-kernel - Re: question about RCU dynticks

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Date:	Mon, 04 May 2015 16:13:50 -0400
From:	Rik van Riel <riel@...hat.com>
To:	paulmck@...ux.vnet.ibm.com
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,
	Peter Zijlstra <peterz@...radead.org>,
	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 05/04/2015 04:02 PM, Paul E. McKenney wrote:
> On Mon, May 04, 2015 at 03:39:25PM -0400, Rik van Riel wrote:
>> On 05/04/2015 02:39 PM, Paul E. McKenney wrote:
>>> On Mon, May 04, 2015 at 11:59:05AM -0400, Rik van Riel wrote:
>>
>>>> In fact, would we be able to simply use tsk->rcu_read_lock_nesting
>>>> as an indicator of whether or not we should bother waiting on that
>>>> task or CPU when doing synchronize_rcu?
>>>
>>> Depends on exactly what you are asking.  If you are asking if I could add
>>> a few more checks to preemptible RCU and speed up grace-period detection
>>> in a number of cases, the answer is very likely "yes".  This is on my
>>> list, but not particularly high priority.  If you are asking whether
>>> CPU 0 could access ->rcu_read_lock_nesting of some task running on
>>> some other CPU, in theory, the answer is "yes", but in practice that
>>> would require putting full memory barriers in both rcu_read_lock()
>>> and rcu_read_unlock(), so the real answer is "no".
>>>
>>> Or am I missing your point?
>>
>> The main question is "how can we greatly reduce the overhead
>> of nohz_full, by simplifying the RCU extended quiescent state
>> code called in the syscall fast path, and maybe piggyback on
>> that to do time accounting for remote CPUs?"
>>
>> Your memory barrier answer above makes it clear we will still
>> want to do the RCU stuff at syscall entry & exit time, at least
>> on x86, where we already have automatic and implicit memory
>> barriers.
> 
> We do need to keep in mind that x86's automatic and implicit memory
> barriers do not order prior stores against later loads.
> 
> Hmmm...  But didn't earlier performance measurements show that the bulk of
> the overhead was the delta-time computations rather than RCU accounting?

The bulk of the overhead was disabling and re-enabling
irqs around the calls to rcu_user_exit and rcu_user_enter :)

Of the remaining time, about 2/3 seems to be the vtime
stuff, and the other 1/3 the rcu code.

I suspect it makes sense to optimize both, though the
vtime code may be the easiest :)

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