linux-kernel - Re: [RFC PATCH 5/6] implement per-cpu&per-domain state machine call

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Date:	Sat, 10 Mar 2012 11:32:16 +0800
From:	Lai Jiangshan <eag0628@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, fweisbec@...il.com,
	patches@...aro.org, tj@...nel.org
Subject: Re: [RFC PATCH 5/6] implement per-cpu&per-domain state machine call_srcu()

On Fri, Mar 9, 2012 at 3:58 AM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> On Tue, Mar 06, 2012 at 04:34:53PM +0100, Peter Zijlstra wrote:
>> On Tue, 2012-03-06 at 23:12 +0800, Lai Jiangshan wrote:
>> > On Tue, Mar 6, 2012 at 7:16 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>> > > On Tue, 2012-03-06 at 17:57 +0800, Lai Jiangshan wrote:
>> > >>         srcu_head is bigger, it is worth, it provides more ability and simplify
>> > >>         the srcu code.
>> > >
>> > > Dubious claim.. memory footprint of various data structures is deemed
>> > > important. rcu_head is 16 bytes, srcu_head is 32 bytes. I think it would
>> > > be real nice not to have two different callback structures and not grow
>> > > them as large.
>> >
>> > CC: tj@...nel.org
>> > It could be better if workqueue also supports 2*sizeof(long) work callbacks.
>>
>> That's going to be very painful if at all possible.
>>
>> > I prefer ability/functionality a little more, it eases the caller's pain.
>> > preemptible callbacks also eases the pressure of the whole system.
>> > But I'm also ok if we limit the srcu-callbacks in softirq.
>>
>> You don't have to use softirq, you could run a complete list from a
>> single worklet. Just keep the single linked rcu_head list and enqueue a
>> static (per-cpu) worker to process the entire list.
>
> I like the idea of SRCU using rcu_head.  I am a little concerned about
> what happens when there are lots of SRCU callbacks, but am willing to
> wait to solve those problems until the situation arises.
>
> But I guess I should ask...  Peter, what do you expect the maximum
> call_srcu() rate to be in your use cases?  If tens of thousands are
> possible, some adjustments will be needed.

Different from RCU(global domain), SRCU is separated
by domains, I think the rate is not high for a given domain,
but the real answer depends on the practice.

>
>                                                        Thanx, Paul
>
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