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

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Message-ID: <1331575097.18960.40.camel@twins>
Date:	Mon, 12 Mar 2012 18:58:17 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Lai Jiangshan <eag0628@...il.com>,
	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 Mon, 2012-03-12 at 10:54 -0700, Paul E. McKenney wrote:
> On Sat, Mar 10, 2012 at 11:09:53AM +0100, Peter Zijlstra wrote:
> > On Thu, 2012-03-08 at 11:58 -0800, Paul E. McKenney wrote:
> > > 
> > > 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. 
> > 
> > The one call-site I currently have is linked to vma lifetimes, so yeah,
> > I guess that that can be lots.
> 
> So the worst case would be if several processes with lots of VMAs were
> to exit at about the same time?  If so, my guess is that call_srcu()
> needs to handle several thousand callbacks showing up within a few
> tens of microseconds.  Is that a reasonable assumption, or am I missing
> an order of magnitude or two in either direction?

That or a process is doing mmap/munmap loops (some file scanners are
known to do this). But yeah, that can be lots.

My current use case doesn't quite trigger since it needs another syscall
to attach something to a vma before vma tear-down actually ends up
calling call_srcu() so in practice it won't be much at all (for now).

Still I think its prudent to make it (srcu callbacks) deal with plenty
callbacks even if initially there won't be many -- who knows what other
people will do while you're not paying attention... :-)
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