linux-kernel - Re: [PATCH 1/3] jump label: add enabled/disabled state to jump label key entries

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Message-ID: <20101125131446.GA27350@Krystal>
Date:	Thu, 25 Nov 2010 08:14:46 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	michael@...erman.id.au, Steven Rostedt <rostedt@...dmis.org>,
	Jason Baron <jbaron@...hat.com>, mingo@...e.hu, hpa@...or.com,
	tglx@...utronix.de, andi@...stfloor.org, roland@...hat.com,
	rth@...hat.com, masami.hiramatsu.pt@...achi.com,
	fweisbec@...il.com, avi@...hat.com, davem@...emloft.net,
	sam@...nborg.org, ddaney@...iumnetworks.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] jump label: add enabled/disabled state to jump
	label key entries

* Peter Zijlstra (peterz@...radead.org) wrote:
[...]
> > What would suit us would be to have an arch callback that is called
> > after all the transforms for a particular jump label key have been made.
> > That way we could optimise the individual patches, and do a sync step at
> > the end, ie. when we want the effect of the patching to be globally
> > visible. 
> 
> I think such a sync-barrier is desired (possibly only on the enable
> path) so we can actually say the tracepoints are on.
> 
> Which would mean sending IPIs to all CPUs and waiting for them to
> acknowledge them. Which, while not quite as expensive as stop_machine,
> its not really cheap either.

Yep, although this can be batched when enabling many tracepoints en
masse. May I suggest that you guys benchmark the two approaches so we
can figure out at how many tracepoints we start hitting a latency wall ?
100, 1000 and 10000 tracepoints should give interesting measurement
points. If we are still below 2 seconds on common hardware when enabling
10000 tracepoints, then the binary search might be fine.

Please note that HPA recommended the use of a perfect hash. It would
make sense, although there seems to be a non-null probability that the
perfect hash cannot be generated. There are techniques that will retry
with a different seed, but the kernel build time then becomes slightly
harder to predict (for very, very rare occurences, so maybe we don't
care).

Thanks,

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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