linux-kernel - Re: [PATCH] Linux Kernel Markers

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Message-ID: <451030A6.6040801@google.com>
Date:	Tue, 19 Sep 2006 11:02:14 -0700
From:	Martin Bligh <mbligh@...gle.com>
To:	prasanna@...ibm.com
CC:	Andrew Morton <akpm@...l.org>,
	"Frank Ch. Eigler" <fche@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Paul Mundt <lethal@...ux-sh.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Jes Sorensen <jes@....com>, Tom Zanussi <zanussi@...ibm.com>,
	Richard J Moore <richardj_moore@...ibm.com>,
	Michel Dagenais <michel.dagenais@...ymtl.ca>,
	Christoph Hellwig <hch@...radead.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	William Cohen <wcohen@...hat.com>, ltt-dev@...fik.org,
	systemtap@...rces.redhat.com, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] Linux Kernel Markers

>>Ah, good point. Though ... how much do we care what the speed of
>>insertion/removal actually is? If we can tolerate it being slow,
>>then just sync everyone up in an IPI to freeze them out whilst
>>doing the insert.
>>
> 
> I guess using IPI occasionally would be acceptable. But I think
> using IPI for each probes will lots of overhead.

Depends how often you're inserting/removing probes, I guess.
Aren't these being done manually, in which case it really can't
be that many? Still doesn't fix the problem Matieu just pointed
out though. Humpf.

>>How about we combine all three ideas together ...
>>
>>1. Load modified copy of the function in question.
>>2. overwrite the first instruction of the routine with an int3 that
>>does what you say (atomically)
>>3. Then overwrite the second instruction with a jump that's faster
>>4. Now atomically overwrite the int3 with a nop, and let the jump
>>take over.
> 
> That's a good solution.

It's not exactly elegant or simple, but I guess it'd work if we have
to go to that extent. Seems like a lot of complexity though, I'd
rather get rid of the int3 trap if we can.

M.
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