linux-kernel - Re: 2.6.22 -mm merge plans

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Date:	Sun, 13 May 2007 22:04:37 +0100
From:	Christoph Hellwig <hch@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	rusty@...tcorp.com.au, wfg@...c.edu
Subject: Re: 2.6.22 -mm merge plans

On Thu, May 10, 2007 at 03:39:36PM -0400, Mathieu Desnoyers wrote:
> * Christoph Hellwig (hch@...radead.org) wrote:
> > On Thu, May 03, 2007 at 01:16:46PM -0400, Mathieu Desnoyers wrote:
> > > > kprobes does this kind of synchronization internally, so the marker
> > > > wrapper should probabl aswell.
> > > > 
> > > 
> > > The problem appears on heavily loaded systems. Doing 50
> > > synchronize_sched() calls in a row can take up to a few seconds on a
> > > 4-way machine. This is why I prefer to do it in the module to which
> > > the callbacks belong.
> > 
> > We recently had a discussion on batch unreistration interface for
> > kprobes.  I'm not very happy with having so different interfaces for
> > different kind of probe registrations.
> > 
> 
> Ok, I've had a look at the kprobes batch registration mechanisms and..
> well, it does not look well suited for the markers. Adding
> supplementary data structures such as linked lists of probes does not
> look like a good match.
> 
> However, I agree with you that providing a similar API is good.
> 
> Therefore, here is my proposal :
> 
> The goal is to do the synchronize just after we unregister the last
> probe handler provided by a given module. Since the unregistration
> functions iterate on every marker present in the kernel, we can keep a
> count of how many probes provided by the same module are still present.
> If we see that we unregistered the last probe pointing to this module,
> we issue a synchronize_sched().
> 
> It adds no data structure and keeps the same order of complexity as what
> is already there, we only have to do 2 passes in the marker structures :
> the first one finds the module associated with the callback and the
> second disables the callbacks and keep a count of the number of
> callbacks associated with the module.
> 
> Mathieu
> 
> P.S.: here is the code.
> 
> 
> Linux Kernel Markers - Architecture Independant code Provide internal
> synchronize_sched() in batch.
> 
> The goal is to do the synchronize just after we unregister the last
> probe handler provided by a given module. Since the unregistration
> functions iterate on every marker present in the kernel, we can keep a
> count of how many probes provided by the same module are still present.
> If we see that we unregistered the last probe pointing to this module,
> we issue a synchronize_sched().
> 
> It adds no data structure and keeps the same order of complexity as what
> is already there, we only have to do 2 passes in the marker structures : 
> the first one finds the module associated with the callback and the 
> second disables the callbacks and keep a count of the number of
> callbacks associated with the module.

Looks good to me, please incorporate this is the next round of the
markers patch series.

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