linux-kernel - Re: [RFC 0/6] uprobes: kill uprobes_srcu/uprobe_srcu

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Date:	Fri, 20 Apr 2012 12:14:21 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-mm <linux-mm@...ck.org>, Andi Kleen <andi@...stfloor.org>,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Anton Arapov <anton@...hat.com>
Subject: Re: [RFC 0/6] uprobes: kill uprobes_srcu/uprobe_srcu_id

On Mon, 2012-04-16 at 23:47 +0200, Oleg Nesterov wrote:
> On 04/16, Peter Zijlstra wrote:
> >
> > On Mon, 2012-04-16 at 01:44 +0200, Oleg Nesterov wrote:
> >
> > > And. I have another reason for down_write() in register/unregister.
> > > I am still not sure this is possible (I had no time to try to
> > > implement), but it seems to me we can kill the uprobe counter in
> > > mm_struct.
> >
> > You mean by making register/unregister down_write, you're exclusive with
> > munmap()
> 
> .. and with register/unregister.
> 
> Why do we need mm->uprobes_state.count? It is writeonly, except we
> check it in the DIE_INT3 notifier before anything else to avoid the
> unnecessary uprobes overhead.

and uprobe_munmap().

> Suppose we kill it, and add the new MMF_HAS_UPROBE flag instead.
> install_breakpoint() sets it unconditionally,
> uprobe_pre_sstep_notifier() checks it.

Argh, why are MMF_flags part of sched.h.. one would expect those to be
in mm.h or mm_types.h.. somewhere near struct mm.

> (And perhaps we can stop right here? I mean how often this can
>  slow down the debugger which installs int3 in the same mm?)
> 
> Now we need to clear MMF_HAS_UPROBE somehowe, when the last
> uprobe goes away. Lets ignore uprobe_map/unmap for simplicity.
>
> 	- We add another flag, MMF_UPROBE_RECALC, it is set by
> 	  remove_breakpoint().
> 
> 	- We change handle_swbp(). Ignoring all details it does:
> 
> 		if (find_uprobe(vaddr))
> 			process_uprobe();
> 		else if (test_bit(MMF_HAS_UPROBE) && test_bit(MMF_UPROBE_RECALC))
> 			recalc_mmf_uprobe_flag();
> 
> 	  where recalc_mmf_uprobe_flag() checks all vmas and either
> 	  clears both flags or MMF_UPROBE_RECALC only.
> 
> 	  This is the really slow O(n) path, but it can only happen after
> 	  unregister, and only if we hit another non-uprobe breakpoint
> 	  in the same mm.
> 
> Something like this. What do you think?

I think I can live with the simple set MMF_HAS_UPROBE and leave it at
that. The better optimization seems to be to not install breakpoints
when ->filter() excludes the task..

It looks like we currently install the breakpoint unconditionally and
only ->filter() once we hit the breakpoint, which is somewhat
sub-optimal.

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