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SubjectRe: [RFC 0/6] uprobes: kill uprobes_srcu/uprobe_srcu_id
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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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