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SubjectRe: Performance regression from switching lock to rw-sem for anon-vma tree
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On Mon, 2013-08-12 at 20:52 +0200, Ingo Molnar wrote:
> * Tim Chen <tim.c.chen@linux.intel.com> wrote:
>
> > On Tue, 2013-08-06 at 16:55 -0700, Davidlohr Bueso wrote:
> >
> > > I got good numbers, recovering the performance drop I noticed with the
> > > i_mmap_mutex to rwsem patches.
> >
> > That's good. I remembered that the earlier version of the patch not
> > only recovered the performance drop, but also provide some boost when
> > you switch from i_mmap_mutex to rwsem for aim7. Do you see similar
> > boost with this version?
> >
> > > Looking forward to a more upstreamable
> > > patchset that deals with this work, including the previous patches.
> > >
> > > One thing that's bugging me about this series though is the huge amount
> > > of duplicated code being introduced to rwsems from mutexes. We can share
> > > common functionality such as mcs locking (perhaps in a new file under
> > > lib/), can_spin_on_owner() and owner_running(), perhaps moving those
> > > functions into sheduler code, were AFAIK they were originally.
> >
> > I think that MCS locking is worth breaking out as its
> > own library. After we've done that, the rest of
> > the duplication are minimal. It is easier
> > to keep them separate as there are some rwsem
> > specific logic that may require tweaking
> > to can_spin_on_owner and owner_running.
>
> That's what I would strongly suggest to be the approach of these patches:
> first the MCS locking factoring out, then changes in rwsem behavior.
>
> I'd suggest the librarization should be done using inlines or so, so that
> we don't touch the current (pretty good) mutex.o code generation. I.e.
> code library only on the source code level.
>
> Done that way we could also apply the librarization first, without having
> to worry about performance aspects. Having the code shared will also make
> sure that an improvement to the mutex slowpaths automatically carries over
> into rwems and vice versa.

Ingo and Davidlohr,

Thanks for your feedbacks. I'll spin off a set of new patches to
incorporate your suggestions later.

Tim



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