Date: Sun, 28 Oct 2012 17:42:34 +0000 From: Attilio Rao <attilio@freebsd.org> To: Andre Oppermann <andre@freebsd.org> Cc: mdf@freebsd.org, src-committers@freebsd.org, John Baldwin <jhb@freebsd.org>, svn-src-user@freebsd.org, Jeff Roberson <jroberson@jroberson.net>, Bruce Evans <brde@optusnet.com.au> Subject: Re: svn commit: r241889 - in user/andre/tcp_workqueue/sys: arm/arm cddl/compat/opensolaris/kern cddl/contrib/opensolaris/uts/common/dtrace cddl/contrib/opensolaris/uts/common/fs/zfs ddb dev/acpica dev/... Message-ID: <CAJ-FndD40whTV=o1P8gMCaq9tKEtRg56xMKdjqnapVR6Awo7og@mail.gmail.com> In-Reply-To: <CAJ-FndAhYk4R2AD3COmkOv3vYQganGXQKuO%2BkMQLaefTQSAejQ@mail.gmail.com> References: <201210221418.q9MEINkr026751@svn.freebsd.org> <201210241136.06154.jhb@freebsd.org> <CAJ-FndAG-Qp%2B1aQvoL7YRj=R151Qe9_wNrUeOAaDsdYao_-zCQ@mail.gmail.com> <201210241414.30723.jhb@freebsd.org> <CAJ-FndAu6BGeMMbtFTLaSqy82mbhM9CVEyJ3Lb1WhAogJr59yA@mail.gmail.com> <CAJ-FndBqRpkBhCntd2aqwVYPu%2B2EHGeuXr5srLtrNNDK-ButxA@mail.gmail.com> <508965B3.2020705@freebsd.org> <CAJ-FndCMMH2Qy=rzzxagNVfgO9vF0xZY6B_vrnjmv_dXKNB5Dg@mail.gmail.com> <5089A913.2040603@freebsd.org> <CAJ-FndApwOo8xmZQyeqq0bGp8P13QWRqgmSDNg1_hbm7nrpOAQ@mail.gmail.com> <508A89EF.5070805@freebsd.org> <CAJ-FndC%2BujncG54_RMwKwdtvNCRJ2QojNuADWn59puwayCHs6A@mail.gmail.com> <CAJ-FndAhYk4R2AD3COmkOv3vYQganGXQKuO%2BkMQLaefTQSAejQ@mail.gmail.com>
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On Sat, Oct 27, 2012 at 5:27 PM, Attilio Rao <attilio@freebsd.org> wrote: > On Sat, Oct 27, 2012 at 3:35 PM, Attilio Rao <attilio@freebsd.org> wrote: >> On 10/26/12, Andre Oppermann <andre@freebsd.org> wrote: >>> On 26.10.2012 08:27, Attilio Rao wrote: >>>> On Thu, Oct 25, 2012 at 10:03 PM, Andre Oppermann <andre@freebsd.org> >>>> wrote: >>>>> On 25.10.2012 18:21, Attilio Rao wrote: >>>>>> Did you see my (and also Jeff) objection to your proposal about this? >>>>>> You are deliberating ignoring this? >>>>> >>>>> >>>>> Well, I'm allowed to have a different opinion, am I? I'm not ignoring >>>>> your objection in the sense as I'm not trying to commit any of this to >>>>> HEAD while it is disputed. >>>>> >>>>> Mind you this whole conversation was started because I was trying to >>>>> solve >>>>> a problem with unfortunate cache line sharing for global mutexes in the >>>>> kernel .bss section on my *personal* svn branch. >>>> >>>> Andre, >>>> I'm sorry if you felt I was being harsh or confrontative. This was >>>> really not my intention and I apologize. >>> >>> I apologize too for being a bit difficult and taking some time understand >>> the differences in __aligned() regarding padding behavior. >>> >>>> Said that, I fail in seeing a proper technical discussion on your side >>>> on how what I propose is "overdoing it" or how do you plan to address >>>> the concerns people are raising with your proposal of bumping all lock >>>> sizes indiscriminately. >>> >>> I'm wary of micro-optimizing and generally prefer clean and for a >>> reader obvious approaches. That said my assumption on the distribution >>> of mutex use cases in the kernel was wrong. By counting from a grep >>> it seems that about half of the mutexes could possibly benefit from >>> being padded and the other half doesn't because it is in structures >>> and next to its data. >> >> Besides that, you are likely misunderstanding something about what I >> propose: what I'm proposing is completely transparent to developers. >> You will just need to declare a mtx like: >> >> struct mtx_unshare Giant; >> >> and then you can use the mtx(9) interface on it without any issue. I >> don't see how this is less clean than what you propose. It just >> enables the alignment/padding on a selection basis rather than >> indiscriminately. >> >>>> However, here is the first half of the patch I'd like to see in: >>>> http:///www.freebsd.org/~attilio/mtx_decoupled.patch >>> > >>>> This is just the part to give the ability to crunch different >>>> structures to the mtx KPI. Please note that from the users perspective >>>> the mtx KPI remains absolutely the same, so there is theoretically no >>>> KPI discontinuity, the support is absolutely transparent. >>> >>> This seems rather complicated. Instead of mtxlock2mtx() wouldn't >>> __containerof() work just as well? The __DEVOLATILE() looks a bit >>> dangerous. Are you sure the compiler won't reorder things it should >>> not? >> >> What do you mean with "rather complicated"? >> For the users of the primitive nothing changes at all. >> For the people that might read the code it is pretty much >> self-explanatory, in particular if you know how lock classes work in >> our locking scheme. Maybe I can add a comment or two to clarify. > > Here we go with further comments tweaks. > Also, in order to make it a complete NOP from KPI perspective I had to > change the way the mtx_assert() wrapper was implemented as in v1 it > wasn't correctly handling the const qualifier. > I think the result is better now and you should refer to this patch for reviews: > http://www.freebsd.org/~attilio/mtx_decoupled2.patch BTW, the mtx_sysuninit() introduction can be avoided by using this other trick: #define MTX_SYSINIT(name, mtx, desc, opts) \ static struct mtx_args name##_args = { \ (mtx), \ (desc), \ (opts) \ }; \ SYSINIT(name##_mtx_sysinit, SI_SUB_LOCK, SI_ORDER_MIDDLE, \ mtx_sysinit, &name##_args); \ SYSUNINIT(name##_mtx_sysuninit, SI_SUB_LOCK, SI_ORDER_MIDDLE, \ _mtx_destroy, __DEVOLATILE(void *, &(mtx)->mtx_lock)) I'm just not sure that I would like the use of __DEVOLATILE() even if it would help in this case when introducing MTX_SYSINIT_UNSHARE() because we will just need to reuse the same code. Also, the more I think about this the more I feel convinced that mtxlock2mtx() should be static in kern_mutex.c. I can simply add a note to _mutex.h as a reminder for it. Attilio -- Peace can only be achieved by understanding - A. Einstein
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