Date: Wed, 24 Oct 2012 16:45:07 +0100 From: Attilio Rao <attilio@freebsd.org> To: Gleb Smirnoff <glebius@freebsd.org> Cc: mdf@freebsd.org, src-committers@freebsd.org, Andre Oppermann <andre@freebsd.org>, John Baldwin <jhb@freebsd.org>, svn-src-user@freebsd.org, 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-FndAF8ghAnFJMCizzmkE7HO0_qUBLOuCyHSwB2PsFwV6tiQ@mail.gmail.com> In-Reply-To: <20121024154302.GH70741@FreeBSD.org> References: <201210221418.q9MEINkr026751@svn.freebsd.org> <CAJ-FndC=zV%2BHN1wr_CnSEY93VHT--w9cYPMhH8P53y%2BLvBSO7g@mail.gmail.com> <CAJ-FndCDeB7w30PgSwOpMbWT6e%2BR7iQHfa8PU8Pn0NLagCiJxA@mail.gmail.com> <201210241136.06154.jhb@freebsd.org> <20121024154302.GH70741@FreeBSD.org>
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On Wed, Oct 24, 2012 at 4:43 PM, Gleb Smirnoff <glebius@freebsd.org> wrote:
> On Wed, Oct 24, 2012 at 11:36:06AM -0400, John Baldwin wrote:
> J> On Wednesday, October 24, 2012 11:24:22 am Attilio Rao wrote:
> J> > On Wed, Oct 24, 2012 at 4:09 PM, Attilio Rao <attilio@freebsd.org> wrote:
> J> > > On Wed, Oct 24, 2012 at 3:45 PM, John Baldwin <jhb@freebsd.org> wrote:
> J> > >> On Wednesday, October 24, 2012 10:34:34 am Attilio Rao wrote:
> J> > >>> On Wed, Oct 24, 2012 at 3:05 PM, John Baldwin <jhb@freebsd.org> wrote:
> J> > >>> > On Tuesday, October 23, 2012 7:20:04 pm Andre Oppermann wrote:
> J> > >>> >> On 24.10.2012 00:15, mdf@FreeBSD.org wrote:
> J> > >>> >> > On Tue, Oct 23, 2012 at 7:41 AM, Andre Oppermann <andre@freebsd.org>
> J> > >> wrote:
> J> > >>> >> >> Struct mtx and MTX_SYSINIT always occur as pair next to each other.
> J> > >>> >> >
> J> > >>> >> > That doesn't matter. Language basics like variable definitions should
> J> > >>> >> > not be obscured by macros. It either takes longer to figure out what
> J> > >>> >> > a variable is (because one needs to look up the definition of the
> J> > >>> >> > macro) or makes it almost impossible (because now e.g. cscope doesn't
> J> > >>> >> > know this is a variable definition.
> J> > >>> >>
> J> > >>> >> Sigh, cscope doesn't expand macros?
> J> > >>> >>
> J> > >>> >> Is there a way to do the cache line alignment in a sane way without
> J> > >>> >> littering __aligned(CACHE_LINE_SIZE) all over the place?
> J> > >>> >
> J> > >>> > I was hoping to do something with an anonymous union or some such like:
> J> > >>> >
> J> > >>> > union mtx_aligned {
> J> > >>> > struct mtx;
> J> > >>> > char[roundup2(sizeof(struct mtx), CACHE_LINE_SIZE)];
> J> > >>> > }
> J> > >>> >
> J> > >>> > I don't know if there is a useful way to define an 'aligned mutex' type
> J> > >>> > that will transparently map to a 'struct mtx', e.g.:
> J> > >>> >
> J> > >>> > typedef struct mtx __aligned(CACHE_LINE_SIZE) aligned_mtx_t;
> J> > >>>
> J> > >>> Unfortunately that doesn't work as I've verified with alc@ few months ago.
> J> > >>> The __aligned() attribute only works with structures definition, not
> J> > >>> objects declaration.
> J> > >>
> J> > >> Are you saying that the typedef doesn't (I expect it doesn't), or that this
> J> > >> doesn't:
> J> > >>
> J> > >> struct mtx foo __aligned(CACHE_LINE_SIZE);
> J> > >
> J> > > I meant to say that such notation won't address the padding issue
> J> > > which is as import as the alignment. Infact, for sensitive locks,
> J> > > having just an aligned object is not really useful if the cacheline
> J> > > gets shared.
> J> > > In the end you will need to use explicit padding or use __aligned in
> J> > > the struct definition, which cannot be used as a general pattern.
> J> >
> J> > The quickest way I see this can be made general is to have a specific
> J> > struct defined in sys/_mutex.h like that
> J> >
> J> > struct mtx_unshare {
> J> > struct mtx lock;
> J> > char _pad[CACHE_LINE_SIZE - sizeof(struct mtx)];
> J> > } __aligned(CACHE_LINE_SIZE);
> J>
> J> I think instead you want my union above that uses roundup2 in case a lock
> J> eats up multiple cache lines:
> J>
> J> union mtx_foo {
> J> struct mtx lock;
> J> char junk[roundup2(sizeof(struct mtx), CACHE_LINE_SIZE)];
> J> } __aligned_CACHE_LINE_SIZE;
> J>
> J> > then let mtx_* functions to accept void ptrs and cast them to struct
> J> > mtx as long as the functions enter.
> J>
> J> Eh, that removes all compile time type checks. That seems very dubious to me.
>
> I think that we should first get benchmarking results, and only then try
> to evolve an API for cache aligned mutexes.
>
> As an option we can allocate mutexes from cache aligned uma zone dynamically,
> to avoid all these syntax acrobatics.
There are several objections to this. Quicker that came to my mind:
- Some locks needs to be ready before UMA subsystem is setup
- On arches where the KVA is already scarce (i386, powerpc, etc.) this
is going to be completely overkill
Attilio
--
Peace can only be achieved by understanding - A. Einstein
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