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