Date: Wed, 24 Oct 2012 14:14:30 -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: <201210241414.30723.jhb@freebsd.org> In-Reply-To: <CAJ-FndAG-Qp%2B1aQvoL7YRj=R151Qe9_wNrUeOAaDsdYao_-zCQ@mail.gmail.com> References: <201210221418.q9MEINkr026751@svn.freebsd.org> <201210241136.06154.jhb@freebsd.org> <CAJ-FndAG-Qp%2B1aQvoL7YRj=R151Qe9_wNrUeOAaDsdYao_-zCQ@mail.gmail.com>
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On Wednesday, October 24, 2012 11:41:24 am Attilio Rao wrote:
> On Wed, Oct 24, 2012 at 4:36 PM, John Baldwin <jhb@freebsd.org> wrote:
> > 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:
>
> Do you think locks can eat more than one cacheline? This would be
> absolutely killer for performance.
Not the lock cookie, but 'struct lock_object', etc. aren't entirely trivial.
If you had a 32-bit platform with a 16-byte cache line size I wouldn't be
surprised if the entire structure spilled over a cacheline.
> > 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.
>
> Well right now fast path already has a fair amount of macros wrapping
> the operations, which don't really enforce any type checks.
Sure they do. They still call a function that takes a 'struct mtx *' even
if it isn't called in the fast path. If you pass a 'struct sx *' to
mtx_lock() it will fail to compile. That needs to stay that way.
--
John Baldwin
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