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/...

[Attilio Rao]

On Wed, Oct 24, 2012 at 7:14 PM, John Baldwin <jhb at freebsd.org> wrote:
> On Wednesday, October 24, 2012 11:41:24 am Attilio Rao wrote:
>> On Wed, Oct 24, 2012 at 4:36 PM, John Baldwin <jhb at 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 at freebsd.org> wrote:
>> >> > On Wed, Oct 24, 2012 at 3:45 PM, John Baldwin <jhb at 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 at freebsd.org> wrote:
>> >> >>> > On Tuesday, October 23, 2012 7:20:04 pm Andre Oppermann wrote:
>> >> >>> >> On 24.10.2012 00:15, mdf at FreeBSD.org wrote:
>> >> >>> >> > On Tue, Oct 23, 2012 at 7:41 AM, Andre Oppermann <andre at 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.

Cache line usually contains 8 words.
struct mtx is madeup only by 4 or 5 (depending if you are on 64 or 32 bits).
I think this is a no-concern and we should not encourage adding more
words to it anyway.

>> > 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.

I think that with some trickery using CTASSERT() and typeof() we may
be able to enforce sanity even with void * arguments.

Attilio


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