svn commit: r242014 - head/sys/kern

[Andre Oppermann]

On 24.10.2012 20:56, Jim Harris wrote:
> On Wed, Oct 24, 2012 at 11:41 AM, Adrian Chadd <adrian at freebsd.org> wrote:
>> On 24 October 2012 11:36, Jim Harris <jimharris at freebsd.org> wrote:
>>
>>>    Pad tdq_lock to avoid false sharing with tdq_load and tdq_cpu_idle.
>>
>> Ok, but..
>>
>>
>>>          struct mtx      tdq_lock;               /* run queue lock. */
>>> +       char            pad[64 - sizeof(struct mtx)];
>>
>> .. don't we have an existing compile time macro for the cache line
>> size, which can be used here?
>
> Yes, but I didn't use it for a couple of reasons:
>
> 1) struct tdq itself is currently using __aligned(64), so I wanted to
> keep it consistent.
> 2) CACHE_LINE_SIZE is currently defined as 128 on x86, due to
> NetBurst-based processors having 128-byte cache sectors a while back.
> I had planned to start a separate thread on arch@ about this today on
> whether this was still appropriate.

See also the discussion on svn-src-all regarding global struct mtx
alignment.

Thank you for proving my point. ;)

Let's go back and see how we can do this the sanest way.  These are
the options I see at the moment:

  1. sprinkle __aligned(CACHE_LINE_SIZE) all over the place
  2. use a macro like MTX_ALIGN that can be SMP/UP aware and in
     the future possibly change to a different compiler dependent
     align attribute
  3. embed __aligned(CACHE_LINE_SIZE) into struct mtx itself so it
     automatically gets aligned in all cases, even when dynamically
     allocated.

Personally I'm undecided between #2 and #3.  #1 is ugly.  In favor
of #3 is that there possibly isn't any case where you'd actually
want the mutex to share a cache line with anything else, even a data
structure.

-- 
Andre