Date: Wed, 24 Oct 2012 21:16:56 +0200 From: Andre Oppermann <andre@freebsd.org> To: Jim Harris <jim.harris@gmail.com> Cc: svn-src-head@freebsd.org, Adrian Chadd <adrian@freebsd.org>, src-committers@freebsd.org, svn-src-all@freebsd.org Subject: Re: svn commit: r242014 - head/sys/kern Message-ID: <50883EA8.1010308@freebsd.org> In-Reply-To: <CAJP=Hc9XmvfW3MrDjvK15OAx1fyfjPk%2BEhqHUOzoEpChu5imtg@mail.gmail.com> References: <201210241836.q9OIafqo073002@svn.freebsd.org> <CAJ-VmonpdJ445hXVaoHqFgS0v7QRwqHWodQrVHm2CN9T661www@mail.gmail.com> <CAJP=Hc9XmvfW3MrDjvK15OAx1fyfjPk%2BEhqHUOzoEpChu5imtg@mail.gmail.com>
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On 24.10.2012 20:56, Jim Harris wrote:
> On Wed, Oct 24, 2012 at 11:41 AM, Adrian Chadd <adrian@freebsd.org> wrote:
>> On 24 October 2012 11:36, Jim Harris <jimharris@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
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