Date: Wed, 24 Oct 2012 20:49:50 +0100 From: Attilio Rao <attilio@freebsd.org> To: Andre Oppermann <andre@freebsd.org> Cc: svn-src-head@freebsd.org, Adrian Chadd <adrian@freebsd.org>, src-committers@freebsd.org, Jim Harris <jim.harris@gmail.com>, svn-src-all@freebsd.org Subject: Re: svn commit: r242014 - head/sys/kern Message-ID: <CAJ-FndAJwuD=oUHyVK4xHoZNrf0r%2Bq2WxZ9rUMW%2B-zzMo_8QuA@mail.gmail.com> In-Reply-To: <50883EA8.1010308@freebsd.org> References: <201210241836.q9OIafqo073002@svn.freebsd.org> <CAJ-VmonpdJ445hXVaoHqFgS0v7QRwqHWodQrVHm2CN9T661www@mail.gmail.com> <CAJP=Hc9XmvfW3MrDjvK15OAx1fyfjPk%2BEhqHUOzoEpChu5imtg@mail.gmail.com> <50883EA8.1010308@freebsd.org>
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On Wed, Oct 24, 2012 at 8:16 PM, Andre Oppermann <andre@freebsd.org> wrote: > 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 This is wrong because it doesn't give padding. > 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 What is this macro supposed to do? I don't understand that from your description. > 3. embed __aligned(CACHE_LINE_SIZE) into struct mtx itself so it > automatically gets aligned in all cases, even when dynamically > allocated. This works but I think it is overkill for structures including sleep mutexes which are the vast majority. So I wouldn't certainly be in favor of such a patch. Attilio -- Peace can only be achieved by understanding - A. Einstein
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