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Date: Thu, 28 Feb 2013 10:13:45 -0500
From: Rik van Riel <riel@redhat.com>
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To: Linus Torvalds <torvalds@linux-foundation.org>
CC: Davidlohr Bueso <davidlohr.bueso@hp.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        "Vinod, Chegu" <chegu_vinod@hp.com>, "Low, Jason" <jason.low2@hp.com>,
        linux-tip-commits@vger.kernel.org,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andrew Morton <akpm@linux-foundation.org>, aquini@redhat.com,
        Michel Lespinasse <walken@google.com>, Ingo Molnar <mingo@kernel.org>
Subject: Re: [tip:core/locking] x86/smp: Move waiting on contended ticket
 lock out of line
References: <20130206150403.006e5294@cuia.bos.redhat.com> <tip-4aef331850b637169ff036ed231f0d236874f310@git.kernel.org> <CA+55aFx6N6SaRN+w=CTXJ=ZSsOE6KeS2R9sT_uhAx9h2j+hjKg@mail.gmail.com> <511BE4A3.8050607@redhat.com> <CA+55aFycMB7OZGQ54ueQ74C42beDBSdQE_fKJDV=TbjTjr_emg@mail.gmail.com> <511C1204.9040608@redhat.com> <CA+55aFwxYK-BLH0crhySHaQvAYHOH-VGUi7WTd1aq-24+WTDYw@mail.gmail.com> <511C24A6.8020409@redhat.com> <CA+55aFws80EEJQskqiP+a5J-HOGu=M1Fe=uCs50ifedVxPxT1Q@mail.gmail.com> <512E376D.70105@redhat.com> <CA+55aFzPxJ3rfZvAvrP9LxCXfDU8AAVvhMZnP-OAoe-ycc70aw@mail.gmail.com> <512E6443.9050603@redhat.com> <CA+55aFwHxv9RH75BoXYcwdKeHx+otOOpg9494TTdFwQe3biOhw@mail.gmail.com> <512E80E3.7060800@redhat.com> <CA+55aFwa0EjGG2NUDYVLVBmXJa2k81YiuNO2yggk=GLRQxhhUQ@mail.gmail.com> <512EC7F0.60103@redhat.com> <1362024397.1867.28.camel@buesod1.americas.hpqcorp.net> <CA+55aFygzjuNYiG4MTPMzimEjQ92rpS-rNJr7HD4OQCAqze5zg@mail.gmail.com>
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On 02/27/2013 11:49 PM, Linus Torvalds wrote:
> On Wed, Feb 27, 2013 at 8:06 PM, Davidlohr Bueso <davidlohr.bueso@hp.com> wrote:
>>
>> The attached file shows how the amount of sys time used by the ipc lock
>> for a 4 and 8 socket box.
>
> I have to say, even with the improvements, that looks pretty
> disgusting. It really makes me wonder if that thing couldn't be done
> better some way. Is it the SysV semaphores that this all ends up
> using, or what?
>
> That said, I think the IPC layer is just about the perfect candidate
> for things like this, because I'm afraid that nobody is ever going to
> fix it.

There's more to it than that. Userspace expects the IPC layer
to provide exclusion and/or serialization. That makes it a
rather poor candidate for parallelism.

Btw, the IPC lock is already fairly fine grained. One ipc
lock is allocated for each set of semaphores allocated through
sys_semget. Looking up those semaphores in the namespace, when
they are used later, is done under RCU.