Re: [tip:core/locking] x86/smp: Move waiting on contended ticketlock out of line

From: Rik van Riel
Date: Thu Feb 28 2013 - 10:15:10 EST

On 02/27/2013 11:49 PM, Linus Torvalds wrote:
On Wed, Feb 27, 2013 at 8:06 PM, Davidlohr Bueso <davidlohr.bueso@xxxxxx> 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.
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