linux-kernel - Re: [PATCH 2/2] [PATCH] sched: Add smp

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Message-ID: <20150428164033.GJ5029@twins.programming.kicks-ass.net>
Date:	Tue, 28 Apr 2015 18:40:33 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Chris Metcalf <cmetcalf@...hip.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Manfred Spraul <manfred@...orfullife.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Kirill Tkhai <ktkhai@...allels.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking
 cycles

On Tue, Apr 28, 2015 at 11:53:21AM -0400, Chris Metcalf wrote:

> The reason we use two 32-bit fields on tilepro is that the only available
> atomic instruction is tns (test and set), which sets a 32-bit "1" value
> into the target memory and returns the old 32-bit value.

And you want a ticket lock as opposed to the test-and-set lock because
with 64 tiles starvation under contention is a real worry?

Where sparc32 (which only has the load-store-unsigned-byte instruction,
which is similar to your tns except it writes 0xff) has the benefit of
not actually having many CPUs you get to do 64 cpus!
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