Date: Tue, 28 Apr 2015 18:40:33 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Chris Metcalf <cmetcalf@ezchip.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Manfred Spraul <manfred@colorfullife.com>,
        Oleg Nesterov <oleg@redhat.com>, Kirill Tkhai <ktkhai@parallels.com>,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking
 cycles
Message-ID: <20150428164033.GJ5029@twins.programming.kicks-ass.net>
References: <20150218155904.GA27687@redhat.com>
 <54E4E479.4050003@colorfullife.com>
 <20150218224317.GC5029@twins.programming.kicks-ass.net>
 <20150219141905.GA11018@redhat.com>
 <54E77CC0.5030401@colorfullife.com>
 <20150220184551.GQ2896@worktop.programming.kicks-ass.net>
 <20150425195602.GA26676@linux.vnet.ibm.com>
 <20150426105213.GA27191@linux.vnet.ibm.com>
 <20150428143357.GF23123@twins.programming.kicks-ass.net>
 <553FACF1.2020405@ezchip.com>
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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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