linux-kernel - Re: [RFC][PATCH 6/8] mm: handle_speculative

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Date:	Fri, 8 Jan 2010 14:43:02 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Christoph Lameter <cl@...ux-foundation.org>
cc:	Peter Zijlstra <peterz@...radead.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Minchan Kim <minchan.kim@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"hugh.dickins" <hugh.dickins@...cali.co.uk>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault()

On Fri, 8 Jan 2010, Christoph Lameter wrote:
> 
> And I made the point that starvation was a hardware issue due to immature
> cacheline handling. Now the software patchup job for the hardware breakage
> is causing regressions for everyone.

Well, in all fairness, (a) existing hardware doesn't do a good job, and 
would have a really hard time doing so in general (ie the whole issue of 
on-die vs directly-between-sockets vs between-complex-fabric), and (b) in 
this case, the problem really was that the x86-64 rwsems were badly 
implemented.

The fact that somebody _thought_ that it might be ok to do them with 
spinlocks and had done some limited testing without ever hitting the 
problem spot (probably never having tested any amount of contention at 
all) is immaterial. We should have had real native rwsemaphores for 
x86-64, and complaining about the fallback sucking under load is kind of 
pointless.

			Linus
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