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Date: Wed, 14 Feb 2001 13:29:16 +0100 (MET)
From: "Maciej W. Rozycki" <macro@ds2.pg.gda.pl>
To: Andrew Morton <andrewm@uow.edu.au>
cc: Ingo Molnar <mingo@chiara.elte.hu>,
        Manfred Spraul <manfred@colorfullife.com>,
        Frank de Lange <frank@unternet.org>,
        Martin Josefsson <gandalf@wlug.westbo.se>,
        linux-kernel@vger.kernel.org
Subject: Re: [patch] 2.4.1, 2.4.2-pre3: APIC lockups
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Organization: Technical University of Gdansk
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On Wed, 14 Feb 2001, Andrew Morton wrote:

> Tell me, please: what tradeoffs are involved in this patch?
> Obviously it works around a pretty fatal problem, but
> what are we giving away?

 The change decreases performance a bit.  For well-behaved systems the
loss is fifteen instructions: a local APIC read (uncached but supposedly
cheap), a global memory read (a cache line invalidation and fetch), seven
stack accesses (cached for sure), a taken branch and five ALU.  With the
version you have I see gcc is actually doing an extra memory read due to
the volatile APIC access presumably -- this is now fixed.

 For misdelivered interrupts the overhead is much, much bigger, involving
acquiring a spinlock and multiple (uncached and possibly slow) I/O APIC
accesses.  We may lower the overhead by undefining APIC_LOCKUP_DEBUG,
which we should do after a bit of testing.  I think we might leave
APIC_MISMATCH_DEBUG intact -- its cost is a single locked instruction
which is negligible IMO.

 Note the original version consisted of two instructions only -- a local
APIC write and "ret", sigh...

  Maciej

-- 
+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+--------------------------------------------------------------+
+        e-mail: macro@ds2.pg.gda.pl, PGP key available        +

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