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SubjectRe: [patch] 2.4.1, 2.4.2-pre3: APIC lockups
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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