Hi all,
I just wanted to direct attention to further somewhat wasteful aspects
in kernel code ;)
Since this extra inb() is in IRQ handler hotpath (called at least a
couple hundred times per second without dynticks patch),
it hurts quite obviously, potentially with several hundred CPU cycles
per I/O due to slow PIC access (haven't actually profiled it, though).
This dummy op can be found in i8259.c of i386/, x86_64/ and mips/.
There seem to be no adverse effects on three systems that I tested it on:
- P4 HT
- P3/700
- Athlon on VIA chipset
Comments?
Further testing on various chipsets by more people would also be useful,
I think...
(however testing might be dangerous, of course, since it's a quite invasive
change and might even corrupt data or so!)
Thanks,
Andreas Mohr
P.S.: the outb() change is just a minor comment correction...
diff -urN linux-2.6.17-mm4.orig/arch/i386/kernel/i8259.c linux-2.6.17-mm4.my/arch/i386/kernel/i8259.c
--- linux-2.6.17-mm4.orig/arch/i386/kernel/i8259.c 2006-06-29 11:57:11.000000000 +0200
+++ linux-2.6.17-mm4.my/arch/i386/kernel/i8259.c 2006-06-29 21:17:33.000000000 +0200
@@ -191,13 +191,13 @@
cached_irq_mask |= irqmask;
handle_real_irq:
+ /* we used to first do a dummy inb() on PIC_SLAVE_IMR/PIC_MASTER_IMR
+ here, but this doesn't seem to actually be necessary... */
if (irq & 8) {
- inb(PIC_SLAVE_IMR); /* DUMMY - (do we need this?) */
outb(cached_slave_mask, PIC_SLAVE_IMR);
outb(0x60+(irq&7),PIC_SLAVE_CMD);/* 'Specific EOI' to slave */
outb(0x60+PIC_CASCADE_IR,PIC_MASTER_CMD); /* 'Specific EOI' to master-IRQ2 */
} else {
- inb(PIC_MASTER_IMR); /* DUMMY - (do we need this?) */
outb(cached_master_mask, PIC_MASTER_IMR);
outb(0x60+irq,PIC_MASTER_CMD); /* 'Specific EOI to master */
}
@@ -272,7 +272,7 @@
* out of.
*/
outb(0xff, PIC_MASTER_IMR); /* mask all of 8259A-1 */
- outb(0xff, PIC_SLAVE_IMR); /* mask all of 8259A-1 */
+ outb(0xff, PIC_SLAVE_IMR); /* mask all of 8259A-2 */
return 0;
}
-
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