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SubjectRe: [PATCH RFC/RFB] x86_64, i386: interrupt dispatch changes
Alexander van Heukelum wrote:
>
> In general: after applying the patch, latencies are more
> often seen by the rdtsctest. It also seems to cause a
> small percentage decrease in speed of hackbench.
> Looking at the latency histograms I believe this is
> a real effect, but I could not do enough boots/runs to
> make this a certainty from the runtimes alone.
>
> At least for this PC, doing hpa's suggested cleanup of
> the stub table is the right way to go for now... A
> second option would be to get rid of the stub table by
> assigning each important vector a unique handler and
> to make sure those handlers do not rely on the vector
> number at all.
>

Hi Alexander,

First of all, great job on the timing analysis. I believe this confirms
the concerns that I had about this technique.

Here is a prototype patch of the compressed IRQ stubs -- this patch
compresses them down to 7 stubs per 32-byte cache line (or part of cache
line) at the expense of a back-to-back jmp which has the potential of
being ugly on some pipelines (we can only get 4 stubs into 32 bytes
without that).

Would you be willing to run your timing test on this patch? This isn't
submission-quality since it commingles multiple changes, and it needs
some cleanup, but it should be useful for testing.

As a side benefit it eliminates some gratuitous differences between the
32- and 64-bit code.

-hpa
diff --git a/arch/x86/include/asm/hw_irq.h b/arch/x86/include/asm/hw_irq.h
index b97aecb..8de644b 100644
--- a/arch/x86/include/asm/hw_irq.h
+++ b/arch/x86/include/asm/hw_irq.h
@@ -109,9 +109,7 @@ extern asmlinkage void smp_invalidate_interrupt(struct pt_regs *);
#endif
#endif

-#ifdef CONFIG_X86_32
-extern void (*const interrupt[NR_VECTORS])(void);
-#endif
+extern void (*__initconst interrupt[NR_VECTORS-FIRST_EXTERNAL_VECTOR])(void);

typedef int vector_irq_t[NR_VECTORS];
DECLARE_PER_CPU(vector_irq_t, vector_irq);
diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
index 28b597e..ffd1a55 100644
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -619,29 +619,38 @@ END(syscall_badsys)
27:;

/*
- * Build the entry stubs and pointer table with
- * some assembler magic.
+ * Build the entry stubs and pointer table with some assembler magic.
+ * We pack 7 stubs into a single 32-byte chunk, which will fit in a
+ * single cache line on all modern x86 implementations.
*/
-.section .rodata,"a"
+ .section ".init.rodata","a"
ENTRY(interrupt)
-.text
-
+ .text
+ .balign 32
ENTRY(irq_entries_start)
RING0_INT_FRAME
-vector=0
-.rept NR_VECTORS
- ALIGN
- .if vector
+vector=FIRST_EXTERNAL_VECTOR
+.rept (NR_VECTORS-FIRST_EXTERNAL_VECTOR+6)/7
+ .balign 32
+ .rept 7
+ .if vector < NR_VECTORS
+ .if vector != FIRST_EXTERNAL_VECTOR
CFI_ADJUST_CFA_OFFSET -4
- .endif
-1: pushl $~(vector)
+ .endif
+1: pushl $(~vector+0x80) /* Note: always in signed byte range */
CFI_ADJUST_CFA_OFFSET 4
- jmp common_interrupt
- .previous
+ .if ((vector-FIRST_EXTERNAL_VECTOR)%7) != 6
+ jmp 2f
+ .endif
+ .previous
.long 1b
- .text
+ .text
vector=vector+1
+ .endif
+ .endr
+2: jmp common_interrupt
.endr
+ .balign 32
END(irq_entries_start)

.previous
@@ -654,6 +663,7 @@ END(interrupt)
*/
ALIGN
common_interrupt:
+ addl $-0x80,(%esp) /* Adjust vector into the [-256,-1] range */
SAVE_ALL
TRACE_IRQS_OFF
movl %esp,%eax
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index ddeeb10..cd0ca70 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -629,6 +629,46 @@ END(stub_rt_sigreturn)
vector already pushed) */
#define XCPT_FRAME _frame ORIG_RAX

+/*
+ * Build the entry stubs and pointer table with some assembler magic.
+ * We pack 7 stubs into a single 32-byte chunk, which will fit in a
+ * single cache line on all modern x86 implementations.
+ */
+ .section ".init.rodata","a"
+ENTRY(interrupt)
+ .text
+ .balign 32
+ENTRY(irq_entries_start)
+ INTR_FRAME
+vector=FIRST_EXTERNAL_VECTOR
+.rept (NR_VECTORS-FIRST_EXTERNAL_VECTOR+6)/7
+ .balign 32
+ .rept 7
+ .if vector < NR_VECTORS
+ .if vector != FIRST_EXTERNAL_VECTOR
+ CFI_ADJUST_CFA_OFFSET -8
+ .endif
+1: pushq $(~vector+0x80) /* Note: always in signed byte range */
+ CFI_ADJUST_CFA_OFFSET 8
+ .if ((vector-FIRST_EXTERNAL_VECTOR)%7) != 6
+ jmp 2f
+ .endif
+ .previous
+ .quad 1b
+ .text
+vector=vector+1
+ .endif
+ .endr
+2: jmp common_interrupt
+.endr
+ CFI_ENDPROC
+ .balign 32
+END(irq_entries_start)
+
+.previous
+END(interrupt)
+.previous
+
/*
* Interrupt entry/exit.
*
@@ -637,11 +677,12 @@ END(stub_rt_sigreturn)
* Entry runs with interrupts off.
*/

-/* 0(%rsp): interrupt number */
+/* 0(%rsp): ~(interrupt number)+0x80 */
.macro interrupt func
+ addq $-0x80,(%rsp) /* Adjust vector to [-256,-1] range */
cld
SAVE_ARGS
- leaq -ARGOFFSET(%rsp),%rdi # arg1 for handler
+ leaq -ARGOFFSET(%rsp),%rdi /* arg1 for handler */
pushq %rbp
/*
* Save rbp twice: One is for marking the stack frame, as usual, and the
@@ -672,7 +713,7 @@ END(stub_rt_sigreturn)
call \func
.endm

-ENTRY(common_interrupt)
+common_interrupt:
XCPT_FRAME
interrupt do_IRQ
/* 0(%rsp): oldrsp-ARGOFFSET */
diff --git a/arch/x86/kernel/irqinit_32.c b/arch/x86/kernel/irqinit_32.c
index 845aa98..607db63 100644
--- a/arch/x86/kernel/irqinit_32.c
+++ b/arch/x86/kernel/irqinit_32.c
@@ -129,7 +129,7 @@ void __init native_init_IRQ(void)
for (i = FIRST_EXTERNAL_VECTOR; i < NR_VECTORS; i++) {
/* SYSCALL_VECTOR was reserved in trap_init. */
if (i != SYSCALL_VECTOR)
- set_intr_gate(i, interrupt[i]);
+ set_intr_gate(i, interrupt[i-FIRST_EXTERNAL_VECTOR]);
}


diff --git a/arch/x86/kernel/irqinit_64.c b/arch/x86/kernel/irqinit_64.c
index ff02353..8670b3c 100644
--- a/arch/x86/kernel/irqinit_64.c
+++ b/arch/x86/kernel/irqinit_64.c
@@ -24,41 +24,6 @@
#include <asm/i8259.h>

/*
- * Common place to define all x86 IRQ vectors
- *
- * This builds up the IRQ handler stubs using some ugly macros in irq.h
- *
- * These macros create the low-level assembly IRQ routines that save
- * register context and call do_IRQ(). do_IRQ() then does all the
- * operations that are needed to keep the AT (or SMP IOAPIC)
- * interrupt-controller happy.
- */
-
-#define IRQ_NAME2(nr) nr##_interrupt(void)
-#define IRQ_NAME(nr) IRQ_NAME2(IRQ##nr)
-
-/*
- * SMP has a few special interrupts for IPI messages
- */
-
-#define BUILD_IRQ(nr) \
- asmlinkage void IRQ_NAME(nr); \
- asm("\n.text\n.p2align\n" \
- "IRQ" #nr "_interrupt:\n\t" \
- "push $~(" #nr ") ; " \
- "jmp common_interrupt\n" \
- ".previous");
-
-#define BI(x,y) \
- BUILD_IRQ(x##y)
-
-#define BUILD_16_IRQS(x) \
- BI(x,0) BI(x,1) BI(x,2) BI(x,3) \
- BI(x,4) BI(x,5) BI(x,6) BI(x,7) \
- BI(x,8) BI(x,9) BI(x,a) BI(x,b) \
- BI(x,c) BI(x,d) BI(x,e) BI(x,f)
-
-/*
* ISA PIC or low IO-APIC triggered (INTA-cycle or APIC) interrupts:
* (these are usually mapped to vectors 0x30-0x3f)
*/
@@ -73,37 +38,6 @@
*
* (these are usually mapped into the 0x30-0xff vector range)
*/
- BUILD_16_IRQS(0x2) BUILD_16_IRQS(0x3)
-BUILD_16_IRQS(0x4) BUILD_16_IRQS(0x5) BUILD_16_IRQS(0x6) BUILD_16_IRQS(0x7)
-BUILD_16_IRQS(0x8) BUILD_16_IRQS(0x9) BUILD_16_IRQS(0xa) BUILD_16_IRQS(0xb)
-BUILD_16_IRQS(0xc) BUILD_16_IRQS(0xd) BUILD_16_IRQS(0xe) BUILD_16_IRQS(0xf)
-
-#undef BUILD_16_IRQS
-#undef BI
-
-
-#define IRQ(x,y) \
- IRQ##x##y##_interrupt
-
-#define IRQLIST_16(x) \
- IRQ(x,0), IRQ(x,1), IRQ(x,2), IRQ(x,3), \
- IRQ(x,4), IRQ(x,5), IRQ(x,6), IRQ(x,7), \
- IRQ(x,8), IRQ(x,9), IRQ(x,a), IRQ(x,b), \
- IRQ(x,c), IRQ(x,d), IRQ(x,e), IRQ(x,f)
-
-/* for the irq vectors */
-static void (*__initdata interrupt[NR_VECTORS - FIRST_EXTERNAL_VECTOR])(void) = {
- IRQLIST_16(0x2), IRQLIST_16(0x3),
- IRQLIST_16(0x4), IRQLIST_16(0x5), IRQLIST_16(0x6), IRQLIST_16(0x7),
- IRQLIST_16(0x8), IRQLIST_16(0x9), IRQLIST_16(0xa), IRQLIST_16(0xb),
- IRQLIST_16(0xc), IRQLIST_16(0xd), IRQLIST_16(0xe), IRQLIST_16(0xf)
-};
-
-#undef IRQ
-#undef IRQLIST_16
-
-
-

/*
* IRQ2 is cascade interrupt to second interrupt controller
diff --git a/arch/x86/lguest/boot.c b/arch/x86/lguest/boot.c
index a5d8e1a..50a7792 100644
--- a/arch/x86/lguest/boot.c
+++ b/arch/x86/lguest/boot.c
@@ -590,7 +590,8 @@ static void __init lguest_init_IRQ(void)
* a straightforward 1 to 1 mapping, so force that here. */
__get_cpu_var(vector_irq)[vector] = i;
if (vector != SYSCALL_VECTOR) {
- set_intr_gate(vector, interrupt[vector]);
+ set_intr_gate(vector,
+ interrupt[vector-FIRST_EXTERNAL_VECTOR]);
set_irq_chip_and_handler_name(i, &lguest_irq_controller,
handle_level_irq,
"level");
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