On Wed, Aug 26 2020 at 13:53, Alexander Graf wrote:
> Commit 633260fa143 ("x86/irq: Convey vector as argument and not in ptregs")
> changed the handover logic of the vector identifier from ~vector in orig_ax
> to purely register based. Unfortunately, this field has another consumer
> in the APIC code which the commit did not touch. The net result was that
> IRQ balancing did not work and instead resulted in interrupt storms, slowing
> down the system.
The net result is an observationof the symptom but that does not explain
what the underlying technical issue is.
> This patch restores the original semantics that orig_ax contains the vector.
> When we receive an interrupt now, the actual vector number stays stored in
> the orig_ax field which then gets consumed by the APIC code.
>
> To ensure that nobody else trips over this in the future, the patch also adds
> comments at strategic places to warn anyone who would refactor the code that
> there is another consumer of the field.
>
> With this patch in place, IRQ balancing works as expected and performance
> levels are restored to previous levels.
There's a lot of 'This patch and we' in that changelog. Care to grep
for 'This patch' in Documentation/process/ ?
> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> index df8c017..22e829c 100644
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -727,7 +727,7 @@ SYM_CODE_START_LOCAL(asm_\cfunc)
> ENCODE_FRAME_POINTER
> movl %esp, %eax
> movl PT_ORIG_EAX(%esp), %edx /* get the vector from stack */
> - movl $-1, PT_ORIG_EAX(%esp) /* no syscall to restart */
> + /* keep vector on stack for APIC's irq_complete_move() */
Yes that's fixing your observed wreckage, but it introduces a worse one.
user space
-> interrupt
push vector into orig_ax (values are in the ranges of 0-127 and -128 -
255
except for the system vectors which do
not go through this code)
handle()
...
exit_to_user_mode_loop()
arch_do_signal()
/* Did we come from a system call? */
if (syscall_get_nr(current, regs) >= 0) {
---> BOOM for any vector 0-127 because syscall_get_nr()
resolves to regs->orig_ax
Going to be fun to debug.
The below nasty hack cures it, but I hate it with a passion. I'll look
deeper for a sane variant.
Thanks,
tglx
---
--- a/arch/x86/kernel/irq.c
+++ b/arch/x86/kernel/irq.c
@@ -246,7 +246,9 @@ DEFINE_IDTENTRY_IRQ(common_interrupt)
desc = __this_cpu_read(vector_irq[vector]);
if (likely(!IS_ERR_OR_NULL(desc))) {
+ regs->orig_ax = (unsigned long)vector;
handle_irq(desc, regs);
+ regs->orig_ax = -1;
} else {
ack_APIC_irq();
