Re: [PATCH 2/4] x86/asm/entry/64: do not TRACE_IRQS fast SYSRET64 path

Ingo Molnar
2015-03-25T18:04:50Z

* Denys Vlasenko <dvlasenk@redhat.com> wrote:

> On 03/25/2015 06:29 PM, Ingo Molnar wrote:
> >
> > * Denys Vlasenko <dvlasenk@redhat.com> wrote:
> >
> >> SYSRET code path has a small irq-off block.
> >> On this code path, TRACE_IRQS_ON can't be called right before interrupts
> >> are enabled for real, we can't clobber registers there.
> >> So current code does it earlier, in a safe place.
> >>
> >> But with this, TRACE_IRQS_OFF/ON frames just two fast instructions,
> >> which is ridiculous: now most of irq-off block is _outside_ of the framing.
> >>
> >> Do the same thing that we do on SYSCALL entry: do not track this irq-off block,
> >> it is very small to ever cause noticeable irq latency.
> >>
> >> Be careful: make sure that "jnz int_ret_from_sys_call_irqs_off" now does
> >> invoke TRACE_IRQS_OFF - move int_ret_from_sys_call_irqs_off label before
> >> TRACE_IRQS_OFF.
> >
> >> @@ -345,8 +346,8 @@ tracesys_phase2:
> >> */
> >> GLOBAL(int_ret_from_sys_call)
> >> DISABLE_INTERRUPTS(CLBR_NONE)
> >> - TRACE_IRQS_OFF
> >> int_ret_from_sys_call_irqs_off:
> >> + TRACE_IRQS_OFF
> >> movl $_TIF_ALLWORK_MASK,%edi
> >> /* edi: mask to check */
> >
> > This latter trick absolutely needs a comment, to keep future lockdep
> > developers from wondering about the mismatch and the weird label
> > placement ...
>
> Unsure how to format it.
>
> How about:
>
>
> DISABLE_INTERRUPTS(CLBR_NONE)
> int_ret_from_sys_call_irqs_off: /* jumps come here with irqs off */
> TRACE_IRQS_OFF

Why not something like 'jumps come here from the irqs-off SYSRET
path'?

>
>
>
> (In truth, there is only one jump as of now, but using pliral
> "jumps" if that would change)

I'd also put a comment to the actual sysret IRQ-disablement that we
are skipping with the annotation. Explain that it's an optimization
for a visible irqs-off path that needs no annotation - and that the
moment something complex is done in that path, this optimization loses
its validity.

Thanks,

Ingo