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

Denys Vlasenko
2015-03-25T17:41:56Z

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

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