On 03/25/2015 07:04 PM, Ingo Molnar wrote: >>> * Denys Vlasenko <dvlas...@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'?
Ok. I'll send v2 for patches 1 and 2. (Patch 1 will be expanded, there is another instance of jump needing the same treatment.) >> >> >> >> (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. Patch does add such a comment: + /* + * We do not frame this tiny irq-off block with TRACE_IRQS_OFF/ON, + * it is too small to ever cause noticeable irq latency. + */ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
