Messages in this thread | | | Date | Wed, 25 Mar 2015 19:04:41 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH 2/4] x86/asm/entry/64: do not TRACE_IRQS fast SYSRET64 path |
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* 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
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