Messages in this thread | | | Date | Wed, 25 Mar 2015 18:41:13 +0100 | From | Denys Vlasenko <> | Subject | Re: [PATCH 2/4] x86/asm/entry/64: do not TRACE_IRQS fast SYSRET64 path |
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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)
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