Messages in this thread | | | Date | Thu, 14 Aug 2008 10:43:30 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: [RFC PATCH] x86 alternatives : fix LOCK_PREFIX race with preemptible kernel and CPU hotplug |
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Mathieu Desnoyers wrote: > * Jeremy Fitzhardinge (jeremy@goop.org) wrote: > >> Mathieu Desnoyers wrote: >> >>> * H. Peter Anvin (hpa@zytor.com) wrote: >>> >>> >>>> Mathieu Desnoyers wrote: >>>> >>>> >>>>> I can't argue about the benefit of using VM CPU pinning to manage >>>>> resources because I don't use it myself, but I ran some tests out of >>>>> curiosity to find if uncontended locks were that cheap, and it turns out >>>>> they aren't. Here are the results : >>>>> Xeon 2.0GHz >>>>> Summary >>>>> make -j1 kernel/ 33.94 +/- 0.07 34.91 +/- 0.27 2.8 % >>>>> hackbench 50 2.99 +/- 0.01 3.74 +/- 0.01 25.1 % >>>>> 1 CPU, replace smp lock prefixes with DS segment selector prefixes >>>>> 1 CPU, noreplace-smp >>>>> >>>>> >>>> For reference, could you also compare replace smp lock with NOPs? >>>> >>>> -hpa >>>> >>>> >>> Sure, here are the updated tables. Basically, they show no significant >>> difference between the NOP and the DS segment selector prefix >>> approaches. >>> >>> >> BTW, are you changing the initial prefix to DS too? Ie, are you doing a >> nop->lock->ds transition, or ds->lock->ds? >> >> J >> > > Yeah, I thought about this case yesterday, good thing you ask. > > include/asm-x86/alternative.h defines LOCK_PREFIX as : > > #define LOCK_PREFIX \ > ".section .smp_locks,\"a\"\n" \ > _ASM_ALIGN "\n" \ > _ASM_PTR "661f\n" /* address */ \ > ".previous\n" \ > "661:\n\tlock; " > > So we have the locked instructions built into the kernel, not the nop'd > one. Therefore, the only transition I am doing for my benchmarks is : > > lock->ds > > but I tried to switch back to SMP and it worked fine. >
Ah, OK. I'd thought we started unlocked, but given that I've just been disassembling the kernel and looking at the lock prefixes, that's a bit of a braino on my part.
BTW, using the ds prefix allows us to undo the hack of dealing with locked instructions with exception handlers. There was a bug where if we do lock->nop, then the address of a faulting instruction changes, so we need exception records for both the locked and unlocked forms. Using ds means the instruction size doesn't change, so we only need one exception record. I don't remember off hand where that happens.
J
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