On 03/02/17 15:21, Andi Kleen wrote: > From: Andi Kleen <ak@xxxxxxxxxxxxxxx> > > The Intel uncore driver can do a lot of PCI config accesses to read > performance counters. I had a situation on a 4S system where it > was spending 40+% of CPU time grabbing the pci_cfg_lock due to that. > > For 64bit x86 with MMCONFIG there isn't really any reason to take > a lock. The access is directly mapped to an underlying MMIO area, > which can fully operate lockless. > > Add a new flag that allows the PCI mid layer to skip the lock > and set it for the 64bit mmconfig code. > > There's a small risk that someone relies on this lock for synchronization, > but I think that's unlikely because there isn't really any useful > synchronization at this individual operation level. Any useful > synchronization would likely need to protect at least a > read-modify-write or similar. So I made it unconditional without opt-in. > > Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx> > --- > arch/x86/pci/mmconfig_64.c | 1 + > drivers/pci/access.c | 14 ++++++++++---- > include/linux/pci.h | 2 ++ > 3 files changed, 13 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/pci/mmconfig_64.c b/arch/x86/pci/mmconfig_64.c > index bea52496aea6..8bf10f41e626 100644 > --- a/arch/x86/pci/mmconfig_64.c > +++ b/arch/x86/pci/mmconfig_64.c > @@ -121,6 +121,7 @@ int __init pci_mmcfg_arch_init(void) > } > > raw_pci_ext_ops = &pci_mmcfg; > + pci_root_ops.ll_allowed = true; > "ll_allowed" is pretty awful naming... you spend almost all the characters telling us nothing. I spend several seconds trying to figure out what "ll" stood for, and without the context of the patch I'd have had to go a massive grep. Just call it "lockless" or something. -hpa
