From: Andi Kleen <[email protected]> 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 <[email protected]> --- 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; return 1; } diff --git a/drivers/pci/access.c b/drivers/pci/access.c index db239547fefd..22552c6606c1 100644 --- a/drivers/pci/access.c +++ b/drivers/pci/access.c @@ -32,11 +32,14 @@ int pci_bus_read_config_##size \ int res; \ unsigned long flags; \ u32 data = 0; \ + bool ll_allowed = bus->ops->ll_allowed; \ if (PCI_##size##_BAD) return PCIBIOS_BAD_REGISTER_NUMBER; \ - raw_spin_lock_irqsave(&pci_lock, flags); \ + if (!ll_allowed) \ + raw_spin_lock_irqsave(&pci_lock, flags); \ res = bus->ops->read(bus, devfn, pos, len, &data); \ *value = (type)data; \ - raw_spin_unlock_irqrestore(&pci_lock, flags); \ + if (!ll_allowed) \ + raw_spin_unlock_irqrestore(&pci_lock, flags); \ return res; \ } @@ -46,10 +49,13 @@ int pci_bus_write_config_##size \ { \ int res; \ unsigned long flags; \ + bool ll_allowed = bus->ops->ll_allowed; \ if (PCI_##size##_BAD) return PCIBIOS_BAD_REGISTER_NUMBER; \ - raw_spin_lock_irqsave(&pci_lock, flags); \ + if (!ll_allowed) \ + raw_spin_lock_irqsave(&pci_lock, flags); \ res = bus->ops->write(bus, devfn, pos, len, value); \ - raw_spin_unlock_irqrestore(&pci_lock, flags); \ + if (!ll_allowed) \ + raw_spin_unlock_irqrestore(&pci_lock, flags); \ return res; \ } diff --git a/include/linux/pci.h b/include/linux/pci.h index e2d1a124216a..9b234cbc7ae1 100644 --- a/include/linux/pci.h +++ b/include/linux/pci.h @@ -612,6 +612,8 @@ struct pci_ops { void __iomem *(*map_bus)(struct pci_bus *bus, unsigned int devfn, int where); int (*read)(struct pci_bus *bus, unsigned int devfn, int where, int size, u32 *val); int (*write)(struct pci_bus *bus, unsigned int devfn, int where, int size, u32 val); + /* Set to true when pci_lock is not needed for read/write */ + bool ll_allowed; }; /* -- 2.9.3
