From 4c21e2f2441dc5fbb957b030333f5a3f2d02dea7 Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Sat, 29 Oct 2005 18:16:40 -0700 Subject: [PATCH] mm: split page table lock Christoph Lameter demonstrated very poor scalability on the SGI 512-way, with a many-threaded application which concurrently initializes different parts of a large anonymous area. This patch corrects that, by using a separate spinlock per page table page, to guard the page table entries in that page, instead of using the mm's single page_table_lock. (But even then, page_table_lock is still used to guard page table allocation, and anon_vma allocation.) In this implementation, the spinlock is tucked inside the struct page of the page table page: with a BUILD_BUG_ON in case it overflows - which it would in the case of 32-bit PA-RISC with spinlock debugging enabled. Splitting the lock is not quite for free: another cacheline access. Ideally, I suppose we would use split ptlock only for multi-threaded processes on multi-cpu machines; but deciding that dynamically would have its own costs. So for now enable it by config, at some number of cpus - since the Kconfig language doesn't support inequalities, let preprocessor compare that with NR_CPUS. But I don't think it's worth being user-configurable: for good testing of both split and unsplit configs, split now at 4 cpus, and perhaps change that to 8 later. There is a benefit even for singly threaded processes: kswapd can be attacking one part of the mm while another part is busy faulting. Signed-off-by: Hugh Dickins Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/Kconfig | 13 +++++++++++++ 1 file changed, 13 insertions(+) (limited to 'mm/Kconfig') diff --git a/mm/Kconfig b/mm/Kconfig index 391ffc54d136..f35a550ba4b9 100644 --- a/mm/Kconfig +++ b/mm/Kconfig @@ -111,3 +111,16 @@ config SPARSEMEM_STATIC config SPARSEMEM_EXTREME def_bool y depends on SPARSEMEM && !SPARSEMEM_STATIC + +# Heavily threaded applications may benefit from splitting the mm-wide +# page_table_lock, so that faults on different parts of the user address +# space can be handled with less contention: split it at this NR_CPUS. +# Default to 4 for wider testing, though 8 might be more appropriate. +# ARM's adjust_pte (unused if VIPT) depends on mm-wide page_table_lock. +# PA-RISC's debug spinlock_t is too large for the 32-bit struct page. +# +config SPLIT_PTLOCK_CPUS + int + default "4096" if ARM && !CPU_CACHE_VIPT + default "4096" if PARISC && DEBUG_SPINLOCK && !64BIT + default "4" -- cgit v1.2.3-59-g8ed1b