From 72b252aed506b8f1a03f7abd29caef4cdf6a043b Mon Sep 17 00:00:00 2001 From: Mel Gorman Date: Fri, 4 Sep 2015 15:47:32 -0700 Subject: mm: send one IPI per CPU to TLB flush all entries after unmapping pages An IPI is sent to flush remote TLBs when a page is unmapped that was potentially accesssed by other CPUs. There are many circumstances where this happens but the obvious one is kswapd reclaiming pages belonging to a running process as kswapd and the task are likely running on separate CPUs. On small machines, this is not a significant problem but as machine gets larger with more cores and more memory, the cost of these IPIs can be high. This patch uses a simple structure that tracks CPUs that potentially have TLB entries for pages being unmapped. When the unmapping is complete, the full TLB is flushed on the assumption that a refill cost is lower than flushing individual entries. Architectures wishing to do this must give the following guarantee. If a clean page is unmapped and not immediately flushed, the architecture must guarantee that a write to that linear address from a CPU with a cached TLB entry will trap a page fault. This is essentially what the kernel already depends on but the window is much larger with this patch applied and is worth highlighting. The architecture should consider whether the cost of the full TLB flush is higher than sending an IPI to flush each individual entry. An additional architecture helper called flush_tlb_local is required. It's a trivial wrapper with some accounting in the x86 case. The impact of this patch depends on the workload as measuring any benefit requires both mapped pages co-located on the LRU and memory pressure. The case with the biggest impact is multiple processes reading mapped pages taken from the vm-scalability test suite. The test case uses NR_CPU readers of mapped files that consume 10*RAM. Linear mapped reader on a 4-node machine with 64G RAM and 48 CPUs 4.2.0-rc1 4.2.0-rc1 vanilla flushfull-v7 Ops lru-file-mmap-read-elapsed 159.62 ( 0.00%) 120.68 ( 24.40%) Ops lru-file-mmap-read-time_range 30.59 ( 0.00%) 2.80 ( 90.85%) Ops lru-file-mmap-read-time_stddv 6.70 ( 0.00%) 0.64 ( 90.38%) 4.2.0-rc1 4.2.0-rc1 vanilla flushfull-v7 User 581.00 611.43 System 5804.93 4111.76 Elapsed 161.03 122.12 This is showing that the readers completed 24.40% faster with 29% less system CPU time. From vmstats, it is known that the vanilla kernel was interrupted roughly 900K times per second during the steady phase of the test and the patched kernel was interrupts 180K times per second. The impact is lower on a single socket machine. 4.2.0-rc1 4.2.0-rc1 vanilla flushfull-v7 Ops lru-file-mmap-read-elapsed 25.33 ( 0.00%) 20.38 ( 19.54%) Ops lru-file-mmap-read-time_range 0.91 ( 0.00%) 1.44 (-58.24%) Ops lru-file-mmap-read-time_stddv 0.28 ( 0.00%) 0.47 (-65.34%) 4.2.0-rc1 4.2.0-rc1 vanilla flushfull-v7 User 58.09 57.64 System 111.82 76.56 Elapsed 27.29 22.55 It's still a noticeable improvement with vmstat showing interrupts went from roughly 500K per second to 45K per second. The patch will have no impact on workloads with no memory pressure or have relatively few mapped pages. It will have an unpredictable impact on the workload running on the CPU being flushed as it'll depend on how many TLB entries need to be refilled and how long that takes. Worst case, the TLB will be completely cleared of active entries when the target PFNs were not resident at all. [sasha.levin@oracle.com: trace tlb flush after disabling preemption in try_to_unmap_flush] Signed-off-by: Mel Gorman Reviewed-by: Rik van Riel Cc: Dave Hansen Acked-by: Ingo Molnar Cc: Linus Torvalds Signed-off-by: Sasha Levin Cc: Michal Hocko Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/rmap.c | 104 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 103 insertions(+), 1 deletion(-) (limited to 'mm/rmap.c') diff --git a/mm/rmap.c b/mm/rmap.c index 171b68768df1..326d5d89e45c 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -62,6 +62,8 @@ #include +#include + #include "internal.h" static struct kmem_cache *anon_vma_cachep; @@ -583,6 +585,89 @@ vma_address(struct page *page, struct vm_area_struct *vma) return address; } +#ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH +static void percpu_flush_tlb_batch_pages(void *data) +{ + /* + * All TLB entries are flushed on the assumption that it is + * cheaper to flush all TLBs and let them be refilled than + * flushing individual PFNs. Note that we do not track mm's + * to flush as that might simply be multiple full TLB flushes + * for no gain. + */ + count_vm_tlb_event(NR_TLB_REMOTE_FLUSH_RECEIVED); + flush_tlb_local(); +} + +/* + * Flush TLB entries for recently unmapped pages from remote CPUs. It is + * important if a PTE was dirty when it was unmapped that it's flushed + * before any IO is initiated on the page to prevent lost writes. Similarly, + * it must be flushed before freeing to prevent data leakage. + */ +void try_to_unmap_flush(void) +{ + struct tlbflush_unmap_batch *tlb_ubc = ¤t->tlb_ubc; + int cpu; + + if (!tlb_ubc->flush_required) + return; + + cpu = get_cpu(); + + trace_tlb_flush(TLB_REMOTE_SHOOTDOWN, -1UL); + + if (cpumask_test_cpu(cpu, &tlb_ubc->cpumask)) + percpu_flush_tlb_batch_pages(&tlb_ubc->cpumask); + + if (cpumask_any_but(&tlb_ubc->cpumask, cpu) < nr_cpu_ids) { + smp_call_function_many(&tlb_ubc->cpumask, + percpu_flush_tlb_batch_pages, (void *)tlb_ubc, true); + } + cpumask_clear(&tlb_ubc->cpumask); + tlb_ubc->flush_required = false; + put_cpu(); +} + +static void set_tlb_ubc_flush_pending(struct mm_struct *mm, + struct page *page) +{ + struct tlbflush_unmap_batch *tlb_ubc = ¤t->tlb_ubc; + + cpumask_or(&tlb_ubc->cpumask, &tlb_ubc->cpumask, mm_cpumask(mm)); + tlb_ubc->flush_required = true; +} + +/* + * Returns true if the TLB flush should be deferred to the end of a batch of + * unmap operations to reduce IPIs. + */ +static bool should_defer_flush(struct mm_struct *mm, enum ttu_flags flags) +{ + bool should_defer = false; + + if (!(flags & TTU_BATCH_FLUSH)) + return false; + + /* If remote CPUs need to be flushed then defer batch the flush */ + if (cpumask_any_but(mm_cpumask(mm), get_cpu()) < nr_cpu_ids) + should_defer = true; + put_cpu(); + + return should_defer; +} +#else +static void set_tlb_ubc_flush_pending(struct mm_struct *mm, + struct page *page) +{ +} + +static bool should_defer_flush(struct mm_struct *mm, enum ttu_flags flags) +{ + return false; +} +#endif /* CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH */ + /* * At what user virtual address is page expected in vma? * Caller should check the page is actually part of the vma. @@ -1220,7 +1305,24 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma, /* Nuke the page table entry. */ flush_cache_page(vma, address, page_to_pfn(page)); - pteval = ptep_clear_flush(vma, address, pte); + if (should_defer_flush(mm, flags)) { + /* + * We clear the PTE but do not flush so potentially a remote + * CPU could still be writing to the page. If the entry was + * previously clean then the architecture must guarantee that + * a clear->dirty transition on a cached TLB entry is written + * through and traps if the PTE is unmapped. + */ + pteval = ptep_get_and_clear(mm, address, pte); + + /* Potentially writable TLBs must be flushed before IO */ + if (pte_dirty(pteval)) + flush_tlb_page(vma, address); + else + set_tlb_ubc_flush_pending(mm, page); + } else { + pteval = ptep_clear_flush(vma, address, pte); + } /* Move the dirty bit to the physical page now the pte is gone. */ if (pte_dirty(pteval)) -- cgit v1.2.3