On Fri, Apr 12, 2024 at 04:14:16AM +0100, Matthew Wilcox wrote: > Suren, what would you think to this? > > diff --git a/mm/memory.c b/mm/memory.c > index 6e2fe960473d..e495adcbe968 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -5821,15 +5821,6 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm, > if (!vma_start_read(vma)) > goto inval; > > - /* > - * find_mergeable_anon_vma uses adjacent vmas which are not locked. > - * This check must happen after vma_start_read(); otherwise, a > - * concurrent mremap() with MREMAP_DONTUNMAP could dissociate the VMA > - * from its anon_vma. > - */ > - if (unlikely(vma_is_anonymous(vma) && !vma->anon_vma)) > - goto inval_end_read; > - > /* Check since vm_start/vm_end might change before we lock the VMA */ > if (unlikely(address < vma->vm_start || address >= vma->vm_end)) > goto inval_end_read; > > That takes a few insns out of the page fault path (good!) at the cost > of one extra trip around the fault handler for the first fault on an > anon vma. It makes the file & anon paths more similar to each other > (good!) > > We'd need some data to be sure it's really a win, but less code is > always good. Intel's 0day got back to me with data and it's ridiculously good. Headline figure: over 3x throughput improvement with vm-scalability https://lore.kernel.org/all/202404261055.c5e24608-oliver.sang@xxxxxxxxx/ I can't see why it's that good. It shouldn't be that good. I'm seeing big numbers here: 4366 ± 2% +565.6% 29061 perf-stat.overall.cycles-between-cache-misses and the code being deleted is only checking vma->vm_ops and vma->anon_vma. Surely that cache line is referenced so frequently during pagefault that deleting a reference here will make no difference at all? We've clearly got an inlining change. viz: 72.57 -72.6 0.00 perf-profile.calltrace.cycles-pp.exc_page_fault.asm_exc_page_fault.do_access 73.28 -72.6 0.70 perf-profile.calltrace.cycles-pp.asm_exc_page_fault.do_access 72.55 -72.5 0.00 perf-profile.calltrace.cycles-pp.do_user_addr_fault.exc_page_fault.asm_exc_page_fault.do_access 69.93 -69.9 0.00 perf-profile.calltrace.cycles-pp.lock_mm_and_find_vma.do_user_addr_fault.exc_page_fault.asm_exc_page_fault.do_access 69.12 -69.1 0.00 perf-profile.calltrace.cycles-pp.down_read_killable.lock_mm_and_find_vma.do_user_addr_fault.exc_page_fault.asm_exc_page_fault 68.78 -68.8 0.00 perf-profile.calltrace.cycles-pp.rwsem_down_read_slowpath.down_read_killable.lock_mm_and_find_vma.do_user_addr_fault.exc_page_fault 65.78 -65.8 0.00 perf-profile.calltrace.cycles-pp._raw_spin_lock_irq.rwsem_down_read_slowpath.down_read_killable.lock_mm_and_find_vma.do_user_addr_fault 65.43 -65.4 0.00 perf-profile.calltrace.cycles-pp.native_queued_spin_lock_slowpath._raw_spin_lock_irq.rwsem_down_read_slowpath.down_read_killable.lock_mm_and_find_vma 11.22 +86.5 97.68 perf-profile.calltrace.cycles-pp.down_write_killable.vm_mmap_pgoff.ksys_mmap_pgoff.do_syscall_64.entry_SYSCALL_64_after_hwframe 11.14 +86.5 97.66 perf-profile.calltrace.cycles-pp.rwsem_down_write_slowpath.down_write_killable.vm_mmap_pgoff.ksys_mmap_pgoff.do_syscall_64 3.17 ± 2% +94.0 97.12 perf-profile.calltrace.cycles-pp.osq_lock.rwsem_optimistic_spin.rwsem_down_write_slowpath.down_write_killable.vm_mmap_pgoff 3.45 ± 2% +94.1 97.59 perf-profile.calltrace.cycles-pp.rwsem_optimistic_spin.rwsem_down_write_slowpath.down_write_killable.vm_mmap_pgoff.ksys_mmap_pgoff 0.00 +98.2 98.15 perf-profile.calltrace.cycles-pp.vm_mmap_pgoff.ksys_mmap_pgoff.do_syscall_64.entry_SYSCALL_64_after_hwframe 0.00 +98.2 98.16 perf-profile.calltrace.cycles-pp.ksys_mmap_pgoff.do_syscall_64.entry_SYSCALL_64_after_hwframe so maybe the compiler has been able to eliminate some loads from contended cachelines? 703147 -87.6% 87147 ± 2% perf-stat.ps.context-switches 663.67 ± 5% +7551.9% 50783 vm-scalability.time.involuntary_context_switches 1.105e+08 -86.7% 14697764 ± 2% vm-scalability.time.voluntary_context_switches indicates to me that we're taking the mmap rwsem far less often (those would be accounted as voluntary context switches). So maybe the cache miss reduction is a consequence of just running for longer before being preempted. I still don't understand why we have to take the mmap_sem less often. Is there perhaps a VMA for which we have a NULL vm_ops, but don't set an anon_vma on a page fault?
