Messages in this thread | | | Date | Fri, 15 Sep 2023 18:51:04 +0800 | Subject | Re: [PATCH v2 6/6] fork: Use __mt_dup() to duplicate maple tree in dup_mmap() | From | Peng Zhang <> |
| |
在 2023/9/8 04:14, Liam R. Howlett 写道: > * Peng Zhang <zhangpeng.00@bytedance.com> [230830 08:58]: >> Use __mt_dup() to duplicate the old maple tree in dup_mmap(), and then >> directly modify the entries of VMAs in the new maple tree, which can >> get better performance. The optimization effect is proportional to the >> number of VMAs. >> >> There is a "spawn" in byte-unixbench[1], which can be used to test the >> performance of fork(). I modified it slightly to make it work with >> different number of VMAs. >> >> Below are the test numbers. There are 21 VMAs by default. The first row >> indicates the number of added VMAs. The following two lines are the >> number of fork() calls every 10 seconds. These numbers are different >> from the test results in v1 because this time the benchmark is bound to >> a CPU. This way the numbers are more stable. >> >> Increment of VMAs: 0 100 200 400 800 1600 3200 6400 >> 6.5.0-next-20230829: 111878 75531 53683 35282 20741 11317 6110 3158 >> Apply this patchset: 114531 85420 64541 44592 28660 16371 9038 4831 >> +2.37% +13.09% +20.23% +26.39% +38.18% +44.66% +47.92% +52.98% > > Thanks! > > Can you include 21 in this table since it's the default? > >> >> [1] https://github.com/kdlucas/byte-unixbench/tree/master >> >> Signed-off-by: Peng Zhang <zhangpeng.00@bytedance.com> >> --- >> kernel/fork.c | 34 ++++++++++++++++++++++++++-------- >> mm/mmap.c | 14 ++++++++++++-- >> 2 files changed, 38 insertions(+), 10 deletions(-) >> >> diff --git a/kernel/fork.c b/kernel/fork.c >> index 3b6d20dfb9a8..e6299adefbd8 100644 >> --- a/kernel/fork.c >> +++ b/kernel/fork.c >> @@ -650,7 +650,6 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, >> int retval; >> unsigned long charge = 0; >> LIST_HEAD(uf); >> - VMA_ITERATOR(old_vmi, oldmm, 0); >> VMA_ITERATOR(vmi, mm, 0); >> >> uprobe_start_dup_mmap(); >> @@ -678,17 +677,39 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, >> goto out; >> khugepaged_fork(mm, oldmm); >> >> - retval = vma_iter_bulk_alloc(&vmi, oldmm->map_count); >> - if (retval) >> + /* Use __mt_dup() to efficiently build an identical maple tree. */ >> + retval = __mt_dup(&oldmm->mm_mt, &mm->mm_mt, GFP_NOWAIT | __GFP_NOWARN); > > Apparently the flags should be GFP_KERNEL here so that compaction can > run. > >> + if (unlikely(retval)) >> goto out; >> >> mt_clear_in_rcu(vmi.mas.tree); >> - for_each_vma(old_vmi, mpnt) { >> + for_each_vma(vmi, mpnt) { >> struct file *file; >> >> vma_start_write(mpnt); >> if (mpnt->vm_flags & VM_DONTCOPY) { >> vm_stat_account(mm, mpnt->vm_flags, -vma_pages(mpnt)); >> + >> + /* >> + * Since the new tree is exactly the same as the old one, >> + * we need to remove the unneeded VMAs. >> + */ >> + mas_store(&vmi.mas, NULL); >> + >> + /* >> + * Even removing an entry may require memory allocation, >> + * and if removal fails, we use XA_ZERO_ENTRY to mark >> + * from which VMA it failed. The case of encountering >> + * XA_ZERO_ENTRY will be handled in exit_mmap(). >> + */ >> + if (unlikely(mas_is_err(&vmi.mas))) { >> + retval = xa_err(vmi.mas.node); >> + mas_reset(&vmi.mas); >> + if (mas_find(&vmi.mas, ULONG_MAX)) >> + mas_store(&vmi.mas, XA_ZERO_ENTRY); >> + goto loop_out; >> + } >> + > > Storing NULL may need extra space as you noted, so we need to be careful > what happens if we don't have that space. We should have a testcase to > test this scenario. > > mas_store_gfp() should be used with GFP_KERNEL. The VMAs use GFP_KERNEL > in this function, see vm_area_dup(). > > Don't use the exit_mmap() path to undo a failed fork. You've added > checks and complications to the exit path for all tasks in the very > unlikely event that we run out of memory when we hit a very unlikely > VM_DONTCOPY flag. > > I see the issue with having a portion of the tree with new VMAs that are > accounted and a portion of the tree that has old VMAs that should not be > looked at. It was clever to use the XA_ZERO_ENTRY as a stop point, but > we cannot add that complication to the exit path and then there is the > OOM race to worry about (maybe, I am not sure since this MM isn't > active yet). I encountered some errors after implementing the scheme you mentioned below. This would also clutter fork.c and mmap.c, as some internal functions would need to be made global.
I thought of another way to put everything into maple tree. In non-RCU mode, we can remove the last half of the tree without allocating any memory. This requires modifications to the internal implementation of mas_store(). Then remove the second half of the tree like this:
mas.index = 0; mas.last = ULONGN_MAX; mas_store(&mas, NULL).
At least in non-RCU mode, we can do this, since we only need to merge some nodes, or move some items to adjacent nodes. However, this will increase the workload significantly.
> > Using what is done in exit_mmap() and do_vmi_align_munmap() as a > prototype, we can do something like the *untested* code below: > > if (unlikely(mas_is_err(&vmi.mas))) { > unsigned long max = vmi.index; > > retval = xa_err(vmi.mas.node); > mas_set(&vmi.mas, 0); > tmp = mas_find(&vmi.mas, ULONG_MAX); > if (tmp) { /* Not the first VMA failed */ > unsigned long nr_accounted = 0; > > unmap_region(mm, &vmi.mas, vma, NULL, mpnt, 0, max, max, > true); > do { > if (vma->vm_flags & VM_ACCOUNT) > nr_accounted += vma_pages(vma); > remove_vma(vma, true); > cond_resched(); > vma = mas_find(&vmi.mas, max - 1); > } while (vma != NULL); > > vm_unacct_memory(nr_accounted); > } > __mt_destroy(&mm->mm_mt); > goto loop_out; > } > > Once exit_mmap() is called, the check for OOM (no vma) will catch that > nothing is left to do. > > It might be worth making an inline function to do this to keep the fork > code clean. We should test this by detecting a specific task name and > returning a failure at a given interval: > > if (!strcmp(current->comm, "fork_test") { > ... > } > > >> continue; >> } >> charge = 0; >> @@ -750,8 +771,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, >> hugetlb_dup_vma_private(tmp); >> >> /* Link the vma into the MT */ >> - if (vma_iter_bulk_store(&vmi, tmp)) >> - goto fail_nomem_vmi_store; >> + mas_store(&vmi.mas, tmp); >> >> mm->map_count++; >> if (!(tmp->vm_flags & VM_WIPEONFORK)) >> @@ -778,8 +798,6 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, >> uprobe_end_dup_mmap(); >> return retval; >> >> -fail_nomem_vmi_store: >> - unlink_anon_vmas(tmp); >> fail_nomem_anon_vma_fork: >> mpol_put(vma_policy(tmp)); >> fail_nomem_policy: >> diff --git a/mm/mmap.c b/mm/mmap.c >> index b56a7f0c9f85..dfc6881be81c 100644 >> --- a/mm/mmap.c >> +++ b/mm/mmap.c >> @@ -3196,7 +3196,11 @@ void exit_mmap(struct mm_struct *mm) >> arch_exit_mmap(mm); >> >> vma = mas_find(&mas, ULONG_MAX); >> - if (!vma) { >> + /* >> + * If dup_mmap() fails to remove a VMA marked VM_DONTCOPY, >> + * xa_is_zero(vma) may be true. >> + */ >> + if (!vma || xa_is_zero(vma)) { >> /* Can happen if dup_mmap() received an OOM */ >> mmap_read_unlock(mm); >> return; >> @@ -3234,7 +3238,13 @@ void exit_mmap(struct mm_struct *mm) >> remove_vma(vma, true); >> count++; >> cond_resched(); >> - } while ((vma = mas_find(&mas, ULONG_MAX)) != NULL); >> + vma = mas_find(&mas, ULONG_MAX); >> + /* >> + * If xa_is_zero(vma) is true, it means that subsequent VMAs >> + * donot need to be removed. Can happen if dup_mmap() fails to >> + * remove a VMA marked VM_DONTCOPY. >> + */ >> + } while (vma != NULL && !xa_is_zero(vma)); >> >> BUG_ON(count != mm->map_count); >> >> -- >> 2.20.1 >> >
| |