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Date: Mon, 31 Jul 2023 20:59:33 +0800
From: Peng Zhang <zhangpeng.00@...edance.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc: linux-mm@...ck.org, avagin@...il.com, npiggin@...il.com,
mathieu.desnoyers@...icios.com, peterz@...radead.org,
michael.christie@...cle.com, surenb@...gle.com, brauner@...nel.org,
willy@...radead.org, akpm@...ux-foundation.org,
linux-fsdevel@...r.kernel.org,
Peng Zhang <zhangpeng.00@...edance.com>, corbet@....net,
linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 11/11] fork: Use __mt_dup() to duplicate maple tree in
dup_mmap()
在 2023/7/27 01:06, Liam R. Howlett 写道:
> * Peng Zhang <zhangpeng.00@...edance.com> [230726 04:10]:
>> 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. dup_mmap() is used by fork(), so this patch
>> optimizes fork(). The optimization effect is proportional to the number
>> of VMAs.
>>
>> Due to the introduction of this method, the optimization in
>> (maple_tree: add a fast path case in mas_wr_slot_store())[1] no longer
>> has an effect here, but it is also an optimization of the maple tree.
>>
>> There is a unixbench test suite[2] where 'spawn' is used to test fork().
>> 'spawn' only has 23 VMAs by default, so I tweaked the benchmark code a
>> bit to use mmap() to control the number of VMAs. Therefore, the
>> performance under different numbers of VMAs can be measured.
>>
>> Insert code like below into 'spawn':
>> for (int i = 0; i < 200; ++i) {
>> size_t size = 10 * getpagesize();
>> void *addr;
>>
>> if (i & 1) {
>> addr = mmap(NULL, size, PROT_READ,
>> MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>> } else {
>> addr = mmap(NULL, size, PROT_WRITE,
>> MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>> }
>> if (addr == MAP_FAILED)
>> ...
>> }
>>
>> Based on next-20230721, use 'spawn' under 23, 203, and 4023 VMAs, test
>> 4 times in 30 seconds each time, and get the following numbers. These
>> numbers are the number of fork() successes in 30s (average of the best
>> 3 out of 4). By the way, based on next-20230725, I reverted [1], and
>> tested it together as a comparison. In order to ensure the reliability
>> of the test results, these tests were run on a physical machine.
>>
>> 23VMAs 223VMAs 4023VMAs
>> revert [1]: 159104.00 73316.33 6787.00
>
> You can probably remove the revert benchmark from this since there is no
> reason to revert the previous change. The change is worth while on its
> own, so it's better to have the numbers more clear by having with and
> without this series.
I will remove it.
>
>>
>> +0.77% +0.42% +0.28%
>> next-20230721: 160321.67 73624.67 6806.33
>>
>> +2.77% +15.42% +29.86%
>> apply this: 164751.67 84980.33 8838.67
>
> What is the difference between using this patch with mas_replace_entry()
> and mas_store_entry()?
I haven't tested and compared them yet, I will compare them when I have
time. It may be compared by simulating fork() in user space.
>
>>
>> It can be seen that the performance improvement is proportional to
>> the number of VMAs. With 23 VMAs, performance improves by about 3%,
>> with 223 VMAs, performance improves by about 15%, and with 4023 VMAs,
>> performance improves by about 30%.
>>
>> [1] https://lore.kernel.org/lkml/20230628073657.75314-4-zhangpeng.00@bytedance.com/
>> [2] https://github.com/kdlucas/byte-unixbench/tree/master
>>
>> Signed-off-by: Peng Zhang <zhangpeng.00@...edance.com>
>> ---
>> kernel/fork.c | 35 +++++++++++++++++++++++++++--------
>> mm/mmap.c | 14 ++++++++++++--
>> 2 files changed, 39 insertions(+), 10 deletions(-)
>>
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index f81149739eb9..ef80025b62d6 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 +677,40 @@ 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);
>> + 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_replace_entry(&vmi.mas,
>> + XA_ZERO_ENTRY);
>> + goto loop_out;
>> + }
>> +
>> continue;
>> }
>> charge = 0;
>> @@ -750,8 +772,7 @@ 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_replace_entry(&vmi.mas, tmp);
>>
>> mm->map_count++;
>> if (!(tmp->vm_flags & VM_WIPEONFORK))
>> @@ -778,8 +799,6 @@ 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 bc91d91261ab..5bfba2fb0e39 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -3184,7 +3184,11 @@ 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;
>> @@ -3222,7 +3226,13 @@ 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
>>
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