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Message-ID: <df1ab51c-810a-f7ce-e591-d4fbbed95dab@loongson.cn>
Date: Sat, 16 May 2020 17:34:35 +0800
From: maobibo <maobibo@...ngson.cn>
To: David Hildenbrand <david@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
linux-mips@...r.kernel.org
Subject: Re: mm/memory.c: Add update local tlb for smp race
On 05/15/2020 09:50 PM, David Hildenbrand wrote:
> On 14.05.20 08:50, Bibo Mao wrote:
>> If there are two threads hitting page fault at the address, one
>> thread updates pte entry and local tlb, the other thread can update
>> local tlb also, rather than give up and let page fault happening
>> again.
>
> Let me suggest
>
> "mm/memory: optimize concurrent page faults at same address
>
> If two threads concurrently fault at the same address, the thread that
> won the race updates the PTE and its local TLB. For now, the other
> thread gives up, simply does nothing, and continues.
>
> It could happen that this second thread triggers another fault, whereby
> it only updates its local TLB while handling the fault. Instead of
> triggering another fault, let's directly update the local TLB of the
> second thread.
> "
>
> If I got the intention of this patch correctly.
>
> Are there any performance numbers to support this patch?
>
> (I can't say too much about the correctness and/or usefulness of this patch)
yes, that is the situation. On MIPS platform software can update TLB,
so update_mmu_cache is used here. This does not happen frequently, and with the three series patches in later mail. I test lat_pagefault in lmbench, here is is result:
with these three series patches,
# ./lat_pagefault -N 10 /tmp/1
Pagefaults on /tmp/1: 1.4973 microseconds
# ./lat_pagefault -P 4 -N 10 /tmp/1
Pagefaults on /tmp/1: 1.5716 microseconds
original version, without these three series patch
# ./lat_pagefault -N 10 /tmp/1
Pagefaults on /tmp/1: 1.6489 microseconds
# ./lat_pagefault -P 4 -N 10 /tmp/1
Pagefaults on /tmp/1: 1.7214 microseconds
>>
>> modified: mm/memory.c
>
> This does not belong into a patch description.
well, I will modify the patch description.
regards
bibo,mao
>
>
>> Signed-off-by: Bibo Mao <maobibo@...ngson.cn>
>> ---
>> mm/memory.c | 30 ++++++++++++++++++++++--------
>> 1 file changed, 22 insertions(+), 8 deletions(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index f703fe8..3a741ce 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -2436,11 +2436,10 @@ static inline bool cow_user_page(struct page *dst, struct page *src,
>> if (!likely(pte_same(*vmf->pte, vmf->orig_pte))) {
>> /*
>> * Other thread has already handled the fault
>> - * and we don't need to do anything. If it's
>> - * not the case, the fault will be triggered
>> - * again on the same address.
>> + * and update local tlb only
>> */
>> ret = false;
>> + update_mmu_cache(vma, addr, vmf->pte);
>> goto pte_unlock;
>> }
>>
>> @@ -2463,8 +2462,9 @@ static inline bool cow_user_page(struct page *dst, struct page *src,
>> vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr, &vmf->ptl);
>> locked = true;
>> if (!likely(pte_same(*vmf->pte, vmf->orig_pte))) {
>> - /* The PTE changed under us. Retry page fault. */
>> + /* The PTE changed under us. update local tlb */
>> ret = false;
>> + update_mmu_cache(vma, addr, vmf->pte);
>> goto pte_unlock;
>> }
>>
>> @@ -2704,6 +2704,7 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
>> }
>> flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte));
>> entry = mk_pte(new_page, vma->vm_page_prot);
>> + entry = pte_mkyoung(entry);
>> entry = maybe_mkwrite(pte_mkdirty(entry), vma);
>> /*
>> * Clear the pte entry and flush it first, before updating the
>> @@ -2752,6 +2753,7 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
>> new_page = old_page;
>> page_copied = 1;
>> } else {
>> + update_mmu_cache(vma, vmf->address, vmf->pte);
>> mem_cgroup_cancel_charge(new_page, memcg, false);
>> }
>>
>> @@ -2812,6 +2814,7 @@ vm_fault_t finish_mkwrite_fault(struct vm_fault *vmf)
>> * pte_offset_map_lock.
>> */
>> if (!pte_same(*vmf->pte, vmf->orig_pte)) {
>> + update_mmu_cache(vmf->vma, vmf->address, vmf->pte);
>> pte_unmap_unlock(vmf->pte, vmf->ptl);
>> return VM_FAULT_NOPAGE;
>> }
>> @@ -2936,6 +2939,7 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>> vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
>> vmf->address, &vmf->ptl);
>> if (!pte_same(*vmf->pte, vmf->orig_pte)) {
>> + update_mmu_cache(vma, vmf->address, vmf->pte);
>> unlock_page(vmf->page);
>> pte_unmap_unlock(vmf->pte, vmf->ptl);
>> put_page(vmf->page);
>> @@ -3341,8 +3345,10 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>> vma->vm_page_prot));
>> vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
>> vmf->address, &vmf->ptl);
>> - if (!pte_none(*vmf->pte))
>> + if (!pte_none(*vmf->pte)) {
>> + update_mmu_cache(vma, vmf->address, vmf->pte);
>> goto unlock;
>> + }
>> ret = check_stable_address_space(vma->vm_mm);
>> if (ret)
>> goto unlock;
>> @@ -3373,13 +3379,16 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>> __SetPageUptodate(page);
>>
>> entry = mk_pte(page, vma->vm_page_prot);
>> + entry = pte_mkyoung(entry);
>> if (vma->vm_flags & VM_WRITE)
>> entry = pte_mkwrite(pte_mkdirty(entry));
>>
>> vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
>> &vmf->ptl);
>> - if (!pte_none(*vmf->pte))
>> + if (!pte_none(*vmf->pte)) {
>> + update_mmu_cache(vma, vmf->address, vmf->pte);
>> goto release;
>> + }
>>
>> ret = check_stable_address_space(vma->vm_mm);
>> if (ret)
>> @@ -3646,11 +3655,14 @@ vm_fault_t alloc_set_pte(struct vm_fault *vmf, struct mem_cgroup *memcg,
>> }
>>
>> /* Re-check under ptl */
>> - if (unlikely(!pte_none(*vmf->pte)))
>> + if (unlikely(!pte_none(*vmf->pte))) {
>> + update_mmu_cache(vma, vmf->address, vmf->pte);
>> return VM_FAULT_NOPAGE;
>> + }
>>
>> flush_icache_page(vma, page);
>> entry = mk_pte(page, vma->vm_page_prot);
>> + entry = pte_mkyoung(entry);
>> if (write)
>> entry = maybe_mkwrite(pte_mkdirty(entry), vma);
>> /* copy-on-write page */
>> @@ -4224,8 +4236,10 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
>> vmf->ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd);
>> spin_lock(vmf->ptl);
>> entry = vmf->orig_pte;
>> - if (unlikely(!pte_same(*vmf->pte, entry)))
>> + if (unlikely(!pte_same(*vmf->pte, entry))) {
>> + update_mmu_cache(vmf->vma, vmf->address, vmf->pte);
>> goto unlock;
>> + }
>> if (vmf->flags & FAULT_FLAG_WRITE) {
>> if (!pte_write(entry))
>> return do_wp_page(vmf);
>>
>
>
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