Re: [Qemu-devel] [RFC PATCH 1/2] mem: make phys_section and phys_map_nodes prepared for RCU

[liu ping fan]

[...]
>>>> +static PhysPageTable *cur_pgtbl;
>>>> +static PhysPageTable *next_pgtbl;
>>>
>>> You shouldn't need cur_pgtbl.  Instead, each AddressSpaceDispatch should
>>> have a pointer to its own cur_pgtbl.  In the commit hook you can then
>>> take a lock, unref the old table, assign cur_pgtbl = next_pgtbl and ref
>>> the new one.
>>>
>> Here is a trick. All AddressSpaceDispatch will update its radix tree
>> at the same time, so the re-claimer will come after all of them commit
>> to drop the old phys_map_node[] which holds the radix trees all at
>> once. Is it OK? Or you still prefer  each AddressSpaceDispatch has its
>> own space for radix tree?
>
> I'm not sure... I prefer to have more stuff in AddressSpaceDispatch to
> make things as streamlined as possible on the read side.
>
When I try to make each AddressSpaceDispatch own a separate radix
tree, I found in subpage_read/write(), we need to retrieve
"phys_sections" for this subpage, ie,
   section = &phys_sections[mmio->sub_section[idx]]; will be changed to
   section = &subpage->as_dispatch->phys_sections[mmio->sub_section[idx]];
It will sacrifice performance.  Is it acceptable?

Regards,
Pingfan

>>> The lock must also be taken in address_space_translate, together with a
>>> relatively expensive ref/unref pair (two atomic operations).  We
>>> probably need real RCU so that we can skip the ref/unref.
>>>
>> Currently, without RCU, do we rely on as->lock? So no need ref/unref
>> on cur_pgtbl?  With RCU, I think we need ref/unref on mr, since
>> mr->ops->read/write can block, and should be excluded from RCU
>> section.
>
> Yes, we need ref/unref on the region, but in many cases that will be a
> no-op (most important, for access to system RAM).  And if we can do the
> dispatch in the RCU section, similar to what I proposed in reviewing
> your patch 2, we have no ref/unref in the common case of accessing RAM.
>
>>>>  #define PHYS_MAP_NODE_NIL (((uint16_t)~0) >> 1)
>>>>
>>>> @@ -111,13 +122,13 @@ static MemoryRegion io_mem_watch;
>>>>
>>>>  static void phys_map_node_reserve(unsigned nodes)
>>>>  {
>>>> -    if (phys_map_nodes_nb + nodes > phys_map_nodes_nb_alloc) {
>>>> +    if (next_pgtbl->phys_map_nodes_nb + nodes > 
>>>> next_pgtbl->phys_map_nodes_nb_alloc) {
>>>>          typedef PhysPageEntry Node[L2_SIZE];
>>>> -        phys_map_nodes_nb_alloc = MAX(phys_map_nodes_nb_alloc * 2, 16);
>>>> -        phys_map_nodes_nb_alloc = MAX(phys_map_nodes_nb_alloc,
>>>> -                                      phys_map_nodes_nb + nodes);
>>>> -        phys_map_nodes = g_renew(Node, phys_map_nodes,
>>>> -                                 phys_map_nodes_nb_alloc);
>>>> +        next_pgtbl->phys_map_nodes_nb_alloc = 
>>>> MAX(next_pgtbl->phys_map_nodes_nb_alloc * 2, 16);
>>>> +        next_pgtbl->phys_map_nodes_nb_alloc = 
>>>> MAX(next_pgtbl->phys_map_nodes_nb_alloc,
>>>> +                                      next_pgtbl->phys_map_nodes_nb + 
>>>> nodes);
>>>> +        next_pgtbl->phys_map_nodes = g_renew(Node, 
>>>> next_pgtbl->phys_map_nodes,
>>>> +                                 next_pgtbl->phys_map_nodes_nb_alloc);
>>>>      }
>>>>  }
>>>>
>>>> @@ -126,22 +137,16 @@ static uint16_t phys_map_node_alloc(void)
>>>>      unsigned i;
>>>>      uint16_t ret;
>>>>
>>>> -    ret = phys_map_nodes_nb++;
>>>> +    ret = next_pgtbl->phys_map_nodes_nb++;
>>>>      assert(ret != PHYS_MAP_NODE_NIL);
>>>> -    assert(ret != phys_map_nodes_nb_alloc);
>>>> +    assert(ret != next_pgtbl->phys_map_nodes_nb_alloc);
>>>>      for (i = 0; i < L2_SIZE; ++i) {
>>>> -        phys_map_nodes[ret][i].is_leaf = 0;
>>>> -        phys_map_nodes[ret][i].ptr = PHYS_MAP_NODE_NIL;
>>>> +        next_pgtbl->phys_map_nodes[ret][i].is_leaf = 0;
>>>> +        next_pgtbl->phys_map_nodes[ret][i].ptr = PHYS_MAP_NODE_NIL;
>>>>      }
>>>>      return ret;
>>>>  }
>>>>
>>>> -static void phys_map_nodes_reset(void)
>>>> -{
>>>> -    phys_map_nodes_nb = 0;
>>>> -}
>>>> -
>>>> -
>>>>  static void phys_page_set_level(PhysPageEntry *lp, hwaddr *index,
>>>>                                  hwaddr *nb, uint16_t leaf,
>>>>                                  int level)
>>>> @@ -152,15 +157,15 @@ static void phys_page_set_level(PhysPageEntry *lp, 
>>>> hwaddr *index,
>>>>
>>>>      if (!lp->is_leaf && lp->ptr == PHYS_MAP_NODE_NIL) {
>>>>          lp->ptr = phys_map_node_alloc();
>>>> -        p = phys_map_nodes[lp->ptr];
>>>> +        p = next_pgtbl->phys_map_nodes[lp->ptr];
>>>>          if (level == 0) {
>>>>              for (i = 0; i < L2_SIZE; i++) {
>>>>                  p[i].is_leaf = 1;
>>>> -                p[i].ptr = phys_section_unassigned;
>>>> +                p[i].ptr = next_pgtbl->phys_section_unassigned;
>>>>              }
>>>>          }
>>>>      } else {
>>>> -        p = phys_map_nodes[lp->ptr];
>>>> +        p = next_pgtbl->phys_map_nodes[lp->ptr];
>>>>      }
>>>>      lp = &p[(*index >> (level * L2_BITS)) & (L2_SIZE - 1)];
>>>>
>>>> @@ -192,11 +197,13 @@ static PhysSection 
>>>> *phys_section_find(AddressSpaceDispatch *d,
>>>>  {
>>>>      PhysPageEntry lp = d->phys_map;
>>>>      PhysPageEntry *p;
>>>> +    PhysSection *phys_sections = cur_pgtbl->phys_sections;
>>>> +    Node *phys_map_nodes = cur_pgtbl->phys_map_nodes;
>>>>      int i;
>>>>
>>>>      for (i = P_L2_LEVELS - 1; i >= 0 && !lp.is_leaf; i--) {
>>>>          if (lp.ptr == PHYS_MAP_NODE_NIL) {
>>>> -            return &phys_sections[phys_section_unassigned];
>>>> +            return &phys_sections[cur_pgtbl->phys_section_unassigned];
>>>>          }
>>>>          p = phys_map_nodes[lp.ptr];
>>>>          lp = p[(index >> (i * L2_BITS)) & (L2_SIZE - 1)];
>>>> @@ -209,6 +216,7 @@ static MemoryRegionSection 
>>>> *address_space_lookup_region(AddressSpace *as,
>>>>  {
>>>>      PhysSection *psection;
>>>>      uint16_t idx;
>>>> +    PhysSection *phys_sections = cur_pgtbl->phys_sections;
>>>>
>>>>      psection = phys_section_find(as->dispatch, addr >> TARGET_PAGE_BITS);
>>>>      if (psection->sub_section) {
>>>> @@ -246,7 +254,7 @@ MemoryRegionSection 
>>>> *address_space_translate(AddressSpace *as, hwaddr addr,
>>>>                  | (addr & iotlb.addr_mask));
>>>>          len = MIN(len, (addr | iotlb.addr_mask) - addr + 1);
>>>>          if (!iotlb.perm[is_write]) {
>>>> -            section = &phys_sections[phys_section_unassigned].section;
>>>> +            section = 
>>>> &cur_pgtbl->phys_sections[cur_pgtbl->phys_section_unassigned].section;
>>>>              break;
>>>>          }
>>>>
>>>> @@ -690,12 +698,12 @@ hwaddr memory_region_section_get_iotlb(CPUArchState 
>>>> *env,
>>>>          iotlb = (memory_region_get_ram_addr(section->mr) & 
>>>> TARGET_PAGE_MASK)
>>>>              + xlat;
>>>>          if (!section->readonly) {
>>>> -            iotlb |= phys_section_notdirty;
>>>> +            iotlb |= cur_pgtbl->phys_section_notdirty;
>>>>          } else {
>>>> -            iotlb |= phys_section_rom;
>>>> +            iotlb |= cur_pgtbl->phys_section_rom;
>>>>          }
>>>>      } else {
>>>> -        iotlb = PHYS_SECTION_ID(container_of(section, PhysSection, 
>>>> section));
>>>> +        iotlb = PHYS_SECTION_ID(container_of(section, PhysSection, 
>>>> section), cur_pgtbl);
>>>>          iotlb += xlat;
>>>>      }
>>>>
>>>> @@ -705,7 +713,7 @@ hwaddr memory_region_section_get_iotlb(CPUArchState 
>>>> *env,
>>>>          if (vaddr == (wp->vaddr & TARGET_PAGE_MASK)) {
>>>>              /* Avoid trapping reads of pages with a write breakpoint. */
>>>>              if ((prot & PAGE_WRITE) || (wp->flags & BP_MEM_READ)) {
>>>> -                iotlb = phys_section_watch + paddr;
>>>> +                iotlb = cur_pgtbl->phys_section_watch + paddr;
>>>>                  *address |= TLB_MMIO;
>>>>                  break;
>>>>              }
>>>> @@ -721,59 +729,40 @@ static int subsection_register(PhysSection 
>>>> *psection, uint32_t start,
>>>>                                 uint32_t end, uint16_t section);
>>>>  static void subsections_init(PhysSection *psection);
>>>>
>>>> -static void destroy_page_desc(uint16_t section_index)
>>>> +/* Call after all listener has been commit.
>>>> +  * we do not walk over tree, just simply drop.
>>>> +  */
>>>> +static void destroy_pagetable(PhysPageTable *pgtbl)
>>>>  {
>>>> -    g_free(phys_sections[section_index].sub_section);
>>>> -}
>>>> -
>>>> -static void destroy_l2_mapping(PhysPageEntry *lp, unsigned level)
>>>> -{
>>>> -    unsigned i;
>>>> -    PhysPageEntry *p;
>>>> +    int i;
>>>>
>>>> -    if (lp->ptr == PHYS_MAP_NODE_NIL) {
>>>> -        return;
>>>> -    }
>>>> +    g_free(pgtbl->phys_map_nodes);
>>>>
>>>> -    p = phys_map_nodes[lp->ptr];
>>>> -    for (i = 0; i < L2_SIZE; ++i) {
>>>> -        if (!p[i].is_leaf) {
>>>> -            destroy_l2_mapping(&p[i], level - 1);
>>>> +    for (i = 0; i < pgtbl->phys_sections_nb_alloc; i++) {
>>>> +        if (pgtbl->phys_sections[i].sub_section) {
>>>> +            g_free(pgtbl->phys_sections[i].sub_section);
>>>>          } else {
>>>> -            destroy_page_desc(p[i].ptr);
>>>> +            memory_region_unref(pgtbl->phys_sections[i].section.mr);
>>>>          }
>>>>      }
>>>> -    lp->is_leaf = 0;
>>>> -    lp->ptr = PHYS_MAP_NODE_NIL;
>>>> -}
>>>> +    g_free(pgtbl->phys_sections);
>>>>
>>>> -static void destroy_all_mappings(AddressSpaceDispatch *d)
>>>> -{
>>>> -    destroy_l2_mapping(&d->phys_map, P_L2_LEVELS - 1);
>>>> -    phys_map_nodes_reset();
>>>> +    g_free(pgtbl);
>>>>  }
>>>
>>> This should be a separate patch.
>>>
>> Sorry, this is part of reclaimer to play destruction of prev pgtbl.
>> Why to separate it?
>
> You are changing the algorithm by which the old pagetable is destroyed.
>  This applies just as well to the current code.
>
> BTW, your patch includes Jan's subpage patch which was broken.  I'll
> drop it from the branch next time I push.
>
> Paolo
>
>>>> -static uint16_t phys_section_add(MemoryRegionSection *section)
>>>> +static uint16_t phys_section_add(MemoryRegionSection *section, 
>>>> PhysPageTable *pgtbl)
>>>
>>> phys_section_add will always add to next_pgtbl.
>>>
>> Ok, will drop @pgtbl.
>>>>  {
>>>> -    assert(phys_sections_nb < TARGET_PAGE_SIZE);
>>>> +    assert(pgtbl->phys_sections_nb < TARGET_PAGE_SIZE);
>>>>
>>>> -    if (phys_sections_nb == phys_sections_nb_alloc) {
>>>> -        phys_sections_nb_alloc = MAX(phys_sections_nb_alloc * 2, 16);
>>>> -        phys_sections = g_renew(PhysSection, phys_sections,
>>>> -                                phys_sections_nb_alloc);
>>>> +    if (pgtbl->phys_sections_nb == pgtbl->phys_sections_nb_alloc) {
>>>> +        pgtbl->phys_sections_nb_alloc = MAX(pgtbl->phys_sections_nb_alloc 
>>>> * 2, 16);
>>>> +        pgtbl->phys_sections = g_renew(PhysSection, pgtbl->phys_sections,
>>>> +                                pgtbl->phys_sections_nb_alloc);
>>>>      }
>>>> -    phys_sections[phys_sections_nb].section = *section;
>>>> -    phys_sections[phys_sections_nb].sub_section = NULL;
>>>> +    pgtbl->phys_sections[pgtbl->phys_sections_nb].section = *section;
>>>> +    pgtbl->phys_sections[pgtbl->phys_sections_nb].sub_section = NULL;
>>>>      memory_region_ref(section->mr);
>>>> -    return phys_sections_nb++;
>>>> -}
>>>> -
>>>> -static void phys_sections_clear(void)
>>>> -{
>>>> -    while (phys_sections_nb > 0) {
>>>> -        PhysSection *phys_section = &phys_sections[--phys_sections_nb];
>>>> -        memory_region_unref(phys_section->section.mr);
>>>> -    }
>>>> +    return pgtbl->phys_sections_nb++;
>>>>  }
>>>>
>>>>  static void register_subsection(AddressSpaceDispatch *d,
>>>> @@ -793,18 +782,18 @@ static void register_subsection(AddressSpaceDispatch 
>>>> *d,
>>>>             psection->section.mr == &io_mem_unassigned);
>>>>
>>>>      if (!psection->sub_section) {
>>>> -        new_section = phys_section_add(&subsection);
>>>> -        psection = &phys_sections[new_section];
>>>> +        new_section = phys_section_add(&subsection, next_pgtbl);
>>>> +        psection = &next_pgtbl->phys_sections[new_section];
>>>>          subsections_init(psection);
>>>>          phys_page_set(d, base >> TARGET_PAGE_BITS, 1, new_section);
>>>>      } else {
>>>> -        new_section = PHYS_SECTION_ID(psection);
>>>> +        new_section = PHYS_SECTION_ID(psection, next_pgtbl);
>>>>      }
>>>>
>>>> -    new_subsection = phys_section_add(section);
>>>> +    new_subsection = phys_section_add(section, next_pgtbl);
>>>>
>>>>      /* phys_section_add invalidates psection, reload it  */
>>>> -    psection = &phys_sections[new_section];
>>>> +    psection = &next_pgtbl->phys_sections[new_section];
>>>>      start = section->offset_within_address_space & ~TARGET_PAGE_MASK;
>>>>      end = start + section->size - 1;
>>>>      subsection_register(psection, start, end, new_subsection);
>>>> @@ -816,7 +805,7 @@ static void register_multipage(AddressSpaceDispatch 
>>>> *d, MemoryRegionSection *sec
>>>>      hwaddr start_addr = section->offset_within_address_space;
>>>>      ram_addr_t size = section->size;
>>>>      hwaddr addr;
>>>> -    uint16_t section_index = phys_section_add(section);
>>>> +    uint16_t section_index = phys_section_add(section, next_pgtbl);
>>>>
>>>>      assert(size);
>>>>
>>>> @@ -1653,7 +1642,7 @@ static void subsections_init(PhysSection *psection)
>>>>  {
>>>>      psection->sub_section = g_malloc0(sizeof(uint16_t) * 
>>>> TARGET_PAGE_SIZE);
>>>>      subsection_register(psection, 0, TARGET_PAGE_SIZE-1,
>>>> -                        phys_section_unassigned);
>>>> +                        next_pgtbl->phys_section_unassigned);
>>>>  }
>>>>
>>>>  static uint16_t dummy_section(MemoryRegion *mr)
>>>> @@ -1665,12 +1654,12 @@ static uint16_t dummy_section(MemoryRegion *mr)
>>>>          .size = UINT64_MAX,
>>>>      };
>>>>
>>>> -    return phys_section_add(&section);
>>>> +    return phys_section_add(&section, next_pgtbl);
>>>>  }
>>>>
>>>>  MemoryRegion *iotlb_to_region(hwaddr index)
>>>>  {
>>>> -    return phys_sections[index & ~TARGET_PAGE_MASK].section.mr;
>>>> +    return cur_pgtbl->phys_sections[index & ~TARGET_PAGE_MASK].section.mr;
>>>>  }
>>>>
>>>>  static void io_mem_init(void)
>>>> @@ -1685,21 +1674,40 @@ static void io_mem_init(void)
>>>>                            "watch", UINT64_MAX);
>>>>  }
>>>>
>>>> +void global_begin(void)
>>>> +{
>>>> +    next_pgtbl = g_new0(PhysPageTable, 1);
>>>> +    next_pgtbl->ref = 1;
>>>> +    next_pgtbl->phys_section_unassigned = 
>>>> dummy_section(&io_mem_unassigned);
>>>> +    next_pgtbl->phys_section_notdirty = dummy_section(&io_mem_notdirty);
>>>> +    next_pgtbl->phys_section_rom = dummy_section(&io_mem_rom);
>>>> +    next_pgtbl->phys_section_watch = dummy_section(&io_mem_watch);
>>>> +}
>>>> +
>>>> +/* other listeners finished */
>>>> +void global_commit(void)
>>>> +{
>>>> +    PhysPageTable *t = cur_pgtbl;
>>>> +
>>>> +    cur_pgtbl = next_pgtbl;
>>>> +    /* Fix me,  currently, we rely on each address space listener agaist 
>>>> its
>>>> +      * reader. So when we come here, no readers will touch old 
>>>> phys_map_node.
>>>> +      * After rcu, should changed to call_rcu()
>>>> +      */
>>>> +    if (__sync_sub_and_fetch(&t->ref, 1) == 0) {
>>>> +        destroy_pagetable(t);
>>>> +    }
>>>
>>> global_commit should not be needed, and global_begin should be simply
>>> the new core_begin.  It should unref next_pgtbl and reallocate a new one.
>>>
>> Good idea, thanks.
>>
>> Regards,
>> Pingfan
>>>
>>>> +}
>>>> +
>>>>  static void mem_begin(MemoryListener *listener)
>>>>  {
>>>>      AddressSpaceDispatch *d = container_of(listener, 
>>>> AddressSpaceDispatch, listener);
>>>>
>>>> -    destroy_all_mappings(d);
>>>>      d->phys_map.ptr = PHYS_MAP_NODE_NIL;
>>>>  }
>>>>
>>>>  static void core_begin(MemoryListener *listener)
>>>>  {
>>>> -    phys_sections_clear();
>>>> -    phys_section_unassigned = dummy_section(&io_mem_unassigned);
>>>> -    phys_section_notdirty = dummy_section(&io_mem_notdirty);
>>>> -    phys_section_rom = dummy_section(&io_mem_rom);
>>>> -    phys_section_watch = dummy_section(&io_mem_watch);
>>>>  }
>>>>
>>>>  static void tcg_commit(MemoryListener *listener)
>>>> @@ -1779,7 +1787,6 @@ void address_space_destroy_dispatch(AddressSpace *as)
>>>>      AddressSpaceDispatch *d = as->dispatch;
>>>>
>>>>      memory_listener_unregister(&d->listener);
>>>> -    destroy_l2_mapping(&d->phys_map, P_L2_LEVELS - 1);
>>>>      g_free(d);
>>>>      as->dispatch = NULL;
>>>>  }
>>>> @@ -2386,7 +2393,7 @@ void stl_phys_notdirty(hwaddr addr, uint32_t val)
>>>>
>>>>      if (!memory_region_is_ram(section->mr) || section->readonly) {
>>>>          if (memory_region_is_ram(section->mr)) {
>>>> -            section = &phys_sections[phys_section_rom].section;
>>>> +            section = 
>>>> &cur_pgtbl->phys_sections[cur_pgtbl->phys_section_rom].section;
>>>>          }
>>>>          io_mem_write(section->mr, addr, val, 4);
>>>>      } else {
>>>> @@ -2422,7 +2429,7 @@ void stq_phys_notdirty(hwaddr addr, uint64_t val)
>>>>
>>>>      if (!memory_region_is_ram(section->mr) || section->readonly) {
>>>>          if (memory_region_is_ram(section->mr)) {
>>>> -            section = &phys_sections[phys_section_rom].section;
>>>> +            section = 
>>>> &cur_pgtbl->phys_sections[cur_pgtbl->phys_section_rom].section;
>>>>          }
>>>>  #ifdef TARGET_WORDS_BIGENDIAN
>>>>          io_mem_write(section->mr, addr, val >> 32, 4);
>>>> @@ -2455,7 +2462,7 @@ static inline void stl_phys_internal(hwaddr addr, 
>>>> uint32_t val,
>>>>
>>>>      if (!memory_region_is_ram(section->mr) || section->readonly) {
>>>>          if (memory_region_is_ram(section->mr)) {
>>>> -            section = &phys_sections[phys_section_rom].section;
>>>> +            section = 
>>>> &cur_pgtbl->phys_sections[cur_pgtbl->phys_section_rom].section;
>>>>          }
>>>>  #if defined(TARGET_WORDS_BIGENDIAN)
>>>>          if (endian == DEVICE_LITTLE_ENDIAN) {
>>>> @@ -2526,7 +2533,7 @@ static inline void stw_phys_internal(hwaddr addr, 
>>>> uint32_t val,
>>>>
>>>>      if (!memory_region_is_ram(section->mr) || section->readonly) {
>>>>          if (memory_region_is_ram(section->mr)) {
>>>> -            section = &phys_sections[phys_section_rom].section;
>>>> +            section = 
>>>> &cur_pgtbl->phys_sections[cur_pgtbl->phys_section_rom].section;
>>>>          }
>>>>  #if defined(TARGET_WORDS_BIGENDIAN)
>>>>          if (endian == DEVICE_LITTLE_ENDIAN) {
>>>> diff --git a/include/exec/memory.h b/include/exec/memory.h
>>>> index b97ace7..cc654fa 100644
>>>> --- a/include/exec/memory.h
>>>> +++ b/include/exec/memory.h
>>>> @@ -992,6 +992,8 @@ void *address_space_map(AddressSpace *as, hwaddr addr,
>>>>  void address_space_unmap(AddressSpace *as, void *buffer, hwaddr len,
>>>>                           int is_write, hwaddr access_len);
>>>>
>>>> +void global_begin(void);
>>>> +void global_commit(void);
>>>>
>>>>  #endif
>>>>
>>>> diff --git a/memory.c b/memory.c
>>>> index 1a86607..da06dfd 100644
>>>> --- a/memory.c
>>>> +++ b/memory.c
>>>> @@ -805,6 +805,7 @@ void memory_region_transaction_commit(void)
>>>>      --memory_region_transaction_depth;
>>>>      if (!memory_region_transaction_depth && memory_region_update_pending) 
>>>> {
>>>>          memory_region_update_pending = false;
>>>> +        global_begin();
>>>>          MEMORY_LISTENER_CALL_GLOBAL(begin, Forward);
>>>>
>>>>          QTAILQ_FOREACH(as, &address_spaces, address_spaces_link) {
>>>> @@ -812,6 +813,7 @@ void memory_region_transaction_commit(void)
>>>>          }
>>>>
>>>>          MEMORY_LISTENER_CALL_GLOBAL(commit, Forward);
>>>> +        global_commit();
>>>>      }
>>>>  }
>>>>
>>>>
>>>
>>
>>
>