diff mbox series

[qemu,v2,11/13] memory: Share FlatView's and dispatch trees between address spaces

Message ID 20170915084030.40988-12-aik@ozlabs.ru
State New
Headers show
Series memory: Reduce memory use | expand

Commit Message

Alexey Kardashevskiy Sept. 15, 2017, 8:40 a.m. UTC 
This allows sharing flat views between address spaces when the same root
memory region is used when creating a new address space.

This adds a global list of flat views and a list of attached address
spaces per a flat view. Each address space references a flat view.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
---
 include/exec/memory.h |   3 +-
 memory.c              | 148 ++++++++++++++++++++++++++++++++++++--------------
 2 files changed, 108 insertions(+), 43 deletions(-)

Comments

Paolo Bonzini Sept. 15, 2017, 9:25 a.m. UTC | #1 
On 15/09/2017 10:40, Alexey Kardashevskiy wrote:
> +
> +static bool flatview_can_share(FlatView *old_view, FlatView *new_view)
> +{
> +    MemoryRegion *old_root = memory_region_unalias_entire(old_view->root);
> +    MemoryRegion *new_root = memory_region_unalias_entire(new_view->root);
> +
> +    if (old_root == new_root) {
> +        return true;
> +    }
> +
> +    if (!old_root->enabled && !new_root->enabled) {
> +        return true;
> +    }
> +
> +    return false;
> +}
> +

I think you can improve this by keeping the root in the address space
(removing the previous patch).  Instead, the FlatView's root can be the
one with resolved aliases.  Then old_root is just old_view->root, and if
an empty FlatView has a NULL root this just becomes

   the_other_view->root == memory_region_unalias_entire(as->root);

> 
> +
> +    /* Build list of unique FlatViews, FV::root is the key */
> +    QTAILQ_FOREACH(old_view, &flat_views, flat_views_link) {
> +        found = false;
> +        QTAILQ_FOREACH(new_view, &fvs_tmp, flat_views_link) {
> +            if (flatview_can_share(old_view, new_view)) {
> +                found = true;
> +                break;
> +            }
> +        }
> +        if (found) {
> +            continue;
> +        }
> +
> +        new_view = generate_memory_topology(old_view->root);
> +        flatview_render_new(old_view, new_view);
> +        QTAILQ_INSERT_TAIL(&fvs_tmp, new_view, flat_views_link);
> +    }

I don't understand the algorithm here.  Why is it visiting &flat_views
rather than the list of address spaces?  I would have thought it enough
to do

   for each address space
        if there is a (new) flat view that we can share
            add a reference
        else
            render a new flat view and add it to fvs_tmp

   flat_views = fvs_tmp;

   for each address space
        old_view = address_space_to_flatview(as);
        find the new flat view to use in fvs_tmp
        address_space_update_topology_pass(..., false);
        address_space_update_topology_pass(..., true);
        QTAILQ_REMOVE(&old_view->address_spaces, ...);
        atomic_rcu_set(&as->current_map, new_view);
        flatview_unref(old_view);
        QTAILQ_INSERT_TAIL(&new_view->address_spaces, ...);


It's also not clear to me what you need the FlatView's address_spaces
list for.  (It's probably something trivial that I'm missing, or maybe
it goes away with the previous change).

> 
>  AddressSpace *address_space_init_shareable(MemoryRegion *root, const char *name)
>  {
>      AddressSpace *as;
>  
> -    QTAILQ_FOREACH(as, &address_spaces, address_spaces_link) {
> -        if (root == address_space_root(as) && as->malloced) {
> -            as->ref_count++;
> -            return as;
> -        }
> -    }
> -
>      as = g_malloc0(sizeof *as);
>      address_space_init(as, root, name);
> -    as->malloced = true;
> +
>      return as;
>  }
>  

This belongs in the next patch; leaving as->malloced in
do_address_space_destroy and the reference count in
address_space_destroy is not a big complication.

Paolo
Alexey Kardashevskiy Sept. 16, 2017, 12:58 a.m. UTC | #2 
On 15/09/17 19:25, Paolo Bonzini wrote:
> On 15/09/2017 10:40, Alexey Kardashevskiy wrote:
>> +
>> +static bool flatview_can_share(FlatView *old_view, FlatView *new_view)
>> +{
>> +    MemoryRegion *old_root = memory_region_unalias_entire(old_view->root);
>> +    MemoryRegion *new_root = memory_region_unalias_entire(new_view->root);
>> +
>> +    if (old_root == new_root) {
>> +        return true;
>> +    }
>> +
>> +    if (!old_root->enabled && !new_root->enabled) {
>> +        return true;
>> +    }
>> +
>> +    return false;
>> +}
>> +
> 
> I think you can improve this by keeping the root in the address space
> (removing the previous patch).  Instead, the FlatView's root can be the
> one with resolved aliases.  Then old_root is just old_view->root, and if
> an empty FlatView has a NULL root this just becomes
> 
>    the_other_view->root == memory_region_unalias_entire(as->root);

Ok!


> 
>>
>> +
>> +    /* Build list of unique FlatViews, FV::root is the key */
>> +    QTAILQ_FOREACH(old_view, &flat_views, flat_views_link) {
>> +        found = false;
>> +        QTAILQ_FOREACH(new_view, &fvs_tmp, flat_views_link) {
>> +            if (flatview_can_share(old_view, new_view)) {
>> +                found = true;
>> +                break;
>> +            }
>> +        }
>> +        if (found) {
>> +            continue;
>> +        }
>> +
>> +        new_view = generate_memory_topology(old_view->root);
>> +        flatview_render_new(old_view, new_view);
>> +        QTAILQ_INSERT_TAIL(&fvs_tmp, new_view, flat_views_link);
>> +    }
> 
> I don't understand the algorithm here.  Why is it visiting &flat_views
> rather than the list of address spaces?  I would have thought it enough
> to do
> 
>    for each address space
>         if there is a (new) flat view that we can share
>             add a reference
>         else
>             render a new flat view and add it to fvs_tmp
> 
>    flat_views = fvs_tmp;
> 
>    for each address space
>         old_view = address_space_to_flatview(as);
>         find the new flat view to use in fvs_tmp
>         address_space_update_topology_pass(..., false);
>         address_space_update_topology_pass(..., true);
>         QTAILQ_REMOVE(&old_view->address_spaces, ...);
>         atomic_rcu_set(&as->current_map, new_view);
>         flatview_unref(old_view);
>         QTAILQ_INSERT_TAIL(&new_view->address_spaces, ...);
> 
> 
> It's also not clear to me what you need the FlatView's address_spaces
> list for.  (It's probably something trivial that I'm missing, or maybe
> it goes away with the previous change).


It is trivial - the first version added a global list of FVs and shared
them among ASes. Every transactoion_commit would produce a new FV and
replace it in all ASes which old FV was shared among. The decision whether
to share FV or not was made in address_space_init() which a very different
place in code.

In v2 I moved the sharing decision to the commit part and noooow having a
global list of FVs and ASes list in each FV seems redundant so I'll remove
it in v3, thanks for pointing out :)

And I'll probably drop FV allocation in address_space_init as it is going
to be rebuild at commit time anyway.


> 
>>
>>  AddressSpace *address_space_init_shareable(MemoryRegion *root, const char *name)
>>  {
>>      AddressSpace *as;
>>  
>> -    QTAILQ_FOREACH(as, &address_spaces, address_spaces_link) {
>> -        if (root == address_space_root(as) && as->malloced) {
>> -            as->ref_count++;
>> -            return as;
>> -        }
>> -    }
>> -
>>      as = g_malloc0(sizeof *as);
>>      address_space_init(as, root, name);
>> -    as->malloced = true;
>> +
>>      return as;
>>  }
>>  
> 
> This belongs in the next patch;

By "this" you mean removal of "malloced", not the AS loop above, right?

> leaving as->malloced in
> do_address_space_destroy and the reference count in
> address_space_destroy is not a big complication.


But why would we want to leave them anyway?


thanks for the quick review!
diff mbox series

Patch

diff --git a/include/exec/memory.h b/include/exec/memory.h
index be8cc1ccd3..04993f8eca 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -308,8 +308,6 @@  struct AddressSpace {
     /* All fields are private. */
     struct rcu_head rcu;
     char *name;
-    int ref_count;
-    bool malloced;
 
     /* Accessed via RCU.  */
     struct FlatView *current_map;
@@ -318,6 +316,7 @@  struct AddressSpace {
     struct MemoryRegionIoeventfd *ioeventfds;
     QTAILQ_HEAD(memory_listeners_as, MemoryListener) listeners;
     QTAILQ_ENTRY(AddressSpace) address_spaces_link;
+    QTAILQ_ENTRY(AddressSpace) flat_view_link;
 };
 
 FlatView *address_space_to_flatview(AddressSpace *as);
diff --git a/memory.c b/memory.c
index 29f8588945..f3da9379df 100644
--- a/memory.c
+++ b/memory.c
@@ -47,6 +47,9 @@  static QTAILQ_HEAD(memory_listeners, MemoryListener) memory_listeners
 static QTAILQ_HEAD(, AddressSpace) address_spaces
     = QTAILQ_HEAD_INITIALIZER(address_spaces);
 
+static QTAILQ_HEAD(FlatViewList, FlatView) flat_views
+    = QTAILQ_HEAD_INITIALIZER(flat_views);
+
 typedef struct AddrRange AddrRange;
 
 /*
@@ -232,6 +235,8 @@  struct FlatView {
     unsigned nr_allocated;
     struct AddressSpaceDispatch *dispatch;
     MemoryRegion *root;
+    QTAILQ_ENTRY(FlatView) flat_views_link;
+    QTAILQ_HEAD(address_spaces, AddressSpace) address_spaces;
 };
 
 typedef struct AddressSpaceOps AddressSpaceOps;
@@ -269,6 +274,7 @@  static FlatView *flatview_alloc(MemoryRegion *mr_root)
     view->ref = 1;
     view->root = mr_root;
     memory_region_ref(mr_root);
+    QTAILQ_INIT(&view->address_spaces);
 
     return view;
 }
@@ -953,28 +959,99 @@  static void flatview_render_new(FlatView *old_view, FlatView *new_view)
     address_space_dispatch_compact(new_view->dispatch);
 }
 
-static void address_space_update_topology(AddressSpace *as)
+static MemoryRegion *memory_region_unalias_entire(MemoryRegion *mr)
 {
-    FlatView *old_view = address_space_get_flatview(as);
-    FlatView *new_view = generate_memory_topology(old_view->root);
-
-    flatview_render_new(old_view, new_view);
-    address_space_update_topology_pass(as, old_view, new_view, false);
-    address_space_update_topology_pass(as, old_view, new_view, true);
-
-    /* Writes are protected by the BQL.  */
-    atomic_rcu_set(&as->current_map, new_view);
-    call_rcu(old_view, flatview_unref, rcu);
-
-    /* Note that all the old MemoryRegions are still alive up to this
-     * point.  This relieves most MemoryListeners from the need to
-     * ref/unref the MemoryRegions they get---unless they use them
-     * outside the iothread mutex, in which case precise reference
-     * counting is necessary.
-     */
-    flatview_unref(old_view);
-
-    address_space_update_ioeventfds(as);
+    while (mr->alias && !mr->alias_offset &&
+           int128_ge(mr->size, mr->alias->size)) {
+        /* The alias is included in its entirety.  Use it as
+         * the "real" root, so that we can share more FlatViews.
+         */
+        mr = mr->alias;
+    }
+
+    return mr;
+}
+
+static bool flatview_can_share(FlatView *old_view, FlatView *new_view)
+{
+    MemoryRegion *old_root = memory_region_unalias_entire(old_view->root);
+    MemoryRegion *new_root = memory_region_unalias_entire(new_view->root);
+
+    if (old_root == new_root) {
+        return true;
+    }
+
+    if (!old_root->enabled && !new_root->enabled) {
+        return true;
+    }
+
+    return false;
+}
+
+static void flatview_update_topology(void)
+{
+    AddressSpace *as, *asnext;
+    FlatView *old_view, *new_view, *vnext;
+    struct FlatViewList fvs_tmp = QTAILQ_HEAD_INITIALIZER(fvs_tmp);
+    bool found;
+
+    /* Build list of unique FlatViews, FV::root is the key */
+    QTAILQ_FOREACH(old_view, &flat_views, flat_views_link) {
+        found = false;
+        QTAILQ_FOREACH(new_view, &fvs_tmp, flat_views_link) {
+            if (flatview_can_share(old_view, new_view)) {
+                found = true;
+                break;
+            }
+        }
+        if (found) {
+            continue;
+        }
+
+        new_view = generate_memory_topology(old_view->root);
+        flatview_render_new(old_view, new_view);
+        QTAILQ_INSERT_TAIL(&fvs_tmp, new_view, flat_views_link);
+    }
+
+    /* Replace old FVs with new ones */
+    QTAILQ_FOREACH_SAFE(old_view, &flat_views, flat_views_link, vnext) {
+        flatview_ref(old_view);
+
+        found = false;
+        QTAILQ_FOREACH(new_view, &fvs_tmp, flat_views_link) {
+            if (flatview_can_share(old_view, new_view)) {
+                found = true;
+                break;
+            }
+        }
+        assert(found);
+
+        QTAILQ_FOREACH_SAFE(as, &old_view->address_spaces, flat_view_link,
+                            asnext) {
+            address_space_update_topology_pass(as, old_view, new_view, false);
+            address_space_update_topology_pass(as, old_view, new_view, true);
+
+            QTAILQ_REMOVE(&old_view->address_spaces, as, flat_view_link);
+            flatview_unref(old_view);
+
+            atomic_rcu_set(&as->current_map, new_view);
+
+            flatview_ref(new_view);
+            QTAILQ_INSERT_TAIL(&new_view->address_spaces, as, flat_view_link);
+        }
+
+        QTAILQ_REMOVE(&flat_views, old_view, flat_views_link);
+
+        flatview_unref(old_view); /* unref from beginning of the scope */
+    }
+
+    /* Copy new FVs to the global list */
+    QTAILQ_FOREACH_SAFE(new_view, &fvs_tmp, flat_views_link, vnext) {
+        QTAILQ_REMOVE(&fvs_tmp, new_view, flat_views_link);
+        flatview_unref(new_view);
+
+        QTAILQ_INSERT_HEAD(&flat_views, new_view, flat_views_link);
+    }
 }
 
 void memory_region_transaction_begin(void)
@@ -994,9 +1071,10 @@  void memory_region_transaction_commit(void)
     if (!memory_region_transaction_depth) {
         if (memory_region_update_pending) {
             MEMORY_LISTENER_CALL_GLOBAL(begin, Forward);
+            flatview_update_topology();
 
             QTAILQ_FOREACH(as, &address_spaces, address_spaces_link) {
-                address_space_update_topology(as);
+                address_space_update_ioeventfds(as);
             }
             memory_region_update_pending = false;
             MEMORY_LISTENER_CALL_GLOBAL(commit, Forward);
@@ -2685,8 +2763,6 @@  void memory_region_invalidate_mmio_ptr(MemoryRegion *mr, hwaddr offset,
 void address_space_init(AddressSpace *as, MemoryRegion *root, const char *name)
 {
     memory_region_transaction_begin();
-    as->ref_count = 1;
-    as->malloced = false;
     as->current_map = flatview_alloc(root);
     as->ioeventfd_nb = 0;
     as->ioeventfds = NULL;
@@ -2694,6 +2770,8 @@  void address_space_init(AddressSpace *as, MemoryRegion *root, const char *name)
     QTAILQ_INSERT_TAIL(&address_spaces, as, address_spaces_link);
     as->name = g_strdup(name ? name : "anonymous");
     memory_region_update_pending |= root->enabled;
+    QTAILQ_INSERT_TAIL(&flat_views, as->current_map, flat_views_link);
+    QTAILQ_INSERT_TAIL(&as->current_map->address_spaces, as, flat_view_link);
     memory_region_transaction_commit();
 }
 
@@ -2704,41 +2782,29 @@  MemoryRegion *address_space_root(AddressSpace *as)
 
 static void do_address_space_destroy(AddressSpace *as)
 {
-    bool do_free = as->malloced;
-
     assert(QTAILQ_EMPTY(&as->listeners));
 
+    QTAILQ_REMOVE(&as->current_map->address_spaces, as, flat_view_link);
+
     flatview_unref(as->current_map);
+
     g_free(as->name);
     g_free(as->ioeventfds);
-    if (do_free) {
-        g_free(as);
-    }
+    g_free(as);
 }
 
 AddressSpace *address_space_init_shareable(MemoryRegion *root, const char *name)
 {
     AddressSpace *as;
 
-    QTAILQ_FOREACH(as, &address_spaces, address_spaces_link) {
-        if (root == address_space_root(as) && as->malloced) {
-            as->ref_count++;
-            return as;
-        }
-    }
-
     as = g_malloc0(sizeof *as);
     address_space_init(as, root, name);
-    as->malloced = true;
+
     return as;
 }
 
 void address_space_destroy(AddressSpace *as)
 {
-    as->ref_count--;
-    if (as->ref_count) {
-        return;
-    }
     /* Flush out anything from MemoryListeners listening in on this */
     memory_region_transaction_begin();
     memory_region_transaction_commit();