mm: memcontrol: rewrite charge API
These patches rework memcg charge lifetime to integrate more naturally
with the lifetime of user pages. This drastically simplifies the code and
reduces charging and uncharging overhead. The most expensive part of
charging and uncharging is the page_cgroup bit spinlock, which is removed
entirely after this series.
Here are the top-10 profile entries of a stress test that reads a 128G
sparse file on a freshly booted box, without even a dedicated cgroup (i.e.
executing in the root memcg). Before:
15.36% cat [kernel.kallsyms] [k] copy_user_generic_string
13.31% cat [kernel.kallsyms] [k] memset
11.48% cat [kernel.kallsyms] [k] do_mpage_readpage
4.23% cat [kernel.kallsyms] [k] get_page_from_freelist
2.38% cat [kernel.kallsyms] [k] put_page
2.32% cat [kernel.kallsyms] [k] __mem_cgroup_commit_charge
2.18% kswapd0 [kernel.kallsyms] [k] __mem_cgroup_uncharge_common
1.92% kswapd0 [kernel.kallsyms] [k] shrink_page_list
1.86% cat [kernel.kallsyms] [k] __radix_tree_lookup
1.62% cat [kernel.kallsyms] [k] __pagevec_lru_add_fn
After:
15.67% cat [kernel.kallsyms] [k] copy_user_generic_string
13.48% cat [kernel.kallsyms] [k] memset
11.42% cat [kernel.kallsyms] [k] do_mpage_readpage
3.98% cat [kernel.kallsyms] [k] get_page_from_freelist
2.46% cat [kernel.kallsyms] [k] put_page
2.13% kswapd0 [kernel.kallsyms] [k] shrink_page_list
1.88% cat [kernel.kallsyms] [k] __radix_tree_lookup
1.67% cat [kernel.kallsyms] [k] __pagevec_lru_add_fn
1.39% kswapd0 [kernel.kallsyms] [k] free_pcppages_bulk
1.30% cat [kernel.kallsyms] [k] kfree
As you can see, the memcg footprint has shrunk quite a bit.
text data bss dec hex filename
37970 9892 400 48262 bc86 mm/memcontrol.o.old
35239 9892 400 45531 b1db mm/memcontrol.o
This patch (of 4):
The memcg charge API charges pages before they are rmapped - i.e. have an
actual "type" - and so every callsite needs its own set of charge and
uncharge functions to know what type is being operated on. Worse,
uncharge has to happen from a context that is still type-specific, rather
than at the end of the page's lifetime with exclusive access, and so
requires a lot of synchronization.
Rewrite the charge API to provide a generic set of try_charge(),
commit_charge() and cancel_charge() transaction operations, much like
what's currently done for swap-in:
mem_cgroup_try_charge() attempts to reserve a charge, reclaiming
pages from the memcg if necessary.
mem_cgroup_commit_charge() commits the page to the charge once it
has a valid page->mapping and PageAnon() reliably tells the type.
mem_cgroup_cancel_charge() aborts the transaction.
This reduces the charge API and enables subsequent patches to
drastically simplify uncharging.
As pages need to be committed after rmap is established but before they
are added to the LRU, page_add_new_anon_rmap() must stop doing LRU
additions again. Revive lru_cache_add_active_or_unevictable().
[hughd@google.com: fix shmem_unuse]
[hughd@google.com: Add comments on the private use of -EAGAIN]
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vladimir Davydov <vdavydov@parallels.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/Documentation/cgroups/memcg_test.txt b/Documentation/cgroups/memcg_test.txt
index 80ac454..bcf750d 100644
--- a/Documentation/cgroups/memcg_test.txt
+++ b/Documentation/cgroups/memcg_test.txt
@@ -24,24 +24,7 @@
a page/swp_entry may be charged (usage += PAGE_SIZE) at
- mem_cgroup_charge_anon()
- Called at new page fault and Copy-On-Write.
-
- mem_cgroup_try_charge_swapin()
- Called at do_swap_page() (page fault on swap entry) and swapoff.
- Followed by charge-commit-cancel protocol. (With swap accounting)
- At commit, a charge recorded in swap_cgroup is removed.
-
- mem_cgroup_charge_file()
- Called at add_to_page_cache()
-
- mem_cgroup_cache_charge_swapin()
- Called at shmem's swapin.
-
- mem_cgroup_prepare_migration()
- Called before migration. "extra" charge is done and followed by
- charge-commit-cancel protocol.
- At commit, charge against oldpage or newpage will be committed.
+ mem_cgroup_try_charge()
2. Uncharge
a page/swp_entry may be uncharged (usage -= PAGE_SIZE) by
@@ -69,19 +52,14 @@
to new page is committed. At failure, charge to old page is committed.
3. charge-commit-cancel
- In some case, we can't know this "charge" is valid or not at charging
- (because of races).
- To handle such case, there are charge-commit-cancel functions.
- mem_cgroup_try_charge_XXX
- mem_cgroup_commit_charge_XXX
- mem_cgroup_cancel_charge_XXX
- these are used in swap-in and migration.
+ Memcg pages are charged in two steps:
+ mem_cgroup_try_charge()
+ mem_cgroup_commit_charge() or mem_cgroup_cancel_charge()
At try_charge(), there are no flags to say "this page is charged".
at this point, usage += PAGE_SIZE.
- At commit(), the function checks the page should be charged or not
- and set flags or avoid charging.(usage -= PAGE_SIZE)
+ At commit(), the page is associated with the memcg.
At cancel(), simply usage -= PAGE_SIZE.