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    Subject[RFC][PATCH 2/5] memcg: uncharge in batched manner

    In massive parallel enviroment, res_counter can be a performance bottleneck.
    This patch is a trial for reducing lock contention.
    One strong techinque to reduce lock contention is reducing calls by
    batching some amount of calls int one.

    Considering charge/uncharge chatacteristic,
    - charge is done one by one via demand-paging.
    - uncharge is done by
    - in chunk at munmap, truncate, exit, execve...
    - one by one via vmscan/paging.

    It seems we hace a chance to batched-uncharge.
    This patch is a base patch for batched uncharge. For avoiding
    scattering memcg's structure, this patch adds memcg batch uncharge
    information to the task. please see start/end usage in next patch.

    Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    ---
    include/linux/memcontrol.h | 12 +++++++
    include/linux/sched.h | 7 ++++
    mm/memcontrol.c | 70 +++++++++++++++++++++++++++++++++++++++++----
    3 files changed, 83 insertions(+), 6 deletions(-)

    Index: mmotm-2.6.31-Aug27/include/linux/memcontrol.h
    ===================================================================
    --- mmotm-2.6.31-Aug27.orig/include/linux/memcontrol.h
    +++ mmotm-2.6.31-Aug27/include/linux/memcontrol.h
    @@ -54,6 +54,10 @@ extern void mem_cgroup_rotate_lru_list(s
    extern void mem_cgroup_del_lru(struct page *page);
    extern void mem_cgroup_move_lists(struct page *page,
    enum lru_list from, enum lru_list to);
    +
    +extern void mem_cgroup_uncharge_batch_start(void);
    +extern void mem_cgroup_uncharge_batch_end(void);
    +
    extern void mem_cgroup_uncharge_page(struct page *page);
    extern void mem_cgroup_uncharge_cache_page(struct page *page);
    extern int mem_cgroup_shmem_charge_fallback(struct page *page,
    @@ -151,6 +155,14 @@ static inline void mem_cgroup_cancel_cha
    {
    }

    +static inline void mem_cgroup_uncharge_batch_start(void)
    +{
    +}
    +
    +static inline void mem_cgroup_uncharge_batch_start(void)
    +{
    +}
    +
    static inline void mem_cgroup_uncharge_page(struct page *page)
    {
    }
    Index: mmotm-2.6.31-Aug27/mm/memcontrol.c
    ===================================================================
    --- mmotm-2.6.31-Aug27.orig/mm/memcontrol.c
    +++ mmotm-2.6.31-Aug27/mm/memcontrol.c
    @@ -1837,7 +1837,35 @@ void mem_cgroup_cancel_charge_swapin(str
    css_put(&mem->css);
    }

    +static bool
    +__do_batch_uncharge(struct mem_cgroup *mem, const enum charge_type ctype)
    +{
    + struct memcg_batch_info *batch = NULL;
    + bool uncharge_memsw;
    + /* If swapout, usage of swap doesn't decrease */
    + if (do_swap_account && (ctype == MEM_CGROUP_CHARGE_TYPE_SWAPOUT))
    + uncharge_memsw = false;
    + else
    + uncharge_memsw = true;

    + if (current->memcg_batch.do_batch) {
    + batch = &current->memcg_batch;
    + if (batch->memcg == NULL) {
    + batch->memcg = mem;
    + css_get(&mem->css);
    + }
    + }
    + if (!batch || batch->memcg != mem) {
    + res_counter_uncharge(&mem->res, PAGE_SIZE);
    + if (uncharge_memsw)
    + res_counter_uncharge(&mem->memsw, PAGE_SIZE);
    + } else {
    + batch->pages += PAGE_SIZE;
    + if (uncharge_memsw)
    + batch->memsw += PAGE_SIZE;
    + }
    + return soft_limit_excess;
    +}
    /*
    * uncharge if !page_mapped(page)
    */
    @@ -1886,12 +1914,8 @@ __mem_cgroup_uncharge_common(struct page
    break;
    }

    - if (!mem_cgroup_is_root(mem)) {
    - res_counter_uncharge(&mem->res, PAGE_SIZE);
    - if (do_swap_account &&
    - (ctype != MEM_CGROUP_CHARGE_TYPE_SWAPOUT))
    - res_counter_uncharge(&mem->memsw, PAGE_SIZE);
    - }
    + if (!mem_cgroup_is_root(mem))
    + __do_batch_uncharge(mem, ctype);
    if (ctype == MEM_CGROUP_CHARGE_TYPE_SWAPOUT)
    mem_cgroup_swap_statistics(mem, true);
    mem_cgroup_charge_statistics(mem, pc, false);
    @@ -1938,6 +1962,40 @@ void mem_cgroup_uncharge_cache_page(stru
    __mem_cgroup_uncharge_common(page, MEM_CGROUP_CHARGE_TYPE_CACHE);
    }

    +void mem_cgroup_uncharge_batch_start(void)
    +{
    + VM_BUG_ON(current->memcg_batch.do_batch);
    + /* avoid batch if killed by OOM */
    + if (test_thread_flag(TIF_MEMDIE))
    + return;
    + current->memcg_batch.do_batch = 1;
    + current->memcg_batch.memcg = NULL;
    + current->memcg_batch.pages = 0;
    + current->memcg_batch.memsw = 0;
    +}
    +
    +void mem_cgroup_uncharge_batch_end(void)
    +{
    + struct mem_cgroup *mem;
    +
    + if (!current->memcg_batch.do_batch)
    + return;
    +
    + current->memcg_batch.do_batch = 0;
    +
    + mem = current->memcg_batch.memcg;
    + if (!mem)
    + return;
    + if (current->memcg_batch.pages)
    + res_counter_uncharge(&mem->res,
    + current->memcg_batch.pages, NULL);
    + if (current->memcg_batch.memsw)
    + res_counter_uncharge(&mem->memsw,
    + current->memcg_batch.memsw, NULL);
    + /* we got css's refcnt */
    + cgroup_release_and_wakeup_rmdir(&mem->css);
    +}
    +
    #ifdef CONFIG_SWAP
    /*
    * called after __delete_from_swap_cache() and drop "page" account.
    Index: mmotm-2.6.31-Aug27/include/linux/sched.h
    ===================================================================
    --- mmotm-2.6.31-Aug27.orig/include/linux/sched.h
    +++ mmotm-2.6.31-Aug27/include/linux/sched.h
    @@ -1540,6 +1540,13 @@ struct task_struct {
    unsigned long trace_recursion;
    #endif /* CONFIG_TRACING */
    unsigned long stack_start;
    +#ifdef CONFIG_CGROUP_MEM_RES_CTLR /* memcg uses this to do batch job */
    + struct memcg_batch_info {
    + bool do_batch;
    + struct mem_cgroup *memcg;
    + long pages, memsw;
    + } memcg_batch;
    +#endif
    };

    /* Future-safe accessor for struct task_struct's cpus_allowed. */


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