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    SubjectRe: [PATCH 2/2] memcg: coalescing charges per cpu
    On Fri, 9 Oct 2009 09:45:36 +0530
    Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

    > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-10-02 14:03:43]:
    >
    > > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    > >
    > > This is a patch for coalescing access to res_counter at charging by
    > > percpu caching. At charge, memcg charges 64pages and remember it in
    > > percpu cache. Because it's cache, drain/flush is done if necessary.
    > >
    > > This version uses public percpu area.
    > > 2 benefits of using public percpu area.
    > > 1. Sum of stocked charge in the system is limited to # of cpus
    > > not to the number of memcg. This shows better synchonization.
    > > 2. drain code for flush/cpuhotplug is very easy (and quick)
    > >
    > > The most important point of this patch is that we never touch res_counter
    > > in fast path. The res_counter is system-wide shared counter which is modified
    > > very frequently. We shouldn't touch it as far as we can for avoiding
    > > false sharing.
    > >
    > > Changelog (new):
    > > - rabased onto the latest mmotm
    > > Changelog (old):
    > > - moved charge size check before __GFP_WAIT check for avoiding unnecesary
    > > - added asynchronous flush routine.
    > > - fixed bugs pointed out by Nishimura-san.
    > >
    > > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    > > ---
    > > mm/memcontrol.c | 126 +++++++++++++++++++++++++++++++++++++++++++++++++++++---
    > > 1 file changed, 120 insertions(+), 6 deletions(-)
    > >
    > > Index: mmotm-2.6.31-Sep28/mm/memcontrol.c
    > > ===================================================================
    > > --- mmotm-2.6.31-Sep28.orig/mm/memcontrol.c
    > > +++ mmotm-2.6.31-Sep28/mm/memcontrol.c
    > > @@ -38,6 +38,7 @@
    > > #include <linux/vmalloc.h>
    > > #include <linux/mm_inline.h>
    > > #include <linux/page_cgroup.h>
    > > +#include <linux/cpu.h>
    > > #include "internal.h"
    > >
    > > #include <asm/uaccess.h>
    > > @@ -275,6 +276,7 @@ enum charge_type {
    > > static void mem_cgroup_get(struct mem_cgroup *mem);
    > > static void mem_cgroup_put(struct mem_cgroup *mem);
    > > static struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *mem);
    > > +static void drain_all_stock_async(void);
    > >
    > > static struct mem_cgroup_per_zone *
    > > mem_cgroup_zoneinfo(struct mem_cgroup *mem, int nid, int zid)
    > > @@ -1137,6 +1139,8 @@ static int mem_cgroup_hierarchical_recla
    > > victim = mem_cgroup_select_victim(root_mem);
    > > if (victim == root_mem) {
    > > loop++;
    > > + if (loop >= 1)
    > > + drain_all_stock_async();
    > > if (loop >= 2) {
    > > /*
    > > * If we have not been able to reclaim
    > > @@ -1258,6 +1262,103 @@ done:
    > > unlock_page_cgroup(pc);
    > > }
    > >
    > > +/* size of first charge trial. "32" comes from vmscan.c's magic value */
    > > +#define CHARGE_SIZE (32 * PAGE_SIZE)
    >
    > Should this then be SWAP_CLUSTER_MAX instead of 32?
    >
    I'm not sure what number is the best here. So, just borrow a number from vmscan.
    (maybe not very bad number.)
    I'd like to keep this as it is for a while but write why I selected this more.
    as..
    ==
    SWAP_CLUSTER_MAX(32) is the number of pages reclaimed by vmscan.c in a turn.
    So, we borrow that number here. Considering scalability, this should
    be proportional to the number of cpus. But it's a TO-DO now.
    ==
    Maybe we cannot update this until we get 32 or 64+cpus machine...


    > > +struct memcg_stock_pcp {
    > > + struct mem_cgroup *cached;
    > > + int charge;
    > > + struct work_struct work;
    > > +};
    > > +static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock);
    > > +static DEFINE_MUTEX(memcg_drain_mutex);
    > > +
    > > +static bool consume_stock(struct mem_cgroup *mem)
    > > +{
    > > + struct memcg_stock_pcp *stock;
    > > + bool ret = true;
    > > +
    > > + stock = &get_cpu_var(memcg_stock);
    > > + if (mem == stock->cached && stock->charge)
    > > + stock->charge -= PAGE_SIZE;
    > > + else
    > > + ret = false;
    > > + put_cpu_var(memcg_stock);
    > > + return ret;
    > > +}
    >
    >
    > Shouldn't consume stock and routines below check for memcg_is_root()?
    >
    This is never called if memcg_is_root(). It's checked by caller.


    > > +
    > > +static void drain_stock(struct memcg_stock_pcp *stock)
    > > +{
    > > + struct mem_cgroup *old = stock->cached;
    > > +
    > > + if (stock->charge) {
    > > + res_counter_uncharge(&old->res, stock->charge);
    > > + if (do_swap_account)
    > > + res_counter_uncharge(&old->memsw, stock->charge);
    > > + }
    > > + stock->cached = NULL;
    > > + stock->charge = 0;
    > > +}
    > > +
    > > +static void drain_local_stock(struct work_struct *dummy)
    > > +{
    > > + struct memcg_stock_pcp *stock = &get_cpu_var(memcg_stock);
    > > + drain_stock(stock);
    > > + put_cpu_var(memcg_stock);
    > > +}
    > > +
    > > +static void refill_stock(struct mem_cgroup *mem, int val)
    > > +{
    > > + struct memcg_stock_pcp *stock = &get_cpu_var(memcg_stock);
    > > +
    > > + if (stock->cached != mem) {
    > > + drain_stock(stock);
    > > + stock->cached = mem;
    > > + }
    >
    > More comments here would help
    >
    Ah, yes. I'll add.


    > > + stock->charge += val;
    > > + put_cpu_var(memcg_stock);
    > > +}
    > > +
    > > +static void drain_all_stock_async(void)
    > > +{
    > > + int cpu;
    > > + /* Contention means someone tries to flush. */
    > > + if (!mutex_trylock(&memcg_drain_mutex))
    > > + return;
    > > + for_each_online_cpu(cpu) {
    > > + struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
    > > + if (work_pending(&stock->work))
    > > + continue;
    > > + INIT_WORK(&stock->work, drain_local_stock);
    > > + schedule_work_on(cpu, &stock->work);
    > > + }
    > > + mutex_unlock(&memcg_drain_mutex);
    > > + /* We don't wait for flush_work */
    > > +}
    > > +
    > > +static void drain_all_stock_sync(void)
    > > +{
    > > + /* called when force_empty is called */
    > > + mutex_lock(&memcg_drain_mutex);
    > > + schedule_on_each_cpu(drain_local_stock);
    > > + mutex_unlock(&memcg_drain_mutex);
    > > +}
    > > +
    > > +static int __cpuinit memcg_stock_cpu_callback(struct notifier_block *nb,
    > > + unsigned long action,
    > > + void *hcpu)
    > > +{
    > > +#ifdef CONFIG_HOTPLUG_CPU
    > > + int cpu = (unsigned long)hcpu;
    > > + struct memcg_stock_pcp *stock;
    > > +
    > > + if (action != CPU_DEAD)
    > > + return NOTIFY_OK;
    > > + stock = &per_cpu(memcg_stock, cpu);
    > > + drain_stock(stock);
    > > +#endif
    > > + return NOTIFY_OK;
    > > +}
    > > +
    > > /*
    > > * Unlike exported interface, "oom" parameter is added. if oom==true,
    > > * oom-killer can be invoked.
    > > @@ -1269,6 +1370,7 @@ static int __mem_cgroup_try_charge(struc
    > > struct mem_cgroup *mem, *mem_over_limit;
    > > int nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
    > > struct res_counter *fail_res;
    > > + int csize = CHARGE_SIZE;
    > >
    > > if (unlikely(test_thread_flag(TIF_MEMDIE))) {
    > > /* Don't account this! */
    > > @@ -1293,23 +1395,25 @@ static int __mem_cgroup_try_charge(struc
    > > return 0;
    > >
    > > VM_BUG_ON(css_is_removed(&mem->css));
    > > + if (mem_cgroup_is_root(mem))
    > > + goto done;
    > >
    > > while (1) {
    > > int ret = 0;
    > > unsigned long flags = 0;
    > >
    > > - if (mem_cgroup_is_root(mem))
    > > - goto done;
    > > - ret = res_counter_charge(&mem->res, PAGE_SIZE, &fail_res);
    > > + if (consume_stock(mem))
    > > + goto charged;
    > > +
    > > + ret = res_counter_charge(&mem->res, csize, &fail_res);
    > > if (likely(!ret)) {
    > > if (!do_swap_account)
    > > break;
    > > - ret = res_counter_charge(&mem->memsw, PAGE_SIZE,
    > > - &fail_res);
    > > + ret = res_counter_charge(&mem->memsw, csize, &fail_res);
    > > if (likely(!ret))
    > > break;
    > > /* mem+swap counter fails */
    > > - res_counter_uncharge(&mem->res, PAGE_SIZE);
    > > + res_counter_uncharge(&mem->res, csize);
    > > flags |= MEM_CGROUP_RECLAIM_NOSWAP;
    > > mem_over_limit = mem_cgroup_from_res_counter(fail_res,
    > > memsw);
    > > @@ -1318,6 +1422,11 @@ static int __mem_cgroup_try_charge(struc
    > > mem_over_limit = mem_cgroup_from_res_counter(fail_res,
    > > res);
    > >
    > > + /* reduce request size and retry */
    > > + if (csize > PAGE_SIZE) {
    > > + csize = PAGE_SIZE;
    > > + continue;
    > > + }
    > > if (!(gfp_mask & __GFP_WAIT))
    > > goto nomem;
    > >
    > > @@ -1347,6 +1456,9 @@ static int __mem_cgroup_try_charge(struc
    > > goto nomem;
    > > }
    > > }
    > > + if (csize > PAGE_SIZE)
    > > + refill_stock(mem, csize - PAGE_SIZE);
    > > +charged:
    > > /*
    > > * Insert ancestor (and ancestor's ancestors), to softlimit RB-tree.
    > > * if they exceeds softlimit.
    > > @@ -2463,6 +2575,7 @@ move_account:
    > > goto out;
    > > /* This is for making all *used* pages to be on LRU. */
    > > lru_add_drain_all();
    > > + drain_all_stock_sync();
    > > ret = 0;
    > > for_each_node_state(node, N_HIGH_MEMORY) {
    > > for (zid = 0; !ret && zid < MAX_NR_ZONES; zid++) {
    > > @@ -3181,6 +3294,7 @@ mem_cgroup_create(struct cgroup_subsys *
    > > root_mem_cgroup = mem;
    > > if (mem_cgroup_soft_limit_tree_init())
    > > goto free_out;
    > > + hotcpu_notifier(memcg_stock_cpu_callback, 0);
    > >
    > > } else {
    > > parent = mem_cgroup_from_cont(cont->parent);
    > >
    >
    > I tested earlier versions of the patchset and they worked absolutely
    > fine for me!
    >
    Thanks! I'll update commentary.

    Regards,
    -Kame



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