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Message-Id: <20091228131440.3a49a943.kamezawa.hiroyu@jp.fujitsu.com>
Date: Mon, 28 Dec 2009 13:14:40 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To: "Kirill A. Shutemov" <kirill@...temov.name>
Cc: containers@...ts.linux-foundation.org, linux-mm@...ck.org,
Paul Menage <menage@...gle.com>,
Li Zefan <lizf@...fujitsu.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Balbir Singh <balbir@...ux.vnet.ibm.com>,
Pavel Emelyanov <xemul@...nvz.org>,
Dan Malek <dan@...eddedalley.com>,
Vladislav Buzov <vbuzov@...eddedalley.com>,
Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
Alexander Shishkin <virtuoso@...nd.org>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 4/4] memcg: implement memory thresholds
On Mon, 28 Dec 2009 05:23:51 +0200
"Kirill A. Shutemov" <kirill@...temov.name> wrote:
> On Mon, Dec 28, 2009 at 4:43 AM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@...fujitsu.com> wrote:
> > On Sun, 27 Dec 2009 04:09:02 +0200
> > "Kirill A. Shutemov" <kirill@...temov.name> wrote:
/*
> >> * Statistics for memory cgroup.
> >> @@ -72,6 +79,8 @@ enum mem_cgroup_stat_index {
> >> MEM_CGROUP_STAT_SWAPOUT, /* # of pages, swapped out */
> >> MEM_CGROUP_STAT_SOFTLIMIT, /* decrements on each page in/out.
> >> used by soft limit implementation */
> >> + MEM_CGROUP_STAT_THRESHOLDS, /* decrements on each page in/out.
> >> + used by threshold implementation */
> >>
> >> MEM_CGROUP_STAT_NSTATS,
> >> };
> >> @@ -182,6 +191,20 @@ struct mem_cgroup_tree {
> >>
> >> static struct mem_cgroup_tree soft_limit_tree __read_mostly;
> >>
> >> +struct mem_cgroup_threshold {
> >> + struct eventfd_ctx *eventfd;
> >> + u64 threshold;
> >> +};
> >> +
> >> +struct mem_cgroup_threshold_ary {
> >> + unsigned int size;
> >> + atomic_t cur;
> >> + struct mem_cgroup_threshold entries[0];
> >> +};
> >> +
> > Why "array" is a choice here ? IOW, why not list ?
>
> We need be able to walk by thresholds in both directions to be fast.
> AFAIK, It's impossible with RCU-protected list.
>
I couldn't read your code correctly. Could you add a comment on
atomic_t cur; /* An array index points to XXXXX */
or use better name ?
> > How many waiters are expected as usual workload ?
>
> Array of thresholds reads every 100 page in/out for every CPU.
> Write access only when registering new threshold.
>
> >> +static bool mem_cgroup_threshold_check(struct mem_cgroup* mem);
> >> +static void mem_cgroup_threshold(struct mem_cgroup* mem);
> >> +
> >> /*
> >> * The memory controller data structure. The memory controller controls both
> >> * page cache and RSS per cgroup. We would eventually like to provide
> >> @@ -233,6 +256,15 @@ struct mem_cgroup {
> >> /* set when res.limit == memsw.limit */
> >> bool memsw_is_minimum;
> >>
> >> + /* protect arrays of thresholds */
> >> + struct mutex thresholds_lock;
> >> +
> >> + /* thresholds for memory usage. RCU-protected */
> >> + struct mem_cgroup_threshold_ary *thresholds;
> >> +
> >> + /* thresholds for mem+swap usage. RCU-protected */
> >> + struct mem_cgroup_threshold_ary *memsw_thresholds;
> >> +
> >> /*
> >> * statistics. This must be placed at the end of memcg.
> >> */
> >> @@ -525,6 +557,8 @@ static void mem_cgroup_charge_statistics(struct mem_cgroup *mem,
> >> __mem_cgroup_stat_add_safe(cpustat,
> >> MEM_CGROUP_STAT_PGPGOUT_COUNT, 1);
> >> __mem_cgroup_stat_add_safe(cpustat, MEM_CGROUP_STAT_SOFTLIMIT, -1);
> >> + __mem_cgroup_stat_add_safe(cpustat, MEM_CGROUP_STAT_THRESHOLDS, -1);
> >> +
> >> put_cpu();
> >> }
> >>
> >> @@ -1510,6 +1544,8 @@ charged:
> >> if (mem_cgroup_soft_limit_check(mem))
> >> mem_cgroup_update_tree(mem, page);
> >> done:
> >> + if (mem_cgroup_threshold_check(mem))
> >> + mem_cgroup_threshold(mem);
> >> return 0;
> >> nomem:
> >> css_put(&mem->css);
> >> @@ -2075,6 +2111,8 @@ __mem_cgroup_uncharge_common(struct page *page, enum charge_type ctype)
> >>
> >> if (mem_cgroup_soft_limit_check(mem))
> >> mem_cgroup_update_tree(mem, page);
> >> + if (mem_cgroup_threshold_check(mem))
> >> + mem_cgroup_threshold(mem);
> >> /* at swapout, this memcg will be accessed to record to swap */
> >> if (ctype != MEM_CGROUP_CHARGE_TYPE_SWAPOUT)
> >> css_put(&mem->css);
> >> @@ -3071,12 +3109,246 @@ static int mem_cgroup_swappiness_write(struct cgroup *cgrp, struct cftype *cft,
> >> return 0;
> >> }
> >>
> >> +static bool mem_cgroup_threshold_check(struct mem_cgroup *mem)
> >> +{
> >> + bool ret = false;
> >> + int cpu;
> >> + s64 val;
> >> + struct mem_cgroup_stat_cpu *cpustat;
> >> +
> >> + cpu = get_cpu();
> >> + cpustat = &mem->stat.cpustat[cpu];
> >> + val = __mem_cgroup_stat_read_local(cpustat, MEM_CGROUP_STAT_THRESHOLDS);
> >> + if (unlikely(val < 0)) {
> >> + __mem_cgroup_stat_set(cpustat, MEM_CGROUP_STAT_THRESHOLDS,
> >> + THRESHOLDS_EVENTS_THRESH);
> >> + ret = true;
> >> + }
> >> + put_cpu();
> >> + return ret;
> >> +}
> >> +
> >> +static void __mem_cgroup_threshold(struct mem_cgroup *memcg, bool swap)
> >> +{
> >> + struct mem_cgroup_threshold_ary *thresholds;
> >> + u64 usage = mem_cgroup_usage(memcg, swap);
> >> + int i, cur;
> >> +
> >> + rcu_read_lock();
> >> + if (!swap) {
> >> + thresholds = rcu_dereference(memcg->thresholds);
> >> + } else {
> >> + thresholds = rcu_dereference(memcg->memsw_thresholds);
> >> + }
> >> +
> >> + if (!thresholds)
> >> + goto unlock;
> >> +
> >> + cur = atomic_read(&thresholds->cur);
> >> +
> >> + /* Check if a threshold crossed in any direction */
> >> +
> >> + for(i = cur; i >= 0 &&
> >> + unlikely(thresholds->entries[i].threshold > usage); i--) {
> >> + atomic_dec(&thresholds->cur);
> >> + eventfd_signal(thresholds->entries[i].eventfd, 1);
> >> + }
> >> +
> >> + for(i = cur + 1; i < thresholds->size &&
> >> + unlikely(thresholds->entries[i].threshold <= usage); i++) {
> >> + atomic_inc(&thresholds->cur);
> >> + eventfd_signal(thresholds->entries[i].eventfd, 1);
> >> + }
Could you add explanation here ?
> >> +unlock:
> >> + rcu_read_unlock();
> >> +}
> >> +
> >> +static void mem_cgroup_threshold(struct mem_cgroup *memcg)
> >> +{
> >> + __mem_cgroup_threshold(memcg, false);
> >> + if (do_swap_account)
> >> + __mem_cgroup_threshold(memcg, true);
> >> +}
> >> +
> >> +static int compare_thresholds(const void *a, const void *b)
> >> +{
> >> + const struct mem_cgroup_threshold *_a = a;
> >> + const struct mem_cgroup_threshold *_b = b;
> >> +
> >> + return _a->threshold - _b->threshold;
> >> +}
> >> +
> >> +static int mem_cgroup_register_event(struct cgroup *cgrp, struct cftype *cft,
> >> + struct eventfd_ctx *eventfd, const char *args)
> >> +{
> >> + struct mem_cgroup *memcg = mem_cgroup_from_cont(cgrp);
> >> + struct mem_cgroup_threshold_ary *thresholds, *thresholds_new;
> >> + int type = MEMFILE_TYPE(cft->private);
> >> + u64 threshold, usage;
> >> + int size;
> >> + int i, ret;
> >> +
> >> + ret = res_counter_memparse_write_strategy(args, &threshold);
> >> + if (ret)
> >> + return ret;
> >> +
> >> + mutex_lock(&memcg->thresholds_lock);
> >> + if (type == _MEM)
> >> + thresholds = memcg->thresholds;
> >> + else if (type == _MEMSWAP)
> >> + thresholds = memcg->memsw_thresholds;
> >> + else
> >> + BUG();
> >> +
> >> + usage = mem_cgroup_usage(memcg, type == _MEMSWAP);
> >> +
> >> + /* Check if a threshold crossed before adding a new one */
> >> + if (thresholds)
> >> + __mem_cgroup_threshold(memcg, type == _MEMSWAP);
> >> +
> >> + if (thresholds)
> >> + size = thresholds->size + 1;
> >> + else
> >> + size = 1;
> >> +
> >> + /* Allocate memory for new array of thresholds */
> >> + thresholds_new = kmalloc(sizeof(*thresholds_new) +
> >> + size * sizeof(struct mem_cgroup_threshold),
> >> + GFP_KERNEL);
> >> + if (!thresholds_new) {
> >> + ret = -ENOMEM;
> >> + goto unlock;
> >> + }
> >> + thresholds_new->size = size;
> >> +
> >> + /* Copy thresholds (if any) to new array */
> >> + if (thresholds)
> >> + memcpy(thresholds_new->entries, thresholds->entries,
> >> + thresholds->size *
> >> + sizeof(struct mem_cgroup_threshold));
> >> + /* Add new threshold */
> >> + thresholds_new->entries[size - 1].eventfd = eventfd;
> >> + thresholds_new->entries[size - 1].threshold = threshold;
> >> +
> >> + /* Sort thresholds. Registering of new threshold isn't time-critical */
> >> + sort(thresholds_new->entries, size,
> >> + sizeof(struct mem_cgroup_threshold),
> >> + compare_thresholds, NULL);
> >> +
> >> + /* Find current threshold */
> >> + atomic_set(&thresholds_new->cur, -1);
> >> + for(i = 0; i < size; i++) {
> >> + if (thresholds_new->entries[i].threshold < usage)
> >> + atomic_inc(&thresholds_new->cur);
> >> + }
> >> +
> >> + /*
> >> + * We need to increment refcnt to be sure that all thresholds
> >> + * will be unregistered before calling __mem_cgroup_free()
> >> + */
> >> + mem_cgroup_get(memcg);
> >> +
> >> + if (type == _MEM)
> >> + rcu_assign_pointer(memcg->thresholds, thresholds_new);
> >> + else
> >> + rcu_assign_pointer(memcg->memsw_thresholds, thresholds_new);
> >> +
> >> + synchronize_rcu();
> >
> > Could you add explanation when you use synchronize_rcu() ?
>
> It uses before freeing old array of thresholds to be sure than nobody uses it.
>
> >> + kfree(thresholds);
> >
> > Can't this be freed by RCU instead of synchronize_rcu() ?
>
> Yes, this can. But I don't think that (un)registering os thresholds is
> time critical.
> I think my variant is more clean.
>
I don't ;) But ok, this is a nitpick. Ignore me but add an explanation
commentary in codes.
> >> +unlock:
> >> + mutex_unlock(&memcg->thresholds_lock);
> >> +
> >> + return ret;
> >> +}
> >> +
> >> +static int mem_cgroup_unregister_event(struct cgroup *cgrp, struct cftype *cft,
> >> + struct eventfd_ctx *eventfd)
> >> +{
> >> + struct mem_cgroup *memcg = mem_cgroup_from_cont(cgrp);
> >> + struct mem_cgroup_threshold_ary *thresholds, *thresholds_new;
> >> + int type = MEMFILE_TYPE(cft->private);
> >> + u64 usage;
> >> + int size = 0;
> >> + int i, j, ret;
> >> +
> >> + mutex_lock(&memcg->thresholds_lock);
> >> + if (type == _MEM)
> >> + thresholds = memcg->thresholds;
> >> + else if (type == _MEMSWAP)
> >> + thresholds = memcg->memsw_thresholds;
> >> + else
> >> + BUG();
> >> +
> >> + /*
> >> + * Something went wrong if we trying to unregister a threshold
> >> + * if we don't have thresholds
> >> + */
> >> + BUG_ON(!thresholds);
> >> +
> >> + usage = mem_cgroup_usage(memcg, type == _MEMSWAP);
> >> +
> >> + /* Check if a threshold crossed before removing */
> >> + __mem_cgroup_threshold(memcg, type == _MEMSWAP);
> >> +
> >> + /* Calculate new number of threshold */
> >> + for(i = 0; i < thresholds->size; i++) {
> >> + if (thresholds->entries[i].eventfd != eventfd)
> >> + size++;
> >> + }
> >> +
> >> + /* Set thresholds array to NULL if we don't have thresholds */
> >> + if (!size) {
> >> + thresholds_new = NULL;
> >> + goto assign;
> >> + }
> >> +
> >> + /* Allocate memory for new array of thresholds */
> >> + thresholds_new = kmalloc(sizeof(*thresholds_new) +
> >> + size * sizeof(struct mem_cgroup_threshold),
> >> + GFP_KERNEL);
> >> + if (!thresholds_new) {
> >> + ret = -ENOMEM;
> >> + goto unlock;
> >> + }
> >> + thresholds_new->size = size;
> >> +
> >> + /* Copy thresholds and find current threshold */
> >> + atomic_set(&thresholds_new->cur, -1);
> >> + for(i = 0, j = 0; i < thresholds->size; i++) {
> >> + if (thresholds->entries[i].eventfd == eventfd)
> >> + continue;
> >> +
> >> + thresholds_new->entries[j] = thresholds->entries[i];
> >> + if (thresholds_new->entries[j].threshold < usage)
> >> + atomic_inc(&thresholds_new->cur);
> > It's better to do atomic set after loop.
>
> We need one more counter to do this. Do you like it?
>
Please add a comment that "cur" is for what or use better name.
Honestly, I don't understand fully how "cur" moves. I'm not sure
whether updating at insert/delete is really necessary or not.
> >> + j++;
> >> + }
> >
> > Hmm..is this "copy array" usual coding style for handling eventfd ?
>
> Since we store only pointer to struct eventfd_ctx, I don't see a problem.
>
Following is just an suggestion after brief look...
IMO, "cur" is not necessary in the 1st version.
Using simple list and do full-scan always will be good as first step.
(And do necessary optimization later.)
Then, size of patch will be dramatically small.
I think the "cur" magic complicates details too much.
Thanks,
-Kame
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