Re: [PATCH v4] hugetlb: Add hugetlb.*.numa_stat file

[Shakeel Butt <shakeelb@google.com>]

On Mon, Nov 8, 2021 at 1:05 PM Mina Almasry <almasrymina@google.com> wrote:
>
> For hugetlb backed jobs/VMs it's critical to understand the numa
> information for the memory backing these jobs to deliver optimal
> performance.
>
> Currently this technically can be queried from /proc/self/numa_maps, but
> there are significant issues with that. Namely:
> 1. Memory can be mapped on unmapped.

or*

> 2. numa_maps are per process and need to be aggregated across all
>    processes in the cgroup. For shared memory this is more involved as
>    the userspace needs to make sure it doesn't double count shared
>    mappings.
> 3. I believe querying numa_maps needs to hold the mmap_lock which adds
>    to the contention on this lock.
>
> For these reasons I propose simply adding hugetlb.*.numa_stat file,
> which shows the numa information of the cgroup similarly to
> memory.numa_stat.
>
> On cgroup-v2:
>    cat /sys/fs/cgroup/unified/test/hugetlb.2MB.numa_stat
>    total=2097152 N0=2097152 N1=0
>
> On cgroup-v1:
>    cat /sys/fs/cgroup/hugetlb/test/hugetlb.2MB.numa_stat
>    total=2097152 N0=2097152 N1=0
>    hierarichal_total=2097152 N0=2097152 N1=0
>
> This patch was tested manually by allocating hugetlb memory and querying
> the hugetlb.*.numa_stat file of the cgroup and its parents.
> 
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Shuah Khan <shuah@kernel.org>
> Cc: Miaohe Lin <linmiaohe@huawei.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Muchun Song <songmuchun@bytedance.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Shakeel Butt <shakeelb@google.com>
> Cc: Jue Wang <juew@google.com>
> Cc: Yang Yao <ygyao@google.com>
> Cc: Joanna Li <joannali@google.com>
> Cc: Cannon Matthews <cannonmatthews@google.com>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
>
> Signed-off-by: Mina Almasry <almasrymina@google.com>
>
[...]

> diff --git a/include/linux/hugetlb_cgroup.h b/include/linux/hugetlb_cgroup.h
> index c137396129db..54ff6ec68ed3 100644
> --- a/include/linux/hugetlb_cgroup.h
> +++ b/include/linux/hugetlb_cgroup.h
> @@ -36,6 +36,11 @@ enum hugetlb_memory_event {
>         HUGETLB_NR_MEMORY_EVENTS,
>  };
>
> +struct hugetlb_cgroup_per_node {
> +       /* hugetlb usage in bytes over all hstates. */
> +       unsigned long usage[HUGE_MAX_HSTATE];

Should we use atomic here? Is this safe for all archs?

[...]

>
>  /*
> @@ -292,7 +321,8 @@ static void __hugetlb_cgroup_commit_charge(int idx, unsigned long nr_pages,
>                 return;
>
>         __set_hugetlb_cgroup(page, h_cg, rsvd);
> -       return;
> +       if (!rsvd && h_cg)

No need to check h_cg.

> +               h_cg->nodeinfo[page_to_nid(page)]->usage[idx] += nr_pages;
>  }
>
>  void hugetlb_cgroup_commit_charge(int idx, unsigned long nr_pages,
> @@ -331,7 +361,8 @@ static void __hugetlb_cgroup_uncharge_page(int idx, unsigned long nr_pages,
>
>         if (rsvd)
>                 css_put(&h_cg->css);
> -
> +       else
> +               h_cg->nodeinfo[page_to_nid(page)]->usage[idx] -= nr_pages;
>         return;
>  }
>
> @@ -421,6 +452,58 @@ enum {
>         RES_RSVD_FAILCNT,
>  };
>
> +static int hugetlb_cgroup_read_numa_stat(struct seq_file *seq, void *dummy)
> +{
> +       int nid;
> +       struct cftype *cft = seq_cft(seq);
> +       int idx = MEMFILE_IDX(cft->private);
> +       bool legacy = MEMFILE_ATTR(cft->private);
> +       struct hugetlb_cgroup *h_cg = hugetlb_cgroup_from_css(seq_css(seq));
> +       struct cgroup_subsys_state *css;
> +       unsigned long usage;
> +
> +       if (legacy) {
> +               /* Add up usage across all nodes for the non-hierarchical total. */
> +               usage = 0;
> +               for_each_node_state(nid, N_MEMORY)
> +                       usage += h_cg->nodeinfo[nid]->usage[idx];
> +               seq_printf(seq, "total=%lu", usage * PAGE_SIZE);
> +
> +               /* Simply print the per-node usage for the non-hierarchical total. */
> +               for_each_node_state(nid, N_MEMORY)
> +                       seq_printf(seq, " N%d=%lu", nid,
> +                                  h_cg->nodeinfo[nid]->usage[idx] * PAGE_SIZE);
> +               seq_putc(seq, '\n');
> +       }
> +
> +       /*
> +        * The hierarchical total is pretty much the value recorded by the
> +        * counter, so use that.
> +        */
> +       seq_printf(seq, "%stotal=%lu", legacy ? "hierarichal_" : "",
> +                  page_counter_read(&h_cg->hugepage[idx]) * PAGE_SIZE);
> +
> +       /*
> +        * For each node, transverse the css tree to obtain the hierarichal
> +        * node usage.
> +        */
> +       for_each_node_state(nid, N_MEMORY) {
> +               usage = 0;
> +               rcu_read_lock();
> +               css_for_each_descendant_pre(css, &h_cg->css) {

This will be slow. Do you really want to traverse the hugetlb-cgroup
tree that many times? We had similar issues with memcg stats as well
but got resolved with rstat.

Though I feel like hugetlb-cgroup is not that worried about
performance. There is no batching in the charging and uncharging
codepaths.

> +                       usage += hugetlb_cgroup_from_css(css)
> +                                        ->nodeinfo[nid]
> +                                        ->usage[idx];
> +               }
> +               rcu_read_unlock();
> +               seq_printf(seq, " N%d=%lu", nid, usage * PAGE_SIZE);
> +       }
> +
> +       seq_putc(seq, '\n');
> +
> +       return 0;
> +}