Date:   Mon, 1 Aug 2022 15:28:15 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Xin Hao <xhao@...ux.alibaba.com>, willy@...radead.org
Cc:     akpm@...ux-foundation.org, adobriyan@...il.com,
        keescook@...omium.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [RFC PATCH V4 1/1] mm: add last level page table numa info to
 /proc/pid/numa_pgtable

On 01.08.22 14:17, Xin Hao wrote:
> In many data center servers, the shared memory architectures is
> Non-Uniform Memory Access (NUMA), remote numa node data access
> often brings a high latency problem, but what we are easy to ignore
> is that the page table remote numa access, It can also leads to a
> performance degradation.

Let me try rewriting:

"
Many data center servers employ Non-Uniform Memory Access (NUMA)
architectures. Remote numa memory access results in high latency. While
memory placement is one issue, sub-optimal page table placement can also
result in surprise performance degradation.
"

> 
> So there add a new interface in /proc, This will help developers to
> get more info about performance issues if they are caused by cross-NUMA.


Why do we only care about "last level page table", why not about the others?

IMHO, we could emit something like "0, 1, 3, 0" instead for a given user
space address, showing the NUMA node the page table belongs to from
highest to lowest page table level.

> 
> Reported-by: kernel test robot <lkp@...el.com>

The kernel test robot reported that we need "/proc/pid/numa_pgtable" ?! :)

Just drop that unless it's a follow-up fix.

> Signed-off-by: Xin Hao <xhao@...ux.alibaba.com>
> ---
>  fs/proc/base.c     |  2 ++
>  fs/proc/internal.h |  1 +
>  fs/proc/task_mmu.c | 87 ++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 90 insertions(+)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 8dfa36a99c74..487e82dd3275 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -3224,6 +3224,7 @@ static const struct pid_entry tgid_base_stuff[] = {
>  	REG("maps",       S_IRUGO, proc_pid_maps_operations),
>  #ifdef CONFIG_NUMA
>  	REG("numa_maps",  S_IRUGO, proc_pid_numa_maps_operations),
> +	REG("numa_pgtable", S_IRUGO, proc_pid_numa_pgtable_operations),
>  #endif
>  	REG("mem",        S_IRUSR|S_IWUSR, proc_mem_operations),
>  	LNK("cwd",        proc_cwd_link),
> @@ -3571,6 +3572,7 @@ static const struct pid_entry tid_base_stuff[] = {
>  #endif
>  #ifdef CONFIG_NUMA
>  	REG("numa_maps", S_IRUGO, proc_pid_numa_maps_operations),
> +	REG("numa_pgtable", S_IRUGO, proc_pid_numa_pgtable_operations),
>  #endif
>  	REG("mem",       S_IRUSR|S_IWUSR, proc_mem_operations),
>  	LNK("cwd",       proc_cwd_link),
> diff --git a/fs/proc/internal.h b/fs/proc/internal.h
> index 06a80f78433d..e7ed9ef097b6 100644
> --- a/fs/proc/internal.h
> +++ b/fs/proc/internal.h
> @@ -296,6 +296,7 @@ struct mm_struct *proc_mem_open(struct inode *inode, unsigned int mode);
> 
>  extern const struct file_operations proc_pid_maps_operations;
>  extern const struct file_operations proc_pid_numa_maps_operations;
> +extern const struct file_operations proc_pid_numa_pgtable_operations;
>  extern const struct file_operations proc_pid_smaps_operations;
>  extern const struct file_operations proc_pid_smaps_rollup_operations;
>  extern const struct file_operations proc_clear_refs_operations;
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 2d04e3470d4c..77b7a49757f5 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1999,4 +1999,91 @@ const struct file_operations proc_pid_numa_maps_operations = {
>  	.release	= proc_map_release,
>  };
> 
> +struct pgtable_numa_private {
> +	struct proc_maps_private proc_maps;
> +	unsigned long node[MAX_NUMNODES];
> +};
> +
> +static int gather_pgtable_numa_stats(pmd_t *pmd, unsigned long addr,
> +				     unsigned long end, struct mm_walk *walk)
> +{
> +	struct pgtable_numa_private *priv = walk->private;
> +	struct page *page;
> +	int nid;
> +
> +	if (pmd_huge(*pmd)) {
> +		page = virt_to_page(pmd);
> +	} else {
> +		page = pmd_page(*pmd);
> +	}
> +
> +	nid = page_to_nid(page);
> +	priv->node[nid]++;
> +
> +	return 0;
> +}
> +
> +static const struct mm_walk_ops show_numa_pgtable_ops = {
> +	.pmd_entry = gather_pgtable_numa_stats,
> +};
> +
> +/*
> + * Display the page talbe allocated per node via /proc.

s/talbe/table/

but the comment somehow doesn't make sense. We don't display a page table.

> + */
> +static int show_numa_pgtable(struct seq_file *m, void *v)
> +{
> +	struct pgtable_numa_private *numa_priv = m->private;
> +	struct vm_area_struct *vma = v;
> +	struct mm_struct *mm = vma->vm_mm;
> +	struct file *file = vma->vm_file;
> +	int nid;
> +
> +	if (!mm)
> +		return 0;
> +
> +	memset(numa_priv->node, 0, sizeof(numa_priv->node));
> +
> +	seq_printf(m, "%08lx ", vma->vm_start);
> +
> +	if (file) {
> +		seq_puts(m, " file=");
> +		seq_file_path(m, file, "\n\t= ");
> +	} else if (vma->vm_start <= mm->brk && vma->vm_end >= mm->start_brk) {
> +		seq_puts(m, " heap");
> +	} else if (is_stack(vma)) {
> +		seq_puts(m, " stack");
> +	}
> +
> +	/* mmap_lock is held by m_start */
> +	walk_page_vma(vma, &show_numa_pgtable_ops, numa_priv);
> +
> +	for_each_node_state(nid, N_MEMORY) {
> +		if (numa_priv->node[nid])
> +			seq_printf(m, " N%d=%lu", nid, numa_priv->node[nid]);
> +	}
> +	seq_putc(m, '\n');
> +
> +	return 0;
> +}
> +
> +static const struct seq_operations proc_pid_numa_pgtable_op = {
> +	.start  = m_start,
> +	.next   = m_next,
> +	.stop   = m_stop,
> +	.show   = show_numa_pgtable,
> +};
> +
> +static int pid_numa_pgtable_open(struct inode *inode, struct file *file)
> +{
> +	return proc_maps_open(inode, file, &proc_pid_numa_pgtable_op,
> +			sizeof(struct pgtable_numa_private));
> +}
> +
> +const struct file_operations proc_pid_numa_pgtable_operations = {
> +	.open		= pid_numa_pgtable_open,
> +	.read		= seq_read,
> +	.llseek		= seq_lseek,
> +	.release	= proc_map_release,
> +};
> +
>  #endif /* CONFIG_NUMA */
> --
> 2.31.0
> 


-- 
Thanks,

David / dhildenb

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