Re: [PATCH v3 6/6] mm: shrinkers: add scan interface for shrinker debugfs

[Muchun Song <songmuchun@bytedance.com>]

On Mon, May 23, 2022 at 01:54:24PM -0700, Roman Gushchin wrote:
> On Sun, May 22, 2022 at 07:35:59PM +0800, Muchun Song wrote:
> > On Mon, May 09, 2022 at 11:38:20AM -0700, Roman Gushchin wrote:
> > > Add a scan interface which allows to trigger scanning of a particular
> > > shrinker and specify memcg and numa node. It's useful for testing,
> > > debugging and profiling of a specific scan_objects() callback.
> > > Unlike alternatives (creating a real memory pressure and dropping
> > > caches via /proc/sys/vm/drop_caches) this interface allows to interact
> > > with only one shrinker at once. Also, if a shrinker is misreporting
> > > the number of objects (as some do), it doesn't affect scanning.
> > > 
> > > Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
> > > ---
> > >  .../admin-guide/mm/shrinker_debugfs.rst       | 39 +++++++++-
> > >  mm/shrinker_debug.c                           | 73 +++++++++++++++++++
> > >  2 files changed, 108 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/Documentation/admin-guide/mm/shrinker_debugfs.rst b/Documentation/admin-guide/mm/shrinker_debugfs.rst
> > > index 6783f8190e63..8fecf81d60ee 100644
> > > --- a/Documentation/admin-guide/mm/shrinker_debugfs.rst
> > > +++ b/Documentation/admin-guide/mm/shrinker_debugfs.rst
> > > @@ -5,14 +5,16 @@ Shrinker Debugfs Interface
> > >  ==========================
> > >  
> > >  Shrinker debugfs interface provides a visibility into the kernel memory
> > > -shrinkers subsystem and allows to get information about individual shrinkers.
> > > +shrinkers subsystem and allows to get information about individual shrinkers
> > > +and interact with them.
> > >  
> > >  For each shrinker registered in the system a directory in **<debugfs>/shrinker/**
> > >  is created. The directory's name is composed from the shrinker's name and an
> > >  unique id: e.g. *kfree_rcu-0* or *sb-xfs:vda1-36*.
> > >  
> > > -Each shrinker directory contains the **count** file, which allows to trigger
> > > -the *count_objects()* callback for each memcg and numa node (if applicable).
> > > +Each shrinker directory contains **count** and **scan** files, which allow to
> > > +trigger *count_objects()* and *scan_objects()* callbacks for each memcg and
> > > +numa node (if applicable).
> > >  
> > >  Usage:
> > >  ------
> > > @@ -43,7 +45,7 @@ Usage:
> > >  
> > >      $ cd sb-btrfs\:vda2-24/
> > >      $ ls
> > > -    count
> > > +    count            scan
> > >  
> > >  3. *Count objects*
> > >  
> > > @@ -98,3 +100,32 @@ Usage:
> > >      2877 84 0
> > >      293 1 0
> > >      735 8 0
> > > +
> > > +4. *Scan objects*
> > > +
> > > +  The expected input format::
> > > +
> > > +    <cgroup inode id> <numa id> <number of objects to scan>
> > > +
> > > +  For a non-memcg-aware shrinker or on a system with no memory
> > > +  cgrups **0** should be passed as cgroup id.
> > > +  ::
> > > +
> > > +    $ cd /sys/kernel/debug/shrinker/
> > > +    $ cd sb-btrfs\:vda2-24/
> > > +
> > > +    $ cat count | head -n 5
> > > +    1 212 0
> > > +    21 97 0
> > > +    55 802 5
> > > +    2367 2 0
> > > +    225 13 0
> > > +
> > > +    $ echo "55 0 200" > scan
> > > +
> > > +    $ cat count | head -n 5
> > > +    1 212 0
> > > +    21 96 0
> > > +    55 752 5
> > > +    2367 2 0
> > > +    225 13 0
> > > diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c
> > > index 28b1c1ab60ef..8f67fef5a643 100644
> > > --- a/mm/shrinker_debug.c
> > > +++ b/mm/shrinker_debug.c
> > > @@ -101,6 +101,77 @@ static int shrinker_debugfs_count_show(struct seq_file *m, void *v)
> > >  }
> > >  DEFINE_SHOW_ATTRIBUTE(shrinker_debugfs_count);
> > >  
> > > +static int shrinker_debugfs_scan_open(struct inode *inode, struct file *file)
> > > +{
> > > +	file->private_data = inode->i_private;
> > > +	return nonseekable_open(inode, file);
> > > +}
> > > +
> > > +static ssize_t shrinker_debugfs_scan_write(struct file *file,
> > > +					   const char __user *buf,
> > > +					   size_t size, loff_t *pos)
> > > +{
> > > +	struct shrinker *shrinker = (struct shrinker *)file->private_data;
> > 
> > Seems we could drop the cast since ->private_data is void * type.
> 
> Yep, fixed. Thanks!
> 
> > 
> > > +	unsigned long nr_to_scan = 0, ino;
> > > +	struct shrink_control sc = {
> > > +		.gfp_mask = GFP_KERNEL,
> > > +	};
> > > +	struct mem_cgroup *memcg = NULL;
> > > +	int nid;
> > > +	char kbuf[72];
> > > +	int read_len = size < (sizeof(kbuf) - 1) ? size : (sizeof(kbuf) - 1);
> > > +	ssize_t ret;
> > > +
> > > +	if (copy_from_user(kbuf, buf, read_len))
> > > +		return -EFAULT;
> > > +	kbuf[read_len] = '\0';
> > > +
> > > +	if (sscanf(kbuf, "%lu %d %lu", &ino, &nid, &nr_to_scan) < 2)
> > > +		return -EINVAL;
> > > +
> > > +	if (nid < 0 || nid >= nr_node_ids)
> > > +		return -EINVAL;
> > > +
> > 
> > Should we break here if nr_to_scan is zero?
> 
> Not a very likely scenario, but ok.
>

Agree.
 
> > 
> > > +	if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
> > > +		memcg = mem_cgroup_get_from_ino(ino);
> > > +		if (!memcg || IS_ERR(memcg))
> > 
> > Should we drop the check of "!memcg" since mem_cgroup_get_from_ino
> > cannot return NULL?
> 
> It can if !CONFIG_MEMCG. You might argue that then shrinker can not have
> the SHRINKER_MEMCG_AWARE flag, but since it's not a hot path at all,
> I'll keep it for extra safety.
>

Make sense.

> > 
> > > +			return -ENOENT;
> > > +
> > > +		if (!mem_cgroup_online(memcg)) {
> > > +			mem_cgroup_put(memcg);
> > > +			return -ENOENT;
> > > +		}
> > > +	} else {
> > > +		if (ino != 0)
> > > +			return -EINVAL;
> > > +		memcg = NULL;
> > 
> > IIUC, memcg is already NULL if we reach here, right?  Then the
> > assignment is not necessary.  Or we cound remove the initialization
> > of 'memcg' where it is definned.
> 
> Right, removed.
> 
> > 
> > > +	}
> > > +
> > > +	ret = down_read_killable(&shrinker_rwsem);
> > > +	if (ret) {
> > > +		mem_cgroup_put(memcg);
> > > +		return ret;
> > > +	}
> > > +
> > > +	sc.nid = nid;
> > > +	sc.memcg = memcg;
> > > +	sc.nr_to_scan = nr_to_scan;
> > > +	sc.nr_scanned = nr_to_scan;
> > > +
> > > +	shrinker->scan_objects(shrinker, &sc);
> > > +
> > > +	up_read(&shrinker_rwsem);
> > > +	mem_cgroup_put(memcg);
> > > +
> > > +	return ret ? ret : size;
> > 
> > Seems "ret" is always equal to 0 here, should we simplify this
> > to "return size"?
> 
> Right.
> 
> Thank you for the review!
>

My pleasure.

Thanks.