Re: [PATCH 2/7] mm: shrinker: Add a .to_text() method for shrinkers

[Qi Zheng]

Hi Kent,

On 2023/11/23 07:25, Kent Overstreet wrote:
> This adds a new callback method to shrinkers which they can use to
> describe anything relevant to memory reclaim about their internal state,
> for example object dirtyness.

I have no objection to this proposal, but maybe it would be better to
place this feature under CONFIG_SHRINKER_DEBUG?

Hi Roman, what do you think?

Also +CC Dave.

More comments below.

> This patch also adds shrinkers_to_text(), which reports on the top 10
> shrinkers - by object count - in sorted order, to be used in OOM
> reporting.
>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Signed-off-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
> ---
>   include/linux/shrinker.h |  6 +++-
>   mm/shrinker.c            | 73 +++++++++++++++++++++++++++++++++++++++-
>   2 files changed, 77 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> index 1a00be90d93a..968c55474e78 100644
> --- a/include/linux/shrinker.h
> +++ b/include/linux/shrinker.h
> @@ -24,6 +24,8 @@ struct shrinker_info {
>   	struct shrinker_info_unit *unit[];
>   };
>   
> +struct seq_buf;
> +
>   /*
>    * This struct is used to pass information from page reclaim to the shrinkers.
>    * We consolidate the values for easier extension later.
> @@ -80,10 +82,12 @@ struct shrink_control {
>    * @flags determine the shrinker abilities, like numa awareness
>    */
>   struct shrinker {
> +	const char *name;
>   	unsigned long (*count_objects)(struct shrinker *,
>   				       struct shrink_control *sc);
>   	unsigned long (*scan_objects)(struct shrinker *,
>   				      struct shrink_control *sc);
> +	void (*to_text)(struct seq_buf *, struct shrinker *);

The "to_text" looks a little strange, how about naming it
"stat_objects"?

>   
>   	long batch;	/* reclaim batch size, 0 = default */
>   	int seeks;	/* seeks to recreate an obj */
> @@ -110,7 +114,6 @@ struct shrinker {
>   #endif
>   #ifdef CONFIG_SHRINKER_DEBUG
>   	int debugfs_id;
> -	const char *name;

The name will only be allocated memory when the CONFIG_SHRINKER_DEBUG is
enabled, otherwise its value is NULL. So you can't move it out and use
it while CONFIG_SHRINKER_DEBUG is disabled.

>   	struct dentry *debugfs_entry;
>   #endif
>   	/* objs pending delete, per node */
> @@ -135,6 +138,7 @@ __printf(2, 3)
>   struct shrinker *shrinker_alloc(unsigned int flags, const char *fmt, ...);
>   void shrinker_register(struct shrinker *shrinker);
>   void shrinker_free(struct shrinker *shrinker);
> +void shrinkers_to_text(struct seq_buf *);
>   
>   static inline bool shrinker_try_get(struct shrinker *shrinker)
>   {
> diff --git a/mm/shrinker.c b/mm/shrinker.c
> index dd91eab43ed3..4976dbac4c83 100644
> --- a/mm/shrinker.c
> +++ b/mm/shrinker.c
> @@ -1,8 +1,9 @@
>   // SPDX-License-Identifier: GPL-2.0
>   #include <linux/memcontrol.h>
> +#include <linux/rculist.h>
>   #include <linux/rwsem.h>
> +#include <linux/seq_buf.h>
>   #include <linux/shrinker.h>
> -#include <linux/rculist.h>
>   #include <trace/events/vmscan.h>
>   
>   #include "internal.h"
> @@ -807,3 +808,73 @@ void shrinker_free(struct shrinker *shrinker)
>   	call_rcu(&shrinker->rcu, shrinker_free_rcu_cb);
>   }
>   EXPORT_SYMBOL_GPL(shrinker_free);
> +
> +void shrinker_to_text(struct seq_buf *out, struct shrinker *shrinker)
> +{
> +	struct shrink_control sc = { .gfp_mask = GFP_KERNEL, };
> +
> +	seq_buf_puts(out, shrinker->name);
> +	seq_buf_printf(out, " objects: %lu\n", shrinker->count_objects(shrinker, &sc));
> +
> +	if (shrinker->to_text) {
> +		shrinker->to_text(out, shrinker);
> +		seq_buf_puts(out, "\n");
> +	}
> +}
> +
> +/**
> + * shrinkers_to_text - Report on shrinkers with highest usage
> + *
> + * This reports on the top 10 shrinkers, by object counts, in sorted order:
> + * intended to be used for OOM reporting.
> + */
> +void shrinkers_to_text(struct seq_buf *out)
> +{
> +	struct shrinker *shrinker;
> +	struct shrinker_by_mem {
> +		struct shrinker	*shrinker;
> +		unsigned long	mem;

The "mem" also looks a little strange, how about naming it
"freeable"?

> +	} shrinkers_by_mem[10];
> +	int i, nr = 0;
> +
> +	if (!mutex_trylock(&shrinker_mutex)) {
> +		seq_buf_puts(out, "(couldn't take shrinker lock)");
> +		return;
> +	}

We now have lockless method (RCU + refcount) to iterate shrinker_list,
please refer to shrink_slab().

Thanks,
Qi

> +
> +	list_for_each_entry(shrinker, &shrinker_list, list) {
> +		struct shrink_control sc = { .gfp_mask = GFP_KERNEL, };
> +		unsigned long mem = shrinker->count_objects(shrinker, &sc);
> +
> +		if (!mem || mem == SHRINK_STOP || mem == SHRINK_EMPTY)
> +			continue;
> +
> +		for (i = 0; i < nr; i++)
> +			if (mem < shrinkers_by_mem[i].mem)
> +				break;
> +
> +		if (nr < ARRAY_SIZE(shrinkers_by_mem)) {
> +			memmove(&shrinkers_by_mem[i + 1],
> +				&shrinkers_by_mem[i],
> +				sizeof(shrinkers_by_mem[0]) * (nr - i));
> +			nr++;
> +		} else if (i) {
> +			i--;
> +			memmove(&shrinkers_by_mem[0],
> +				&shrinkers_by_mem[1],
> +				sizeof(shrinkers_by_mem[0]) * i);
> +		} else {
> +			continue;
> +		}
> +
> +		shrinkers_by_mem[i] = (struct shrinker_by_mem) {
> +			.shrinker = shrinker,
> +			.mem = mem,
> +		};
> +	}
> +
> +	for (i = nr - 1; i >= 0; --i)
> +		shrinker_to_text(out, shrinkers_by_mem[i].shrinker);
> +
> +	mutex_unlock(&shrinker_mutex);
> +}