On Mon, May 23, 2022 at 11:24:10AM -0700, Roman Gushchin wrote:
> On Sun, May 22, 2022 at 06:36:56PM +0800, Muchun Song wrote:
> > On Mon, May 09, 2022 at 11:38:16AM -0700, Roman Gushchin wrote:
> > > This commit introduces the /sys/kernel/debug/shrinker debugfs
> > > interface which provides an ability to observe the state of
> > > individual kernel memory shrinkers.
> > >
> > > Because the feature adds some memory overhead (which shouldn't be
> > > large unless there is a huge amount of registered shrinkers), it's
> > > guarded by a config option (enabled by default).
> > >
> > > This commit introduces the "count" interface for each shrinker
> > > registered in the system.
> > >
> > > The output is in the following format:
> >
> > Hi Roman,
>
> Hi Muchun!
>
> Thank you for taking a look!
>
> >
> > Shoud we print a title to show what those numbers mean? In this case,
> > it is more understandable.
>
> No, I don't think so: this interface is not supposed to be used by
> an average user and those who will be using it can refer to the provided
> documentation. Printing the header each time will add some overhead for
> no good reason.
>
Got it. Make sense.
> > > <cgroup inode id> <nr of objects on node 0> <nr of objects on node 1>...
> > > <cgroup inode id> <nr of objects on node 0> <nr of objects on node 1>...
> > > ...
> > >
> > > To reduce the size of output on machines with many thousands cgroups,
> > > if the total number of objects on all nodes is 0, the line is omitted.
> > >
> > > If the shrinker is not memcg-aware or CONFIG_MEMCG is off, 0 is
> > > printed as cgroup inode id. If the shrinker is not numa-aware, 0's are
> > > printed for all nodes except the first one.
> > >
> > > This commit gives debugfs entries simple numeric names, which are not
> > > very convenient. The following commit in the series will provide
> > > shrinkers with more meaningful names.
> > >
> > > Signed-off-by: Roman Gushchin <roman.gushchin@xxxxxxxxx>
> > > ---
> > > include/linux/shrinker.h | 19 ++++-
> > > lib/Kconfig.debug | 9 +++
> > > mm/Makefile | 1 +
> > > mm/shrinker_debug.c | 171 +++++++++++++++++++++++++++++++++++++++
> > > mm/vmscan.c | 6 +-
> > > 5 files changed, 203 insertions(+), 3 deletions(-)
> > > create mode 100644 mm/shrinker_debug.c
> > >
> > > diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> > > index 76fbf92b04d9..2ced8149c513 100644
> > > --- a/include/linux/shrinker.h
> > > +++ b/include/linux/shrinker.h
> > > @@ -72,6 +72,10 @@ struct shrinker {
> > > #ifdef CONFIG_MEMCG
> > > /* ID in shrinker_idr */
> > > int id;
> > > +#endif
> > > +#ifdef CONFIG_SHRINKER_DEBUG
> > > + int debugfs_id;
> > > + struct dentry *debugfs_entry;
> > > #endif
> > > /* objs pending delete, per node */
> > > atomic_long_t *nr_deferred;
> > > @@ -94,4 +98,17 @@ extern int register_shrinker(struct shrinker *shrinker);
> > > extern void unregister_shrinker(struct shrinker *shrinker);
> > > extern void free_prealloced_shrinker(struct shrinker *shrinker);
> > > extern void synchronize_shrinkers(void);
> > > -#endif
> > > +
> > > +#ifdef CONFIG_SHRINKER_DEBUG
> > > +extern int shrinker_debugfs_add(struct shrinker *shrinker);
> > > +extern void shrinker_debugfs_remove(struct shrinker *shrinker);
> > > +#else /* CONFIG_SHRINKER_DEBUG */
> > > +static inline int shrinker_debugfs_add(struct shrinker *shrinker)
> > > +{
> > > + return 0;
> > > +}
> > > +static inline void shrinker_debugfs_remove(struct shrinker *shrinker)
> > > +{
> > > +}
> > > +#endif /* CONFIG_SHRINKER_DEBUG */
> > > +#endif /* _LINUX_SHRINKER_H */
> > > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > > index 3fd7a2e9eaf1..5fa65a649798 100644
> > > --- a/lib/Kconfig.debug
> > > +++ b/lib/Kconfig.debug
> > > @@ -733,6 +733,15 @@ config SLUB_STATS
> > > out which slabs are relevant to a particular load.
> > > Try running: slabinfo -DA
> > >
> > > +config SHRINKER_DEBUG
> > > + default y
> > > + bool "Enable shrinker debugging support"
> > > + depends on DEBUG_FS
> > > + help
> > > + Say Y to enable the shrinker debugfs interface which provides
> > > + visibility into the kernel memory shrinkers subsystem.
> > > + Disable it to avoid an extra memory footprint.
> > > +
> > > config HAVE_DEBUG_KMEMLEAK
> > > bool
> > >
> > > diff --git a/mm/Makefile b/mm/Makefile
> > > index 298c9991ab75..8083fa85a348 100644
> > > --- a/mm/Makefile
> > > +++ b/mm/Makefile
> > > @@ -133,3 +133,4 @@ obj-$(CONFIG_PAGE_REPORTING) += page_reporting.o
> > > obj-$(CONFIG_IO_MAPPING) += io-mapping.o
> > > obj-$(CONFIG_HAVE_BOOTMEM_INFO_NODE) += bootmem_info.o
> > > obj-$(CONFIG_GENERIC_IOREMAP) += ioremap.o
> > > +obj-$(CONFIG_SHRINKER_DEBUG) += shrinker_debug.o
> > > diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c
> > > new file mode 100644
> > > index 000000000000..fd1f805a581a
> > > --- /dev/null
> > > +++ b/mm/shrinker_debug.c
> > > @@ -0,0 +1,171 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +#include <linux/idr.h>
> > > +#include <linux/slab.h>
> > > +#include <linux/debugfs.h>
> > > +#include <linux/seq_file.h>
> > > +#include <linux/shrinker.h>
> > > +#include <linux/memcontrol.h>
> > > +
> > > +/* defined in vmscan.c */
> > > +extern struct rw_semaphore shrinker_rwsem;
> > > +extern struct list_head shrinker_list;
> > > +
> > > +static DEFINE_IDA(shrinker_debugfs_ida);
> > > +static struct dentry *shrinker_debugfs_root;
> > > +
> > > +static unsigned long shrinker_count_objects(struct shrinker *shrinker,
> > > + struct mem_cgroup *memcg,
> > > + unsigned long *count_per_node)
> > > +{
> > > + unsigned long nr, total = 0;
> > > + int nid;
> > > +
> > > + for_each_node(nid) {
> > > + if (nid == 0 || (shrinker->flags & SHRINKER_NUMA_AWARE)) {
> > > + struct shrink_control sc = {
> > > + .gfp_mask = GFP_KERNEL,
> > > + .nid = nid,
> > > + .memcg = memcg,
> > > + };
> > > +
> > > + nr = shrinker->count_objects(shrinker, &sc);
> > > + if (nr == SHRINK_EMPTY)
> > > + nr = 0;
> > > + } else {
> > > + nr = 0;
> >
> > For efficiency, we could break here, right?
>
> Not really, we need to fill count_per_node[] with zeros.
>
I thought count_per_node[] was initialized with zero by the caller
when allocated. However, I am wrong. Because it'll be reused
in each loop. You are right.
> >
> > > + }
> > > +
> > > + count_per_node[nid] = nr;
> > > + total += nr;
> > > + }
> > > +
> > > + return total;
> > > +}
> > > +
> > > +static int shrinker_debugfs_count_show(struct seq_file *m, void *v)
> > > +{
> > > + struct shrinker *shrinker = (struct shrinker *)m->private;
> >
> > Maybe we cound drop the cast since m->private is a void * type.
>
> Ok.
>
> >
> > > + unsigned long *count_per_node = NULL;
> >
> > Do not need to be initialized, right?
>
> Right, will fix in v4.
>
> >
> > > + struct mem_cgroup *memcg;
> > > + unsigned long total;
> > > + bool memcg_aware;
> > > + int ret, nid;
> > > +
> > > + count_per_node = kcalloc(nr_node_ids, sizeof(unsigned long), GFP_KERNEL);
> > > + if (!count_per_node)
> > > + return -ENOMEM;
> > > +
> > > + ret = down_read_killable(&shrinker_rwsem);
> > > + if (ret) {
> > > + kfree(count_per_node);
> > > + return ret;
> > > + }
> > > + rcu_read_lock();
> > > +
> > > + memcg_aware = shrinker->flags & SHRINKER_MEMCG_AWARE;
> > > +
> > > + memcg = mem_cgroup_iter(NULL, NULL, NULL);
> > > + do {
> > > + if (memcg && !mem_cgroup_online(memcg))
> > > + continue;
> > > +
> > > + total = shrinker_count_objects(shrinker,
> > > + memcg_aware ? memcg : NULL,
> > > + count_per_node);
> > > + if (total) {
> > > + seq_printf(m, "%lu", mem_cgroup_ino(memcg));
> > > + for_each_node(nid)
> > > + seq_printf(m, " %lu", count_per_node[nid]);
> > > + seq_puts(m, "\n");
> >
> > seq_putc(m, '\n') is more efficient.
>
> Ok.
>
> >
> > > + }
> > > +
> > > + if (!memcg_aware) {
> > > + mem_cgroup_iter_break(NULL, memcg);
> > > + break;
> > > + }
> > > +
> > > + if (signal_pending(current)) {
> > > + mem_cgroup_iter_break(NULL, memcg);
> > > + ret = -EINTR;
> > > + break;
> > > + }
> > > +
> > > + cond_resched();
> >
> > We are in rcu read lock, cannot be scheduled, right?
>
> This is a good one, thanks. Fixed.
>
> >
> > > + } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
> > > +
> > > + rcu_read_unlock();
> > > + up_read(&shrinker_rwsem);
> > > +
> > > + kfree(count_per_node);
> > > + return ret;
> > > +}
> > > +DEFINE_SHOW_ATTRIBUTE(shrinker_debugfs_count);
> > > +
> > > +int shrinker_debugfs_add(struct shrinker *shrinker)
> > > +{
> > > + struct dentry *entry;
> > > + char buf[16];
> > > + int id;
> > > +
> > > + lockdep_assert_held(&shrinker_rwsem);
> > > +
> > > + /* debugfs isn't initialized yet, add debugfs entries later. */
> > > + if (!shrinker_debugfs_root)
> > > + return 0;
> > > +
> > > + id = ida_alloc(&shrinker_debugfs_ida, GFP_KERNEL);
> > > + if (id < 0)
> > > + return id;
> > > + shrinker->debugfs_id = id;
> > > +
> > > + snprintf(buf, sizeof(buf), "%d", id);
> > > +
> > > + /* create debugfs entry */
> > > + entry = debugfs_create_dir(buf, shrinker_debugfs_root);
> > > + if (IS_ERR(entry)) {
> > > + ida_free(&shrinker_debugfs_ida, id);
> > > + return PTR_ERR(entry);
> > > + }
> > > + shrinker->debugfs_entry = entry;
> > > +
> > > + debugfs_create_file("count", 0220, entry, shrinker,
> > > + &shrinker_debugfs_count_fops);
> > > + return 0;
> > > +}
> > > +
> > > +void shrinker_debugfs_remove(struct shrinker *shrinker)
> > > +{
> > > + lockdep_assert_held(&shrinker_rwsem);
> > > +
> > > + if (!shrinker->debugfs_entry)
> > > + return;
> > > +
> > > + debugfs_remove_recursive(shrinker->debugfs_entry);
> > > + ida_free(&shrinker_debugfs_ida, shrinker->debugfs_id);
> > > +}
> > > +
> > > +static int __init shrinker_debugfs_init(void)
> > > +{
> > > + struct shrinker *shrinker;
> > > + int ret;
> > > +
> > > + if (!debugfs_initialized())
> > > + return -ENODEV;
> > > +
> >
> > Redundant check since it is checked in debugfs_create_dir().
> > So I think we could remove this.
> >
> > > + shrinker_debugfs_root = debugfs_create_dir("shrinker", NULL);
> >
> > We should use IS_ERR() to detect the error code. So the following
> > check is wrong.
>
> Right, will fix in the next version.
>
> >
> > > + if (!shrinker_debugfs_root)
> > > + return -ENOMEM;
> > > +
> > > + /* Create debugfs entries for shrinkers registered at boot */
> > > + ret = down_write_killable(&shrinker_rwsem);
> >
> > How could we kill this process? IIUC, late_initcall() is called
> > from early init process, there is no way to kill this. Right?
> > If yes, I think we could just use down_write().
>
> Ok, agree.
>
> Thanks!
>
