Re: [PATCH] drivers/base/memory.c: cache blocks in radix tree to accelerate lookup

Michal Hocko
2020-01-09T21:30:21Z

On Thu 09-01-20 15:19:52, Scott Cheloha wrote:
> Searching for a particular memory block by id is an O(n) operation
> because each memory block's underlying device is kept in an unsorted
> linked list on the subsystem bus.
>
> We can cut the lookup cost to O(log n) if we cache the memory blocks in
> a radix tree. With a radix tree cache in place both memory subsystem
> initialization and memory hotplug run palpably faster on systems with a
> large number of memory blocks.
>
> Signed-off-by: Scott Cheloha <cheloha@linux.ibm.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Acked-by: Nathan Lynch <nathanl@linux.ibm.com>
> Acked-by: Michal Hocko <mhocko@suse.com>
> ---
> v2 incorporates suggestions from David Hildenbrand.
>
> v3 changes:
> - Rebase atop "drivers/base/memory.c: drop the mem_sysfs_mutex"
>
> - Be conservative: don't use radix_tree_for_each_slot() in
> walk_memory_blocks() yet. It introduces RCU which could
> change behavior. Walking the tree "by hand" with
> find_memory_block_by_id() is slower but keeps the patch
> simple.
>
> v4 changes:
> - Rewrite commit message to explicitly note the time
> complexity improvements.
>
> - Provide anecdotal accounts of time-savings in the changelog
> (see below).
>
> mhocko@suse.com has asked for additional details on time
> savings, so here are some results I've collected when measuring
> memory_dev_init() with/without the patch.

This data should be part of the changelog. Thanks!

> 1. A 32GB POWER9 VM with 16MB memblocks has 2048 blocks:
>
> # Unpatched
> [ 0.005121] adding memory block 0... ok
> [...]
> [ 0.095230] adding memory block 1024... ok
> [...]
> [ 0.304248] adding memory block 2047... ok
> [ 0.304508] added all memory blocks
>
> # Patched
> [ 0.004701] adding memory block 0... ok
> [...]
> [ 0.033383] adding memory block 1024... ok
> [...]
> [ 0.061387] adding memory block 2047... ok
> [ 0.061414] added all memory blocks
>
> Unpatched, memory_dev_init() runs in about 0.299 seconds. Patched,
> it runs in about 0.057 seconds. Savings of .242 seconds, or nearly
> a quarter of a second.
>
> 2. A 32TB POWER9 LPAR with 256MB memblocks has 131072 blocks:
>
> # Unpatched
> [ 13.703907] memory_dev_init: adding blocks
> [ 13.703931] memory_dev_init: added block 0
> [ 13.762678] memory_dev_init: added block 1024
> [ 13.910359] memory_dev_init: added block 2048
> [ 14.146941] memory_dev_init: added block 3072
> [...]
> [ 218.516235] memory_dev_init: added block 57344
> [ 229.310467] memory_dev_init: added block 58368
> [ 240.590857] memory_dev_init: added block 59392
> [ 252.351665] memory_dev_init: added block 60416
> [...]
> [ 2152.023248] memory_dev_init: added block 128000
> [ 2196.464430] memory_dev_init: added block 129024
> [ 2241.746515] memory_dev_init: added block 130048
> [ 2287.406099] memory_dev_init: added all blocks
>
> # Patched
> [ 13.696898] memory_dev_init: adding blocks
> [ 13.696920] memory_dev_init: added block 0
> [ 13.710966] memory_dev_init: added block 1024
> [ 13.724865] memory_dev_init: added block 2048
> [ 13.738802] memory_dev_init: added block 3072
> [...]
> [ 14.520999] memory_dev_init: added block 57344
> [ 14.536355] memory_dev_init: added block 58368
> [ 14.551747] memory_dev_init: added block 59392
> [ 14.567128] memory_dev_init: added block 60416
> [...]
> [ 15.595638] memory_dev_init: added block 126976
> [ 15.611761] memory_dev_init: added block 128000
> [ 15.627889] memory_dev_init: added block 129024
> [ 15.644048] memory_dev_init: added block 130048
> [ 15.660035] memory_dev_init: added all blocks
>
> Unpatched, memory_dev_init() runs in about 2275 seconds,
> or ~37 minutes. Patched, memory_dev_init() runs in about
> 1.97 seconds. Savings of ~37 minutes.
>
> I did not actually measure walk_memory_blocks(), but during
> boot on this machine without the patch I got the following
> (abbreviated) traces:
>
> [ 2347.494986] [c000000014c5bb60] [c000000000869af4] walk_memory_blocks+0x94/0x160
> [ 2527.625378] [c000000014c5bb60] [c000000000869af4] walk_memory_blocks+0x94/0x160
> [ 2707.761977] [c000000014c5bb60] [c000000000869af4] walk_memory_blocks+0x94/0x160
> [ 2887.899975] [c000000014c5bb60] [c000000000869af4] walk_memory_blocks+0x94/0x160
> [ 3068.028318] [c000000014c5bb60] [c000000000869af4] walk_memory_blocks+0x94/0x160
> [ 3248.158764] [c000000014c5bb60] [c000000000869af4] walk_memory_blocks+0x94/0x160
> [ 3428.287296] [c000000014c5bb60] [c000000000869af4] walk_memory_blocks+0x94/0x160
> [ 3608.425357] [c000000014c5bb60] [c000000000869af4] walk_memory_blocks+0x94/0x160
> [ 3788.554572] [c000000014c5bb60] [c000000000869af4] walk_memory_blocks+0x94/0x160
> [ 3968.695071] [c000000014c5bb60] [c000000000869af4] walk_memory_blocks+0x94/0x160
> [ 4148.823970] [c000000014c5bb60] [c000000000869af4] walk_memory_blocks+0x94/0x160
>
> Those traces disappeared with the patch, so I'm pretty sure
> this patch shaves ~30 minutes off of walk_memory_blocks()
> at boot.
>
> Given the above results I think it is safe to say that this patch will
> dramatically improve boot times on large POWER systems.
>
> drivers/base/memory.c | 36 +++++++++++++++++++++++-------------
> 1 file changed, 23 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/base/memory.c b/drivers/base/memory.c
> index 799b43191dea..8902930d5ef2 100644
> --- a/drivers/base/memory.c
> +++ b/drivers/base/memory.c
> @@ -19,6 +19,7 @@
> #include <linux/memory.h>
> #include <linux/memory_hotplug.h>
> #include <linux/mm.h>
> +#include <linux/radix-tree.h>
> #include <linux/stat.h>
> #include <linux/slab.h>
>
> @@ -56,6 +57,13 @@ static struct bus_type memory_subsys = {
> .offline = memory_subsys_offline,
> };
>
> +/*
> + * Memory blocks are cached in a local radix tree to avoid
> + * a costly linear search for the corresponding device on
> + * the subsystem bus.
> + */
> +static RADIX_TREE(memory_blocks, GFP_KERNEL);
> +
> static BLOCKING_NOTIFIER_HEAD(memory_chain);
>
> int register_memory_notifier(struct notifier_block *nb)
> @@ -572,20 +580,14 @@ int __weak arch_get_memory_phys_device(unsigned long start_pfn)
> /* A reference for the returned memory block device is acquired. */
> static struct memory_block *find_memory_block_by_id(unsigned long block_id)
> {
> - struct device *dev;
> + struct memory_block *mem;
>
> - dev = subsys_find_device_by_id(&memory_subsys, block_id, NULL);
> - return dev ? to_memory_block(dev) : NULL;
> + mem = radix_tree_lookup(&memory_blocks, block_id);
> + if (mem)
> + get_device(&mem->dev);
> + return mem;
> }
>
> -/*
> - * For now, we have a linear search to go find the appropriate
> - * memory_block corresponding to a particular phys_index. If
> - * this gets to be a real problem, we can always use a radix
> - * tree or something here.
> - *
> - * This could be made generic for all device subsystems.
> - */
> struct memory_block *find_memory_block(struct mem_section *section)
> {
> unsigned long block_id = base_memory_block_id(__section_nr(section));
> @@ -628,9 +630,15 @@ int register_memory(struct memory_block *memory)
> memory->dev.offline = memory->state == MEM_OFFLINE;
>
> ret = device_register(&memory->dev);
> - if (ret)
> + if (ret) {
> put_device(&memory->dev);
> -
> + return ret;
> + }
> + ret = radix_tree_insert(&memory_blocks, memory->dev.id, memory);
> + if (ret) {
> + put_device(&memory->dev);
> + device_unregister(&memory->dev);
> + }
> return ret;
> }
>
> @@ -688,6 +696,8 @@ static void unregister_memory(struct memory_block *memory)
> if (WARN_ON_ONCE(memory->dev.bus != &memory_subsys))
> return;
>
> + WARN_ON(radix_tree_delete(&memory_blocks, memory->dev.id) == NULL);
> +
> /* drop the ref. we got via find_memory_block() */
> put_device(&memory->dev);
> device_unregister(&memory->dev);
> --
> 2.24.1

--
Michal Hocko
SUSE Labs