Re: [PATCH v7 1/3] mm: Shuffle initial free memory to improve memory-side-cache utilization

[Michal Hocko <mhocko@kernel.org>]

On Mon 07-01-19 15:21:10, Dan Williams wrote:
[...]

Thanks a lot for the additional information. And...

> Introduce shuffle_free_memory(), and its helper shuffle_zone(), to
> perform a Fisher-Yates shuffle of the page allocator 'free_area' lists
> when they are initially populated with free memory at boot and at
> hotplug time. Do this based on either the presence of a
> page_alloc.shuffle=Y command line parameter, or autodetection of a
> memory-side-cache (to be added in a follow-on patch).

... to make it opt-in and also provide an opt-out to override for the
auto-detected case.

> The shuffling is done in terms of CONFIG_SHUFFLE_PAGE_ORDER sized free
> pages where the default CONFIG_SHUFFLE_PAGE_ORDER is MAX_ORDER-1 i.e.
> 10, 4MB this trades off randomization granularity for time spent
> shuffling.

But I do not really think we want to make this a config option. Who do
you expect will tune this? I would rather wait for those usecases to be
called out and we can give them a command line parameter to do so rather
than something hardcoded during compile time and as such really unusable
for any consumer of the pre-built kernels.

I do not have a problem with the default section though.

> MAX_ORDER-1 was chosen to be minimally invasive to the page
> allocator while still showing memory-side cache behavior improvements,
> and the expectation that the security implications of finer granularity
> randomization is mitigated by CONFIG_SLAB_FREELIST_RANDOM.
> 
> The performance impact of the shuffling appears to be in the noise
> compared to other memory initialization work. Also the bulk of the work
> is done in the background as a part of deferred_init_memmap().
> 
> This initial randomization can be undone over time so a follow-on patch
> is introduced to inject entropy on page free decisions. It is reasonable
> to ask if the page free entropy is sufficient, but it is not enough due
> to the in-order initial freeing of pages. At the start of that process
> putting page1 in front or behind page0 still keeps them close together,
> page2 is still near page1 and has a high chance of being adjacent. As
> more pages are added ordering diversity improves, but there is still
> high page locality for the low address pages and this leads to no
> significant impact to the cache conflict rate.
> 
> [1]: https://itpeernetwork.intel.com/intel-optane-dc-persistent-memory-operating-modes/
> [2]: https://lkml.org/lkml/2018/9/22/54
> [3]: https://lkml.org/lkml/2018/10/12/309

Please turn lkml.org links into http://lkml.kernel.org/r/$msg_id

[....]
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index cc4a507d7ca4..8c37a023a790 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1272,6 +1272,10 @@ void sparse_init(void);
>  #else
>  #define sparse_init()	do {} while (0)
>  #define sparse_index_init(_sec, _nid)  do {} while (0)
> +static inline int pfn_present(unsigned long pfn)
> +{
> +	return 1;
> +}

Does this really make sense? Shouldn't this default to pfn_valid on
!sparsemem?

[...]
> +config SHUFFLE_PAGE_ALLOCATOR
> +	bool "Page allocator randomization"
> +	depends on ACPI_NUMA
> +	default SLAB_FREELIST_RANDOM
> +	help
> +	  Randomization of the page allocator improves the average
> +	  utilization of a direct-mapped memory-side-cache. See section
> +	  5.2.27 Heterogeneous Memory Attribute Table (HMAT) in the ACPI
> +	  6.2a specification for an example of how a platform advertises
> +	  the presence of a memory-side-cache. There are also incidental
> +	  security benefits as it reduces the predictability of page
> +	  allocations to compliment SLAB_FREELIST_RANDOM, but the
> +	  default granularity of shuffling on 4MB (MAX_ORDER) pages is
> +	  selected based on cache utilization benefits.
> +
> +	  While the randomization improves cache utilization it may
> +	  negatively impact workloads on platforms without a cache. For
> +	  this reason, by default, the randomization is enabled only
> +	  after runtime detection of a direct-mapped memory-side-cache.
> +	  Otherwise, the randomization may be force enabled with the
> +	  'page_alloc.shuffle' kernel command line parameter.
> +
> +	  Say Y if unsure.

Do we really need to make this a choice? Are any of the tiny systems
going to be NUMA? Why cannot we just make it depend on ACPI_NUMA?

> +config SHUFFLE_PAGE_ORDER
> +	depends on SHUFFLE_PAGE_ALLOCATOR
> +	int "Page allocator shuffle order"
> +	range 0 10
> +	default 10
> +	help
> +	  Specify the granularity at which shuffling (randomization) is
> +	  performed. By default this is set to MAX_ORDER-1 to minimize
> +	  runtime impact of randomization and with the expectation that
> +	  SLAB_FREELIST_RANDOM mitigates heap attacks on smaller
> +	  object granularities.
> +

and no, do not make this configurable here as already mentioned.
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 022d4cbb3618..3602f7a2eab4 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -17,6 +17,7 @@
>  #include <linux/poison.h>
>  #include <linux/pfn.h>
>  #include <linux/debugfs.h>
> +#include <linux/shuffle.h>
>  #include <linux/kmemleak.h>
>  #include <linux/seq_file.h>
>  #include <linux/memblock.h>
> @@ -1929,9 +1930,16 @@ static unsigned long __init free_low_memory_core_early(void)
>  	 *  low ram will be on Node1
>  	 */
>  	for_each_free_mem_range(i, NUMA_NO_NODE, MEMBLOCK_NONE, &start, &end,
> -				NULL)
> +				NULL) {
> +		pg_data_t *pgdat;
> +
>  		count += __free_memory_core(start, end);
>  
> +		for_each_online_pgdat(pgdat)
> +			shuffle_free_memory(pgdat, PHYS_PFN(start),
> +					PHYS_PFN(end));
> +	}
> +
>  	return count;
>  }
>  
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index b9a667d36c55..7caffb9a91ab 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -23,6 +23,7 @@
>  #include <linux/highmem.h>
>  #include <linux/vmalloc.h>
>  #include <linux/ioport.h>
> +#include <linux/shuffle.h>
>  #include <linux/delay.h>
>  #include <linux/migrate.h>
>  #include <linux/page-isolation.h>
> @@ -895,6 +896,8 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages, int online_typ
>  	zone->zone_pgdat->node_present_pages += onlined_pages;
>  	pgdat_resize_unlock(zone->zone_pgdat, &flags);
>  
> +	shuffle_zone(zone, pfn, zone_end_pfn(zone));
> +
>  	if (onlined_pages) {
>  		node_states_set_node(nid, &arg);
>  		if (need_zonelists_rebuild)
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index cde5dac6229a..2adcd6da8a07 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -61,6 +61,7 @@
>  #include <linux/sched/rt.h>
>  #include <linux/sched/mm.h>
>  #include <linux/page_owner.h>
> +#include <linux/shuffle.h>
>  #include <linux/kthread.h>
>  #include <linux/memcontrol.h>
>  #include <linux/ftrace.h>
> @@ -1634,6 +1635,8 @@ static int __init deferred_init_memmap(void *data)
>  	}
>  	pgdat_resize_unlock(pgdat, &flags);
>  
> +	shuffle_zone(zone, first_init_pfn, zone_end_pfn(zone));
> +
>  	/* Sanity check that the next zone really is unpopulated */
>  	WARN_ON(++zid < MAX_NR_ZONES && populated_zone(++zone));

I would prefer if would have less placess to place the shuffling. Why
cannot we have a single place for the bootup and one for onlining part?
page_alloc_init_late sounds like a good place for the later. You can
miss some early allocations but are those of a big interest?

I haven't checked the actual shuffling algorithm, I will trust you on
that part ;)
-- 
Michal Hocko
SUSE Labs