Re: [PATCH 10/10] mm/migrate: new zone_reclaim_mode to enable reclaim migration

[Yang Shi <shy828301@gmail.com>]

On Thu, Mar 4, 2021 at 4:01 PM Dave Hansen <dave.hansen@linux.intel.com> wrote:
>
>
> From: Dave Hansen <dave.hansen@linux.intel.com>
>
> Some method is obviously needed to enable reclaim-based migration.
>
> Just like traditional autonuma, there will be some workloads that
> will benefit like workloads with more "static" configurations where
> hot pages stay hot and cold pages stay cold.  If pages come and go
> from the hot and cold sets, the benefits of this approach will be
> more limited.
>
> The benefits are truly workload-based and *not* hardware-based.
> We do not believe that there is a viable threshold where certain
> hardware configurations should have this mechanism enabled while
> others do not.
>
> To be conservative, earlier work defaulted to disable reclaim-
> based migration and did not include a mechanism to enable it.
> This proposes extending the existing "zone_reclaim_mode" (now
> now really node_reclaim_mode) as a method to enable it.
>
> We are open to any alternative that allows end users to enable
> this mechanism or disable it it workload harm is detected (just
> like traditional autonuma).
>
> Once this is enabled page demotion may move data to a NUMA node
> that does not fall into the cpuset of the allocating process.
> This could be construed to violate the guarantees of cpusets.
> However, since this is an opt-in mechanism, the assumption is
> that anyone enabling it is content to relax the guarantees.

I think we'd better have the cpuset violation paragraph along with new
zone reclaim mode text so that the users are aware of the potential
violation. I don't think commit log is the to-go place for any plain
users.

>
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Yang Shi <yang.shi@linux.alibaba.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Huang Ying <ying.huang@intel.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: osalvador <osalvador@suse.de>
>
> changes since 20200122:
>  * Changelog material about relaxing cpuset constraints
> ---
>
>  b/Documentation/admin-guide/sysctl/vm.rst |    9 +++++++++
>  b/include/linux/swap.h                    |    3 ++-
>  b/include/uapi/linux/mempolicy.h          |    1 +
>  b/mm/vmscan.c                             |    6 ++++--
>  4 files changed, 16 insertions(+), 3 deletions(-)
>
> diff -puN Documentation/admin-guide/sysctl/vm.rst~RECLAIM_MIGRATE Documentation/admin-guide/sysctl/vm.rst
> --- a/Documentation/admin-guide/sysctl/vm.rst~RECLAIM_MIGRATE   2021-03-04 15:36:26.078806355 -0800
> +++ b/Documentation/admin-guide/sysctl/vm.rst   2021-03-04 15:36:26.093806355 -0800
> @@ -976,6 +976,7 @@ This is value OR'ed together of
>  1      Zone reclaim on
>  2      Zone reclaim writes dirty pages out
>  4      Zone reclaim swaps pages
> +8      Zone reclaim migrates pages
>  =      ===================================
>
>  zone_reclaim_mode is disabled by default.  For file servers or workloads
> @@ -1000,3 +1001,11 @@ of other processes running on other node
>  Allowing regular swap effectively restricts allocations to the local
>  node unless explicitly overridden by memory policies or cpuset
>  configurations.
> +
> +Page migration during reclaim is intended for systems with tiered memory
> +configurations.  These systems have multiple types of memory with varied
> +performance characteristics instead of plain NUMA systems where the same
> +kind of memory is found at varied distances.  Allowing page migration
> +during reclaim enables these systems to migrate pages from fast tiers to
> +slow tiers when the fast tier is under pressure.  This migration is
> +performed before swap.
> diff -puN include/linux/swap.h~RECLAIM_MIGRATE include/linux/swap.h
> --- a/include/linux/swap.h~RECLAIM_MIGRATE      2021-03-04 15:36:26.082806355 -0800
> +++ b/include/linux/swap.h      2021-03-04 15:36:26.093806355 -0800
> @@ -382,7 +382,8 @@ extern int sysctl_min_slab_ratio;
>  static inline bool node_reclaim_enabled(void)
>  {
>         /* Is any node_reclaim_mode bit set? */
> -       return node_reclaim_mode & (RECLAIM_ZONE|RECLAIM_WRITE|RECLAIM_UNMAP);
> +       return node_reclaim_mode & (RECLAIM_ZONE |RECLAIM_WRITE|
> +                                   RECLAIM_UNMAP|RECLAIM_MIGRATE);
>  }
>
>  extern void check_move_unevictable_pages(struct pagevec *pvec);
> diff -puN include/uapi/linux/mempolicy.h~RECLAIM_MIGRATE include/uapi/linux/mempolicy.h
> --- a/include/uapi/linux/mempolicy.h~RECLAIM_MIGRATE    2021-03-04 15:36:26.084806355 -0800
> +++ b/include/uapi/linux/mempolicy.h    2021-03-04 15:36:26.094806355 -0800
> @@ -69,5 +69,6 @@ enum {
>  #define RECLAIM_ZONE   (1<<0)  /* Run shrink_inactive_list on the zone */
>  #define RECLAIM_WRITE  (1<<1)  /* Writeout pages during reclaim */
>  #define RECLAIM_UNMAP  (1<<2)  /* Unmap pages during reclaim */
> +#define RECLAIM_MIGRATE        (1<<3)  /* Migrate to other nodes during reclaim */
>
>  #endif /* _UAPI_LINUX_MEMPOLICY_H */
> diff -puN mm/vmscan.c~RECLAIM_MIGRATE mm/vmscan.c
> --- a/mm/vmscan.c~RECLAIM_MIGRATE       2021-03-04 15:36:26.087806355 -0800
> +++ b/mm/vmscan.c       2021-03-04 15:36:26.096806355 -0800
> @@ -1073,6 +1073,9 @@ static bool migrate_demote_page_ok(struc
>         VM_BUG_ON_PAGE(PageHuge(page), page);
>         VM_BUG_ON_PAGE(PageLRU(page), page);
>
> +       if (!(node_reclaim_mode & RECLAIM_MIGRATE))
> +               return false;
> +
>         /* It is pointless to do demotion in memcg reclaim */
>         if (cgroup_reclaim(sc))
>                 return false;
> @@ -1082,8 +1085,7 @@ static bool migrate_demote_page_ok(struc
>         if (PageTransHuge(page) && !thp_migration_supported())
>                 return false;
>
> -       // FIXME: actually enable this later in the series
> -       return false;
> +       return true;
>  }
>
>  /* Check if a page is dirty or under writeback */
> _
>