Re: [PATCH -V8 1/6] NUMA balancing: optimize page placement for memory tiering system

[Yang Shi]

On Mon, Sep 13, 2021 at 6:37 PM Huang Ying <ying.huang@xxxxxxxxx> wrote:
>
> With the advent of various new memory types, some machines will have
> multiple types of memory, e.g. DRAM and PMEM (persistent memory).  The
> memory subsystem of these machines can be called memory tiering
> system, because the performance of the different types of memory are
> usually different.
>
> In such system, because of the memory accessing pattern changing etc,
> some pages in the slow memory may become hot globally.  So in this
> patch, the NUMA balancing mechanism is enhanced to optimize the page
> placement among the different memory types according to hot/cold
> dynamically.
>
> In a typical memory tiering system, there are CPUs, fast memory and
> slow memory in each physical NUMA node.  The CPUs and the fast memory
> will be put in one logical node (called fast memory node), while the
> slow memory will be put in another (faked) logical node (called slow
> memory node).  That is, the fast memory is regarded as local while the
> slow memory is regarded as remote.  So it's possible for the recently
> accessed pages in the slow memory node to be promoted to the fast
> memory node via the existing NUMA balancing mechanism.
>
> The original NUMA balancing mechanism will stop to migrate pages if the free
> memory of the target node will become below the high watermark.  This
> is a reasonable policy if there's only one memory type.  But this
> makes the original NUMA balancing mechanism almost not work to optimize page
> placement among different memory types.  Details are as follows.
>
> It's the common cases that the working-set size of the workload is
> larger than the size of the fast memory nodes.  Otherwise, it's
> unnecessary to use the slow memory at all.  So in the common cases,
> there are almost always no enough free pages in the fast memory nodes,
> so that the globally hot pages in the slow memory node cannot be
> promoted to the fast memory node.  To solve the issue, we have 2
> choices as follows,
>
> a. Ignore the free pages watermark checking when promoting hot pages
>    from the slow memory node to the fast memory node.  This will
>    create some memory pressure in the fast memory node, thus trigger
>    the memory reclaiming.  So that, the cold pages in the fast memory
>    node will be demoted to the slow memory node.
>
> b. Make kswapd of the fast memory node to reclaim pages until the free
>    pages are a little more (about 10MB) than the high watermark.  Then,
>    if the free pages of the fast memory node reaches high watermark, and
>    some hot pages need to be promoted, kswapd of the fast memory node
>    will be waken up to demote some cold pages in the fast memory node to
>    the slow memory node.  This will free some extra space in the fast
>    memory node, so the hot pages in the slow memory node can be
>    promoted to the fast memory node.
>
> The choice "a" will create the memory pressure in the fast memory
> node.  If the memory pressure of the workload is high, the memory
> pressure may become so high that the memory allocation latency of the
> workload is influenced, e.g. the direct reclaiming may be triggered.
>
> The choice "b" works much better at this aspect.  If the memory
> pressure of the workload is high, the hot pages promotion will stop
> earlier because its allocation watermark is higher than that of the
> normal memory allocation.  So in this patch, choice "b" is
> implemented.
>
> In addition to the original page placement optimization among sockets,
> the NUMA balancing mechanism is extended to be used to optimize page
> placement according to hot/cold among different memory types.  So the
> sysctl user space interface (numa_balancing) is extended in a backward
> compatible way as follow, so that the users can enable/disable these
> functionality individually.
>
> The sysctl is converted from a Boolean value to a bits field.  The
> definition of the flags is,
>
> - 0x0: NUMA_BALANCING_DISABLED
> - 0x1: NUMA_BALANCING_NORMAL
> - 0x2: NUMA_BALANCING_MEMORY_TIERING

Thanks for coming up with the patches. TBH the first question off the
top of my head is all the complexity is really worthy for real life
workload at the moment? And the interfaces (sysctl knob files exported
to users) look complicated for the users. I don't know if the users
know how to set an optimal value for their workloads.

I don't disagree the NUMA balancing needs optimization and improvement
for tiering memory, the question we need answer is how far we should
go for now and what the interfaces should look like. Does it make
sense to you?

IMHO I'd prefer the most simple and straightforward approach at the
moment. For example, we could just skip high water mark check for PMEM
promotion.

>
> TODO:
>
> - Update ABI document: Documentation/sysctl/kernel.txt
>
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Yang Shi <shy828301@xxxxxxxxx>
> Cc: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Wei Xu <weixugc@xxxxxxxxxx>
> Cc: osalvador <osalvador@xxxxxxx>
> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> ---
>  include/linux/sched/sysctl.h | 10 ++++++++++
>  kernel/sched/core.c          | 10 ++++------
>  kernel/sysctl.c              |  7 ++++---
>  mm/migrate.c                 | 19 +++++++++++++++++--
>  mm/vmscan.c                  | 16 ++++++++++++++++
>  5 files changed, 51 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/sched/sysctl.h b/include/linux/sched/sysctl.h
> index 304f431178fd..bc54c1d75d6d 100644
> --- a/include/linux/sched/sysctl.h
> +++ b/include/linux/sched/sysctl.h
> @@ -35,6 +35,16 @@ enum sched_tunable_scaling {
>         SCHED_TUNABLESCALING_END,
>  };
>
> +#define NUMA_BALANCING_DISABLED                0x0
> +#define NUMA_BALANCING_NORMAL          0x1
> +#define NUMA_BALANCING_MEMORY_TIERING  0x2
> +
> +#ifdef CONFIG_NUMA_BALANCING
> +extern int sysctl_numa_balancing_mode;
> +#else
> +#define sysctl_numa_balancing_mode     0
> +#endif
> +
>  /*
>   *  control realtime throttling:
>   *
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 1bba4128a3e6..e61c2d415601 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4228,6 +4228,8 @@ DEFINE_STATIC_KEY_FALSE(sched_numa_balancing);
>
>  #ifdef CONFIG_NUMA_BALANCING
>
> +int sysctl_numa_balancing_mode;
> +
>  void set_numabalancing_state(bool enabled)
>  {
>         if (enabled)
> @@ -4240,20 +4242,16 @@ void set_numabalancing_state(bool enabled)
>  int sysctl_numa_balancing(struct ctl_table *table, int write,
>                           void *buffer, size_t *lenp, loff_t *ppos)
>  {
> -       struct ctl_table t;
>         int err;
> -       int state = static_branch_likely(&sched_numa_balancing);
>
>         if (write && !capable(CAP_SYS_ADMIN))
>                 return -EPERM;
>
> -       t = *table;
> -       t.data = &state;
> -       err = proc_dointvec_minmax(&t, write, buffer, lenp, ppos);
> +       err = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
>         if (err < 0)
>                 return err;
>         if (write)
> -               set_numabalancing_state(state);
> +               set_numabalancing_state(*(int *)table->data);
>         return err;
>  }
>  #endif
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 083be6af29d7..666c58455355 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -115,6 +115,7 @@ static int sixty = 60;
>
>  static int __maybe_unused neg_one = -1;
>  static int __maybe_unused two = 2;
> +static int __maybe_unused three = 3;
>  static int __maybe_unused four = 4;
>  static unsigned long zero_ul;
>  static unsigned long one_ul = 1;
> @@ -1803,12 +1804,12 @@ static struct ctl_table kern_table[] = {
>  #ifdef CONFIG_NUMA_BALANCING
>         {
>                 .procname       = "numa_balancing",
> -               .data           = NULL, /* filled in by handler */
> -               .maxlen         = sizeof(unsigned int),
> +               .data           = &sysctl_numa_balancing_mode,
> +               .maxlen         = sizeof(int),
>                 .mode           = 0644,
>                 .proc_handler   = sysctl_numa_balancing,
>                 .extra1         = SYSCTL_ZERO,
> -               .extra2         = SYSCTL_ONE,
> +               .extra2         = &three,
>         },
>  #endif /* CONFIG_NUMA_BALANCING */
>         {
> diff --git a/mm/migrate.c b/mm/migrate.c
> index a6a7743ee98f..a159a36dd412 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -50,6 +50,7 @@
>  #include <linux/ptrace.h>
>  #include <linux/oom.h>
>  #include <linux/memory.h>
> +#include <linux/sched/sysctl.h>
>
>  #include <asm/tlbflush.h>
>
> @@ -2110,16 +2111,30 @@ static int numamigrate_isolate_page(pg_data_t *pgdat, struct page *page)
>  {
>         int page_lru;
>         int nr_pages = thp_nr_pages(page);
> +       int order = compound_order(page);
>
> -       VM_BUG_ON_PAGE(compound_order(page) && !PageTransHuge(page), page);
> +       VM_BUG_ON_PAGE(order && !PageTransHuge(page), page);
>
>         /* Do not migrate THP mapped by multiple processes */
>         if (PageTransHuge(page) && total_mapcount(page) > 1)
>                 return 0;
>
>         /* Avoid migrating to a node that is nearly full */
> -       if (!migrate_balanced_pgdat(pgdat, nr_pages))
> +       if (!migrate_balanced_pgdat(pgdat, nr_pages)) {
> +               int z;
> +
> +               if (!(sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING) ||
> +                   !numa_demotion_enabled)
> +                       return 0;
> +               if (next_demotion_node(pgdat->node_id) == NUMA_NO_NODE)
> +                       return 0;
> +               for (z = pgdat->nr_zones - 1; z >= 0; z--) {
> +                       if (populated_zone(pgdat->node_zones + z))
> +                               break;
> +               }
> +               wakeup_kswapd(pgdat->node_zones + z, 0, order, ZONE_MOVABLE);
>                 return 0;
> +       }
>
>         if (isolate_lru_page(page))
>                 return 0;
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index f441c5946a4c..7fe737fd0e03 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -56,6 +56,7 @@
>
>  #include <linux/swapops.h>
>  #include <linux/balloon_compaction.h>
> +#include <linux/sched/sysctl.h>
>
>  #include "internal.h"
>
> @@ -3775,6 +3776,12 @@ static bool pgdat_watermark_boosted(pg_data_t *pgdat, int highest_zoneidx)
>         return false;
>  }
>
> +/*
> + * Keep the free pages on fast memory node a little more than the high
> + * watermark to accommodate the promoted pages.
> + */
> +#define NUMA_BALANCING_PROMOTE_WATERMARK       (10UL * 1024 * 1024 >> PAGE_SHIFT)
> +
>  /*
>   * Returns true if there is an eligible zone balanced for the request order
>   * and highest_zoneidx
> @@ -3796,6 +3803,15 @@ static bool pgdat_balanced(pg_data_t *pgdat, int order, int highest_zoneidx)
>                         continue;
>
>                 mark = high_wmark_pages(zone);
> +               if (sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING &&
> +                   numa_demotion_enabled &&
> +                   next_demotion_node(pgdat->node_id) != NUMA_NO_NODE) {
> +                       unsigned long promote_mark;
> +
> +                       promote_mark = min(NUMA_BALANCING_PROMOTE_WATERMARK,
> +                                          pgdat->node_present_pages >> 6);
> +                       mark += promote_mark;
> +               }
>                 if (zone_watermark_ok_safe(zone, order, mark, highest_zoneidx))
>                         return true;
>         }
> --
> 2.30.2
>