Re: [PATCH 3/3] vmevent: Implement special low-memory attribute

[Pekka Enberg]

On Tue, May 1, 2012 at 4:26 PM, Anton Vorontsov
<anton.vorontsov@xxxxxxxxxx> wrote:
> This is specially "blended" attribute, the event triggers when kernel
> decides that we're close to the low memory threshold. Userspace should
> not expect very precise meaning of low memory situation, mostly, it's
> just a guess on the kernel's side.
>
> Well, this is the same as userland should not know or care how exactly
> kernel manages the memory, or assume that memory management behaviour
> is a part of the "ABI". So, all the 'low memory' is just guessing, but
> we're trying to do our best. It might be that we will end up with two
> or three variations of 'low memory' thresholds, and all of them would
> be useful for different use cases.
>
> For this implementation, we assume that there's a low memory situation
> for the N pages threshold when we have neither N pages of completely
> free pages, nor we have N reclaimable pages in the cache. This
> effectively means, that if userland expects to allocate N pages, it
> would consume all the free pages, and any further allocations (above
> N) would start draining caches.
>
> In the worst case, prior to hitting the threshold, we might have only
> N pages in cache, and nearly no memory as free pages.
>
> The same 'low memory' meaning is used in the current Android Low
> Memory Killer driver.
>
> Signed-off-by: Anton Vorontsov <anton.vorontsov@xxxxxxxxxx>

I don't see why we couldn't add this. Minchan, thoughts?

> ---
>  include/linux/vmevent.h              |    7 ++++++
>  mm/vmevent.c                         |   40 ++++++++++++++++++++++++++++++++++
>  tools/testing/vmevent/vmevent-test.c |   12 +++++++++-
>  3 files changed, 58 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/vmevent.h b/include/linux/vmevent.h
> index aae0d24..9bfa244 100644
> --- a/include/linux/vmevent.h
> +++ b/include/linux/vmevent.h
> @@ -10,6 +10,13 @@ enum {
>        VMEVENT_ATTR_NR_AVAIL_PAGES     = 1UL,
>        VMEVENT_ATTR_NR_FREE_PAGES      = 2UL,
>        VMEVENT_ATTR_NR_SWAP_PAGES      = 3UL,
> +       /*
> +        * This is specially blended attribute, the event triggers
> +        * when kernel decides that we're close to the low memory threshold.
> +        * Don't expect very precise meaning of low memory situation, mostly,
> +        * it's just a guess on the kernel's side.
> +        */
> +       VMEVENT_ATTR_LOWMEM_PAGES       = 4UL,
>
>        VMEVENT_ATTR_MAX                /* non-ABI */
>  };
> diff --git a/mm/vmevent.c b/mm/vmevent.c
> index b312236..d278a25 100644
> --- a/mm/vmevent.c
> +++ b/mm/vmevent.c
> @@ -68,10 +68,50 @@ static u64 vmevent_attr_avail_pages(struct vmevent_watch *watch,
>        return totalram_pages;
>  }
>
> +/*
> + * Here's some implementation details for the "low memory" meaning.
> + *
> + * (The explanation is not in the header file as userland should not
> + * know these details, nor it should assume that the meaning will
> + * always be the same. As well as it should not know how exactly kernel
> + * manages the memory, or assume that memory management behaviour is a
> + * part of the "ABI". So, all the 'low memory' is just guessing, but
> + * we're trying to do our best.)
> + *
> + * For this implementation, we assume that there's a low memory situation
> + * for the N pages threshold when we have neither N pages of completely
> + * free pages, nor we have N reclaimable pages in the cache. This
> + * effectively means, that if userland expects to allocate N pages, it
> + * would consume all the free pages, and any further allocations (above
> + * N) would start draining caches.
> + *
> + * In the worst case, prior hitting the threshold, we might have only
> + * N pages in cache, and nearly no memory as free pages.
> + */
> +static u64 vmevent_attr_lowmem_pages(struct vmevent_watch *watch,
> +                                    struct vmevent_attr *attr)
> +{
> +       int free = global_page_state(NR_FREE_PAGES);
> +       int file = global_page_state(NR_FILE_PAGES) -
> +                  global_page_state(NR_SHMEM); /* TODO: account locked pages */
> +       int val = attr->value;
> +
> +       /*
> +        * For convenience we return 0 or attr value (instead of 0/1), it
> +        * makes it easier for vmevent_match() to cope with blended
> +        * attributes, plus userland might use the value to find out which
> +        * threshold triggered.
> +        */
> +       if (free < val && file < val)
> +               return val;
> +       return 0;
> +}
> +
>  static vmevent_attr_sample_fn attr_samplers[] = {
>        [VMEVENT_ATTR_NR_AVAIL_PAGES]   = vmevent_attr_avail_pages,
>        [VMEVENT_ATTR_NR_FREE_PAGES]    = vmevent_attr_free_pages,
>        [VMEVENT_ATTR_NR_SWAP_PAGES]    = vmevent_attr_swap_pages,
> +       [VMEVENT_ATTR_LOWMEM_PAGES]     = vmevent_attr_lowmem_pages,
>  };
>
>  static u64 vmevent_sample_attr(struct vmevent_watch *watch, struct vmevent_attr *attr)
> diff --git a/tools/testing/vmevent/vmevent-test.c b/tools/testing/vmevent/vmevent-test.c
> index fd9a174..c61aed7 100644
> --- a/tools/testing/vmevent/vmevent-test.c
> +++ b/tools/testing/vmevent/vmevent-test.c
> @@ -33,7 +33,7 @@ int main(int argc, char *argv[])
>
>        config = (struct vmevent_config) {
>                .sample_period_ns       = 1000000000L,
> -               .counter                = 6,
> +               .counter                = 7,
>                .attrs                  = {
>                        {
>                                .type   = VMEVENT_ATTR_NR_FREE_PAGES,
> @@ -59,6 +59,13 @@ int main(int argc, char *argv[])
>                                .type   = VMEVENT_ATTR_NR_SWAP_PAGES,
>                        },
>                        {
> +                               .type   = VMEVENT_ATTR_LOWMEM_PAGES,
> +                               .state  = VMEVENT_ATTR_STATE_VALUE_LT |
> +                                         VMEVENT_ATTR_STATE_VALUE_EQ |
> +                                         VMEVENT_ATTR_STATE_ONE_SHOT,
> +                               .value  = phys_pages / 2,
> +                       },
> +                       {
>                                .type   = 0xffff, /* invalid */
>                        },
>                },
> @@ -108,6 +115,9 @@ int main(int argc, char *argv[])
>                        case VMEVENT_ATTR_NR_SWAP_PAGES:
>                                printf("  VMEVENT_ATTR_NR_SWAP_PAGES: %Lu\n", attr->value);
>                                break;
> +                       case VMEVENT_ATTR_LOWMEM_PAGES:
> +                               printf("  VMEVENT_ATTR_LOWMEM_PAGES: %Lu\n", attr->value);
> +                               break;
>                        default:
>                                printf("  Unknown attribute: %Lu\n", attr->value);
>                        }
> --
> 1.7.9.2
>
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