Re: [PATCH 2/3] slab_arena: Provide slab_arena_madvise_create to madvice slabs

[Vladimir Davydov <vdavydov.dev@gmail.com>]

[Cc += Alexander re tarantool/small backward compatibility]

On Wed, May 01, 2019 at 06:50:05PM +0300, Cyrill Gorcunov wrote:
> The purpose of this helper is to be able to eliminate
> some of slab arenas from dumping, especially when huge
> amount of memory involves.
> 
> For this sake one should call slab_arena_madvise_create
> right after area's creation. Strictly speaking this helper
> may be called in any time but then some of already created
> slabs which are not inside preallocated area won't be
> madvised.
> 
> Part of #3509

Again, please fix the link.

> ---
> https://github.com/tarantool/tarantool/issues/3509
> 
>  small/slab_arena.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++
>  small/slab_arena.h | 27 +++++++++++++++++++++++++
>  2 files changed, 76 insertions(+)
> 
> diff --git a/small/slab_arena.c b/small/slab_arena.c
> index 26e2620..12f6198 100644
> --- a/small/slab_arena.c
> +++ b/small/slab_arena.c
> @@ -149,6 +149,9 @@ slab_arena_create(struct slab_arena *arena, struct quota *quota,
>  	arena->used = 0;
>  
>  	arena->flags = flags;
> +#ifdef TARANTOOL_SMALL_HAS_MADVISE
> +	pm_atomic_store(&arena->advice, MADV_NORMAL);
> +#endif
>  
>  	if (arena->prealloc) {
>  		arena->arena = mmap_checked(arena->prealloc,
> @@ -205,6 +208,12 @@ slab_map(struct slab_arena *arena)
>  		__sync_sub_and_fetch(&arena->used, arena->slab_size);
>  		quota_release(arena->quota, arena->slab_size);
>  	}
> +
> +#ifdef TARANTOOL_SMALL_HAS_MADVISE
> +	if (ptr && pm_atomic_load(&arena->advice) != MADV_NORMAL)
> +		madvise(ptr, arena->slab_size, arena->advice);
> +#endif
> +

Why use atomic? The arena cannot be used until it's fully initialized.

>  	VALGRIND_MAKE_MEM_UNDEFINED(ptr, arena->slab_size);
>  	return ptr;
>  }
> @@ -226,3 +235,43 @@ slab_arena_mprotect(struct slab_arena *arena)
>  	if (arena->arena)
>  		mprotect(arena->arena, arena->prealloc, PROT_READ);
>  }
> +
> +#ifdef TARANTOOL_SMALL_HAS_MADVISE
> +/**
> + * slab_arena_madvise_create - Initialize madvise permanent strategy
> + * @arena:	slab arena to madvise
> + * @advice:	strategy (MADV_DONTDUMP, ...)
> + *
> + * This function setups @slab_arena strategy so that new memory
> + * allocation with mmap() call gets madvise() call with @advice.
> + *
> + * Note that we don't allow to change strategy once choosen because
> + * otherwise we need to walk over all possible slab cache entry.
> + *
> + * This function should be called right after the slab arena creation,
> + * otherwise if called later some of the non-preallocated slabs won't be
> + * madvise'd.
> + */
> +int
> +slab_arena_madvise_create(struct slab_arena *arena, int advice)

I don't see much point in changing madvise settings on the fly.
Let's instead allow to pass DONTDUMP flag at creation time.
This means we have to combine MAP_ and MADV_ flags. I guess it
was a bad idea to pass MAP_ flags directly to small_create() in
the first place. What about introducing small-specific flags like

  SLAB_ARENA_SHARED
  SLAB_ARENA_DONTDUMP

Come to think of it, I assume we don't even need to introduce
SLAB_ARENA_SHARED - we always pass MAP_PRIVATE to small_create().
Let's drop it, perhaps?

It will break backward compatibility, but I don't think it really
matters, as tarantool/small is only used by tarantool - it isn't
a big deal if we change its API. We only need to bump the version
number, I guess.

Alexander, please amend me if I'm wrong.

> +{
> +	int expected = MADV_NORMAL;
> +
> +	if (!pm_atomic_compare_exchange_strong(&arena->advice,
> +					       &expected, advice)) {
> +		return -EBUSY;
> +	}
> +
> +	if (arena->arena)
> +		return madvise(arena->arena, arena->prealloc, advice);
> +	return 0;
> +}
> +#else
> +int
> +slab_arena_madvise_create(struct slab_arena *arena, int advice)
> +{
> +	(void)arena;
> +	(void)advice;
> +	return -EINVAL;
> +}
> +#endif /* TARANTOOL_SMALL_HAS_MADVISE */
> diff --git a/small/slab_arena.h b/small/slab_arena.h
> index 364252f..4d86545 100644
> --- a/small/slab_arena.h
> +++ b/small/slab_arena.h
> @@ -31,8 +31,10 @@
>   * SUCH DAMAGE.
>   */
>  #include "lf_lifo.h"
> +#include "config.h"
>  #include <sys/mman.h>
>  #include <limits.h>
> +#include <errno.h>
>  
>  #if defined(__cplusplus)
>  extern "C" {
> @@ -97,6 +99,12 @@ struct slab_arena {
>  	 * mmap() flags: MAP_SHARED or MAP_PRIVATE
>  	 */
>  	int flags;
> +#ifdef TARANTOOL_SMALL_HAS_MADVISE
> +	/**
> +	 * madvise() flag
> +	 */
> +	int advice;
> +#endif
>  };
>  
>  /** Initialize an arena.  */
> @@ -120,6 +128,25 @@ slab_unmap(struct slab_arena *arena, void *ptr);
>  void
>  slab_arena_mprotect(struct slab_arena *arena);
>  
> +/** madvise() all slabs. */
> +int
> +slab_arena_madvise_create(struct slab_arena *arena, int advice);
> +
> +/** handy helpers for slab_arena_madvise_create. */
> +#ifdef TARANTOOL_SMALL_HAS_MADVISE
> +static inline int
> +slab_arena_madvise_dontdump(struct slab_arena *arena)
> +{
> +	return slab_arena_madvise_create(arena, MADV_DONTDUMP);
> +}
> +#else
> +static inline int
> +slab_arena_madvise_dontdump(struct slab_arena *arena)
> +{
> +	return -EINVAL;
> +}
> +#endif
> +
>  /**
>   * Align a size - round up to nearest divisible by the given alignment.
>   * Alignment must be a power of 2