talloc_stackframe() and talloc_tos()

Rusty Russell rusty at rustcorp.com.au
Wed Jun 27 06:12:58 MDT 2012


        I've been looking into talloc_pool, talloc_stackframe and
talloc_tos(), because dbwrap() uses talloc_tos and I want to convert S4
to use dbwrap, so I can get ntdb as an option for every database.  I've
been hacking for a few days, and am taking tomorrow off, so I though I'd
post what I have:

(1) talloc_pool() (in libtalloc) implements a bump allocator.  This only
    makes sense if you have a pop-on-pop-off style workload.  You create
    a pool of a given size, and that's used as a simple allocator.  If
    it fills, we fall back to malloc.

(2) talloc_stackframe() is a samba-specific wrapper which uses
    a talloc_pool if the first call uses talloc_stackframe_pool().
    If you free a talloc_stackframe(), it frees every stackframe
    allocated afterwards, too.

(3) talloc_tos() returns the current top stackframe (ie. last
    talloc_stackframe() call).

Good facts:
(1) talloc_pool is fast [1]
        talloc: 11545659 ops/sec
        talloc_pool: 30377035 ops/sec
        malloc: 12960209 ops/sec

(2) Yes, even faster than alloc_mmap:
        talloc (alloc_mmap): 16943997 ops/sec

(3) For simple functions (eg. ones which do a single allocation) using
    talloc_tos() is faster and simpler than (the more general pattern)
    of using talloc_stackframe() then allocating off that: one
    allocation vs two, eg:

        char *fname = talloc_asprintf(talloc_tos(), "%s/%s", dir, path);
        int fd = open(fname, O_RDONLY);


        TALLOC_CTX *tmp_ctx = talloc_stackframe();
        char *fname = talloc_asprintf(tmp_ctx, "%s/%s", dir, path);
        int fd = open(fname, O_RDONLY);

Bad facts:
(1) Libraries need to set up talloc_stackframe() at every entry point,
    otherwise talloc_tos() does a hack where we leak memory.

(2) Only smbd calls talloc_stackframe_pool(), so other code ends up
    using non-pool allocations (ie. normal ones).

(3) There's no way to know how full a stackframe got, so tuning is hard
    (smbd uses 8192 bytes).

(4) You can talloc_steal() out of a talloc_pool(), leaving the pool
    stuck.  If you turn on debugging for this, you see many cases where
    this is done.

(5) talloc_tos() allocations are often not freed, but left for someone
    in the callchain to free.  This could lead to more memory being used
    than necessary in pathalogical cases.

(6) talloc_stackframe() and talloc_tos() mis-usage is hard to see, since
    a failure to free is not noticed at runtime.

What I'm trying to do about it:
- Add a talloc_pool_stats() call to show how successful a pool was.

- Change talloc_stackframe_pool() to auto-size.  The caller hands a
  static struct (carefully internally threadsafe), and we grow/shrink
  based on usage.  This should make it easier to use elsewhere.

- Add --check-talloc-stack option to configure (developer mode only) which
  does sanity checking, using gcc's -finstrument-functions option to
  insert calls on every function entry and exit.

  This checks that we always (and only) free a talloc_stackframe() in
  the function which allocated it, makes us panic if we ever don't have
  a stackframe in talloc_tos().

- Make talloc_steal() out of a pool log a warning.  Unfortunately, 
  samba3 does this quite a bit: if these can't be fixed, the stackframe
  size will have to be capped quite low, since this prevents immediate
  freeing.  I'm still working through them :(

- Make our talloc_log_fns call smb_panic if we're in developer mode.
  This makes me fix the problems.

- Pass a talloc context into S3's lp_* functions which return a string.
  This is almost always talloc_tos(), but avoids us doing a gratuitous
  talloc_strdup() in the few dozen places where we want another parent

- Plus misc fixes and cleanups.

You can follow the fun in my "talloc-stack-wip" branch... I've just
reshuffled it, so various things might be a bit broken at the moment,
but should make interesting reading:



[1] -O2 on an Ubuntu 32-bit x86 box.  On sn-devel, results are similar:
        talloc: 18524433 ops/sec
        talloc_pool: 43509747 ops/sec
        malloc: 21832590 ops/sec
        talloc (alloc_mmap): 21503065 ops/sec

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