dynamic-debug metadata is bloated; the __dyndbg linker section, filled by DYNAMIC_DEBUG_METADATA, has 3 fields with 90%, 84%, 45% repeated values, consuming ~150kb on my laptop. Instead of an arduous restructuring of the data, lets stuff it into zram, get whatever compression it gives us for free, and be able to map and unmap individual records on demand. Once map/unmap suffices for internal needs, the whole __dyndbg section can be reclaimed.
From this, it seems practical to push it into zswap, and then to keep active/enabled callsites mapped into ram, like a cache. But thats a lot of stuff I can only handwave about. Please fill in details or call BS.
This patchset makes a decent start down that path, but theres a boatload of chatter in the commit-logs about options and maybes, and brave projections about whats possible. Blah Blah. Theres almost certainly some subtle/ornot thinkos. Please point them out. I probably have some terminology cleanup to do too. I punted with lazy names a lot & welcome better ones some // comments too - not for submission I'll try to summarize here. - The easiest thing is to stuff whole struct _ddebug contents, into an anonymous struct, along with a new zhandle, into a new zpool. This is done in patch 1. this defers the unionization, so it just enlarges the ram footprint. - patch 2 attempts to distinguish zhandles from pointers, by making zhandles odd, and testing with %2. This works fine before unionization, falls over afterwards. - The unionization is in patch 3, and it works, at least casually. The fix-on-2 is to add an explicit flag; is_zhandle, and drop the %2 test, which fails when it encounters an odd pointer in the __dyndbg section. the structs/unions must be reworked to actually save any ram - for non-JUMP_LABEL builds, putting whole struct _ddebug into zram is problematic; it means pulling out whole record just to check the am-i-printing flag, a horrible cost. - pulling flags into a separate flags vector would solve that, and would be 'tight' on ram; no paholes, good grouping of flags for callsites enabled with module=foo function=bar file=buz etc. But it creates new ones Im stuck on. - an 'opaque' descriptor might be the magic needed, but Im pretty clueless on how to proceed. Anyone got any pointers to good examples in the tree ? This patchset is on top of other dyndbg patches, which are now in gregkh's char-misc git tree which can be found at git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc.git in the char-misc-next branch. Jim Cromie (3): dyndbg: WIP replace __dyndbg section with a zs-pool copy. dyndbg: zhandle+1 plus info tweaks, BUG_ONs dyndbg: fixup/correct assumptions re ptr-vals include/linux/dynamic_debug.h | 9 ++ lib/dynamic_debug.c | 172 ++++++++++++++++++++++++++++++++-- 2 files changed, 171 insertions(+), 10 deletions(-) -- 2.26.2