Messages in this thread | | | Date | Wed, 22 Sep 2021 11:51:37 -0700 | From | Josh Poimboeuf <> | Subject | Re: [RFC PATCH] x86, vmlinux.lds: Add debug option to force all data sections aligned |
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On Wed, Jul 28, 2021 at 03:21:40PM +0800, Feng Tang wrote: > 0day has reported many strange performance changes (regression or > improvement), in which there was no obvious relation between the culprit > commit and the benchmark at the first look, and it causes people to doubt > the test itself is wrong. > > Upon further check, many of these cases are caused by the change to the > alignment of kernel text or data, as whole text/data of kernel are linked > together, change in one domain can affect alignments of other domains. > > To help to quickly identify if the strange performance change is caused > by _data_ alignment. add a debug option to force the data sections from > all .o files aligned on THREAD_SIZE, so that change in one domain won't > affect other modules' data alignment. > > We have used this option to check some strange kernel changes [1][2][3], > and those performance changes were gone after enabling it, which proved > they are data alignment related. > > Similarly, there is another kernel debug option to check text alignment > related performance changes: CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_64B, > which forces all function's start address to be 64 bytes alinged. > > This option depends on CONFIG_DYNAMIC_DEBUG==n, as '__dyndbg' subsection > of .data has a hard requirement of ALIGN(8), shown in the 'vmlinux.lds': > > " > . = ALIGN(8); __start___dyndbg = .; KEEP(*(__dyndbg)) __stop___dyndbg = .; > " > > It contains all pointers to 'struct _ddebug', and dynamic_debug_init() > will "pointer++" to loop accessing these pointers, which will be broken > with this option enabled. > > [1]. https://lore.kernel.org/lkml/20200205123216.GO12867@shao2-debian/ > [2]. https://lore.kernel.org/lkml/20200305062138.GI5972@shao2-debian/ > [3]. https://lore.kernel.org/lkml/20201112140625.GA21612@xsang-OptiPlex-9020/ > > Signed-off-by: Feng Tang <feng.tang@intel.com> > --- > arch/x86/Kconfig.debug | 13 +++++++++++++ > arch/x86/kernel/vmlinux.lds.S | 7 ++++++- > 2 files changed, 19 insertions(+), 1 deletion(-)
Hi Feng,
Thanks for the interesting LPC presentation about alignment-related performance issues (which mentioned this patch).
https://linuxplumbersconf.org/event/11/contributions/895/
I wonder if we can look at enabling some kind of data section alignment unconditionally instead of just making it a debug option. Have you done any performance and binary size comparisons?
On a similar vein I think we should re-explore permanently enabling cacheline-sized function alignment i.e. making something like CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_64B the default. Ingo did some research on that a while back:
https://lkml.kernel.org/r/20150519213820.GA31688@gmail.com
At the time, the main reported drawback of -falign-functions=64 was that even small functions got aligned. But now I think that can be mitigated with some new options like -flimit-function-alignment and/or -falign-functions=64,X (for some carefully-chosen value of X).
-- Josh
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