2016-02-18 3:29 GMT+09:00 Alexander Potapenko <glider@xxxxxxxxxx>: > On Tue, Feb 16, 2016 at 7:37 PM, Alexander Potapenko <glider@xxxxxxxxxx> wrote: >> On Mon, Feb 1, 2016 at 3:55 AM, Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote: >>> On Thu, Jan 28, 2016 at 02:27:44PM +0100, Alexander Potapenko wrote: >>>> On Thu, Jan 28, 2016 at 1:51 PM, Alexander Potapenko <glider@xxxxxxxxxx> wrote: >>>> > >>>> > On Jan 28, 2016 8:40 AM, "Joonsoo Kim" <iamjoonsoo.kim@xxxxxxx> wrote: >>>> >> >>>> >> Hello, >>>> >> >>>> >> On Wed, Jan 27, 2016 at 07:25:10PM +0100, Alexander Potapenko wrote: >>>> >> > Stack depot will allow KASAN store allocation/deallocation stack traces >>>> >> > for memory chunks. The stack traces are stored in a hash table and >>>> >> > referenced by handles which reside in the kasan_alloc_meta and >>>> >> > kasan_free_meta structures in the allocated memory chunks. >>>> >> >>>> >> Looks really nice! >>>> >> >>>> >> Could it be more generalized to be used by other feature that need to >>>> >> store stack trace such as tracepoint or page owner? >>>> > Certainly yes, but see below. >>>> > >>>> >> If it could be, there is one more requirement. >>>> >> I understand the fact that entry is never removed from depot makes things >>>> >> very simpler, but, for general usecases, it's better to use reference >>>> >> count >>>> >> and allow to remove. Is it possible? >>>> > For our use case reference counting is not really necessary, and it would >>>> > introduce unwanted contention. >>> >>> Okay. >>> >>>> > There are two possible options, each having its advantages and drawbacks: we >>>> > can let the clients store the refcounters directly in their stacks (more >>>> > universal, but harder to use for the clients), or keep the counters in the >>>> > depot but add an API that does not change them (easier for the clients, but >>>> > potentially error-prone). >>>> > I'd say it's better to actually find at least one more user for the stack >>>> > depot in order to understand the requirements, and refactor the code after >>>> > that. >>> >>> I re-think the page owner case and it also may not need refcount. >>> For now, just moving this stuff to /lib would be helpful for other future user. >> I agree this code may need to be moved to /lib someday, but I wouldn't >> hurry with that. >> Right now it is quite KASAN-specific, and it's unclear yet whether >> anyone else is going to use it. >> I suggest we keep it in mm/kasan for now, and factor the common parts >> into /lib when the need arises. >> >>> BTW, is there any performance number? I guess that it could affect >>> the performance. >> I've compared the performance of KASAN with SLAB allocator on a small >> synthetic benchmark in two modes: with stack depot enabled and with >> kasan_save_stack() unconditionally returning 0. >> In the former case 8% more time was spent in the kernel than in the latter case. >> >> If I am not mistaking, for SLUB allocator the bookkeeping (enabled >> with the slub_debug=UZ boot options) take only 1.5 time, so the >> difference is worth looking into (at least before we switch SLUB to >> stack depot). > > I've made additional measurements. > Previously I had been using a userspace benchmark that created and > destroyed pipes in a loop > (https://github.com/google/sanitizers/blob/master/address-sanitizer/kernel_buildbot/slave/bench_pipes.c). > > Now I've made a kernel module that allocated and deallocated memory > chunks of different sizes in a loop. > There were two modes of operation: > 1) all the allocations were made from the same function, therefore all > allocation/deallocation stacks were similar and there always was a hit > in the stackdepot hashtable > 2) The allocations were made from 2^16 different stacks. > > In the first case SLAB+stackdepot turned out to be 13% faster than > SLUB+slub_debug, in the second SLAB was 11% faster. I don't know what version of kernel you tested but, until recently, slub_debug=UZ has a side effect not to using fastpath of SLUB. So, comparison between them isn't appropriate. Today's linux-next branch would have some improvements on this area so use it to compare them. > Note that in both cases and for both allocators most of the time (more > than 90%) was spent in the x86 stack unwinder, which is common for > both approaches. If more than 90% time is spent in stack unwinder which is common for both cases, how something is better than the other by 13%? > Yet another observation regarding stackdepot: under a heavy load > (running Trinity for a hour, 101M allocations) the depot saturates at > around 20K records with the hashtable miss rate of 0.02%. > That said, I still cannot justify the results of the userspace > benchmark, but the slowdown of the stackdepot approach for SLAB sounds > acceptable, especially given the memory gain compared to SLUB > bookkeeping (which requires 128 bytes per memory allocation) and the > fact we'll be dealing with the fast path most of the time. In fact, I don't have much concern about performance because saving memory has enough merit to be merged. Anyway, it looks acceptable even for performance. > It will certainly be nice to compare SLUB+slub_debug to > SLUB+stackdepot once we start switching SLUB to stackdepot. Okay. 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