On Wed 04-05-16 11:14:50, Joonsoo Kim wrote: > On Tue, May 03, 2016 at 10:53:56AM +0200, Michal Hocko wrote: > > On Tue 03-05-16 14:23:04, Joonsoo Kim wrote: [...] > > > Memory saving looks as following. (Boot 4GB memory system with page_owner) > > > > > > 92274688 bytes -> 25165824 bytes > > > > It is not clear to me whether this is after a fresh boot or some workload > > which would grow the stack depot as well. What is a usual cap for the > > memory consumption. > > It is static allocation size after a fresh boot. I didn't add size of > dynamic allocation memory so it could be larger a little. See below line. > > > > > 72% reduction in static allocation size. Even if we should add up size of > > > dynamic allocation memory, it would not that big because stacktrace is > > > mostly duplicated. This would be true only if most of the allocation stacks are basically same after the boot which I am not really convinced is true. But you are right that the number of sublicates will grow only a little. I was interested about how much is that little ;) > > > Note that implementation looks complex than someone would imagine because > > > there is recursion issue. stackdepot uses page allocator and page_owner > > > is called at page allocation. Using stackdepot in page_owner could re-call > > > page allcator and then page_owner. That is a recursion. > > > > This is rather fragile. How do we check there is no lock dependency > > introduced later on - e.g. split_page called from a different > > locking/reclaim context than alloc_pages? Would it be safer to > > There is no callsite that calls set_page_owner() with > __GFP_DIRECT_RECLAIM. So, there would be no lock/context dependency > now. I am confused now. prep_new_page is called with the gfp_mask of the original request, no? > split_page() doesn't call set_page_owner(). Instead, it calls > split_page_owner() and just copies previous entry. Since it doesn't > require any new stackdepot entry, it is safe in any context. Ohh, you are right. I have missed patch 4 (http://lkml.kernel.org/r/1462252984-8524-5-git-send-email-iamjoonsoo.kim@xxxxxxx) > > use ~__GFP_DIRECT_RECLAIM for those stack allocations? Or do you think > > there would be too many failed allocations? This alone wouldn't remove a > > need for the recursion detection but it sounds less tricky. > > > > > To detect and > > > avoid it, whenever we obtain stacktrace, recursion is checked and > > > page_owner is set to dummy information if found. Dummy information means > > > that this page is allocated for page_owner feature itself > > > (such as stackdepot) and it's understandable behavior for user. > > > > > > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> > > > > I like the idea in general I just wish this would be less subtle. Few > > more comments below. > > > > [...] > > > -void __set_page_owner(struct page *page, unsigned int order, gfp_t gfp_mask) > > > +static inline bool check_recursive_alloc(struct stack_trace *trace, > > > + unsigned long ip) > > > { > > > - struct page_ext *page_ext = lookup_page_ext(page); > > > + int i, count; > > > + > > > + if (!trace->nr_entries) > > > + return false; > > > + > > > + for (i = 0, count = 0; i < trace->nr_entries; i++) { > > > + if (trace->entries[i] == ip && ++count == 2) > > > + return true; > > > + } > > > > This would deserve a comment I guess. Btw, don't we have a better and > > more robust way to detect the recursion? Per task_struct flag or > > something like that? > > Okay. I will add a comment. > > I already considered task_struct flag and I know that it is a better > solution. But, I don't think that this debugging feature deserve to > use such precious flag. This implementation isn't efficient but I > think that it is at least robust. I guess there are many holes in task_structs where a single bool would comfortably fit in. But I do not consider this to be a large issue. It is just the above looks quite ugly. > > [...] > > > +static noinline depot_stack_handle_t save_stack(gfp_t flags) > > > +{ > > > + unsigned long entries[PAGE_OWNER_STACK_DEPTH]; > > > struct stack_trace trace = { > > > .nr_entries = 0, > > > - .max_entries = ARRAY_SIZE(page_ext->trace_entries), > > > - .entries = &page_ext->trace_entries[0], > > > - .skip = 3, > > > + .entries = entries, > > > + .max_entries = PAGE_OWNER_STACK_DEPTH, > > > + .skip = 0 > > > }; > > [...] > > > void __dump_page_owner(struct page *page) > > > { > > > struct page_ext *page_ext = lookup_page_ext(page); > > > + unsigned long entries[PAGE_OWNER_STACK_DEPTH]; > > > > This is worrying because of the excessive stack consumption while we > > might be in a deep call chain already. Can we preallocate a hash table > > for few buffers when the feature is enabled? This would require locking > > of course but chances are that contention wouldn't be that large. > > Make sense but I'm not sure that excessive stack consumption would > cause real problem. For example, if direct reclaim is triggered during > allocation, it may go more deeper than this path. Do we really consume 512B of stack during reclaim. That sounds more than worrying to me. > I'd like to postpone to handle this issue until stack breakage is > reported due to this feature. I dunno, but I would expect that a debugging feature wouldn't cause problems like that. It is more than sad when you cannot debug your issue just because of the stack consumption... -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>