On Mon, Sep 5, 2022 at 1:58 AM Marco Elver <elver@xxxxxxxxxx> wrote: > > On Mon, 5 Sept 2022 at 10:12, Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Sun 04-09-22 18:32:58, Suren Baghdasaryan wrote: > > > On Thu, Sep 1, 2022 at 12:15 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > [...] > > > > Yes, tracking back the call trace would be really needed. The question > > > > is whether this is really prohibitively expensive. How much overhead are > > > > we talking about? There is no free lunch here, really. You either have > > > > the overhead during runtime when the feature is used or on the source > > > > code level for all the future development (with a maze of macros and > > > > wrappers). > > > > > > As promised, I profiled a simple code that repeatedly makes 10 > > > allocations/frees in a loop and measured overheads of code tagging, > > > call stack capturing and tracing+BPF for page and slab allocations. > > > Summary: > > > > > > Page allocations (overheads are compared to get_free_pages() duration): > > > 6.8% Codetag counter manipulations (__lazy_percpu_counter_add + __alloc_tag_add) > > > 8.8% lookup_page_ext > > > 1237% call stack capture > > > 139% tracepoint with attached empty BPF program > > > > Yes, I am not surprised that the call stack capturing is really > > expensive comparing to the allocator fast path (which is really highly > > optimized and I suspect that with 10 allocation/free loop you mostly get > > your memory from the pcp lists). Is this overhead still _that_ visible > > for somehow less microoptimized workloads which have to take slow paths > > as well? > > > > Also what kind of stack unwinder is configured (I guess ORC)? This is > > not my area but from what I remember the unwinder overhead varies > > between ORC and FP. > > > > And just to make it clear. I do realize that an overhead from the stack > > unwinding is unavoidable. And code tagging would logically have lower > > overhead as it performs much less work. But the main point is whether > > our existing stack unwiding approach is really prohibitively expensive > > to be used for debugging purposes on production systems. I might > > misremember but I recall people having bigger concerns with page_owner > > memory footprint than the actual stack unwinder overhead. > > This is just to point out that we've also been looking at cheaper > collection of the stack trace (for KASAN and other sanitizers). The > cheapest way to unwind the stack would be a system with "shadow call > stack" enabled. With compiler support it's available on arm64, see > CONFIG_SHADOW_CALL_STACK. For x86 the hope is that at one point the > kernel will support CET, which newer Intel and AMD CPUs support. > Collecting the call stack would then be a simple memcpy. Thanks for the note Marco! I'll check out the CONFIG_SHADOW_CALL_STACK on Android.
