On Thu, Mar 31, 2022 at 10:54:08AM +0100, Mark Rutland wrote: > On Wed, Mar 23, 2022 at 04:32:51PM +0100, andrey.konovalov@xxxxxxxxx wrote: > > From: Andrey Konovalov <andreyknvl@xxxxxxxxxx> > > > > kasan, arm64, scs, stacktrace: collect stack traces from Shadow Call Stack > > > > Currently, KASAN always uses the normal stack trace collection routines, > > which rely on the unwinder, when saving alloc and free stack traces. > > > > Instead of invoking the unwinder, collect the stack trace by copying > > frames from the Shadow Call Stack whenever it is enabled. This reduces > > boot time by 30% for all KASAN modes when Shadow Call Stack is enabled. > > That is an impressive number. TBH, I'm shocked that this has *that* much of an > improvement, and I suspect this means we're doing something unnecssarily > expensive in the regular unwinder. I've had a quick look into this, to see what we could do to improve the regular unwinder, but I can't reproduce that 30% number. In local testing the worst can I could get to was 6-13% (with both the stacktrace *and* stackdepot logic hacked out entirely). I'm testing with clang 13.0.0 from the llvm.org binary releases, with defconfig + SHADOW_CALL_STACK + KASAN_<option>, using a very recent snapshot of mainline (commit d888c83fcec75194a8a48ccd283953bdba7b2550). I'm booting a KVM-accelerated QEMU VM on ThunderX2 with "init=/sbin/reboot -- -f" in the kernel bootargs, timing the whole run from the outside with "perf stat --null". The 6% figure is if I count boot as a whole including VM startup and teardown (i.e. an under-estimate of the proportion), the 13% figure is if I subtract a baseline timing from a run without KASAN (i.e. an over-estimate of the proportion). Could you let me know how you're measuring this, and which platform+config you're using? I'll have a play with some configs in case there's a pathological configuration, but if you could let me know how/what you're testing that'd be a great help. Thanks, Mark.
