On Mon, 2 Aug 2021 02:16:14 +0200 Luigi Rizzo <lrizzo@xxxxxxxxxx> wrote: > > Well, it isn't cost-free. find_vma() is called a lot and a surprising > > number of systems apparently run with CONFIG_DEBUG_VM. Why do you > > think this cost is justified? > > I assume you are concerned with the cost of mmap_assert_locked() ? > > I'd say the justification is the same as for all asserts: > at some point some code change may miss the required lock, and the > asserts are there to catch elusive race conditions, > > There are in fact already instances of mmap_locked_assert() > right before find_vma() in walk_page_range(), and a couple before > calls to __get_user_pages(). > > As for the cost, I'd think that if CONFIG_DEBUG_VM is set, > one does it on purpose to catch errors and is prepared to pay > the cost (in this case the atomic_read(counter) in rwsem_is_locked(), > the counter should be hot). > > FWIW I have instrumented find_vma() on a fast machine using kstats > > https://github.com/luigirizzo/lr-cstats > > (load the module then enable the trace with > echo "trace pcpu:find_vma bits 3" > /sys/kernel/debug/kstats/_control > and monitor the time with > watch "grep CPUS /sys/kernel/debug/kstats/find_vma" > > I didn't run anything especially intensive except some network > benchmarks, but I have collected ~2M samples with the following > distribution of find_vma() time in nanoseconds in 3 configs: > > CONFIGURATION p10 p50 p90 p95 p98 > > no-debug 89 109 214 332 605 > debug 331 369 603 862 1338 > debug+this patch 337 369 603 863 1339 > > As you can see, just compiling a debug kernel, even without this patch, > makes the function 3x more expensive. The effect of this patch is > not measurable (the differences are below measurement error). Cool, thanks, that's convincing.
