On Tue, 23 Jun 2020 08:47:06 +0900 Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote: > On Mon, 22 Jun 2020 09:01:48 -0400 > Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > > > On Mon, 22 Jun 2020 08:27:53 +0800 > > Ming Lei <ming.lei@xxxxxxxxxx> wrote: > > > > > Can you kprobe guys improve the implementation for covering this case? > > > For example, put probe on 3) in case the above situation is recognized. > > > > To do so would require solving the halting problem. > > > > https://en.wikipedia.org/wiki/Halting_problem > > > > Or perhaps reading the DWARF output of the compiler to determine if it > > optimized the location you are looking for. > > As far as I can see, gcc-9.3 doesn't generate this information :( > Maybe the optimizer forgot to push the tail-call callsite information > to dwarf generator when making a recursive tail-call to a loop. > > > The first case is impossible to solve, the second would take a lot of > > work, (are you going to fund it?) > > What I can provide is "--skip-prologue" option for the perf-probe > which will be similar to the "-P" option. If the compiler correctly > generates the information, we can enable it automatically. But > as far as I can see, it doesn't. > > [OT] DWARF has its option(and GNU extension) but it seems not correctly > implemented yet. > > http://www.dwarfstd.org/ShowIssue.php?issue=100909.2 Oops, sorry, I missed the following sentences. "Tail calls are jump-like instructions which transfer control to the start of some subprogram, but the call site location address isn't visible in the unwind information." "Tail recursion is a call to the current function which is compiled as a loop into the middle of the current function." "The DW_TAG_call_site entries describe normal and tail calls." This means, the gcc is correctly implemented and this __blkdev_put() case is NOT covered by DT_TAG_call_site. So we can not detect it from the debuginfo. Thank you, -- Masami Hiramatsu <mhiramat@xxxxxxxxxx>
