Hi Brendan, On Wed, 16 Jul 2014 23:29:10 -0700, Brendan Gregg wrote: > G'Day David, > > On Wed, Jul 16, 2014 at 9:41 PM, David Ahern <dsah...@gmail.com> wrote: >> >> >> On 07/16/2014 06:51 PM, Brendan Gregg wrote: >>> >>> G'Day, >>> >>> I'm not sure where else to ask this; I don't think this functionality >>> is in perf_events yet... >>> >>> kprobes is supposed to be able to handle string arguments, but I've >>> not been able to find a single working example. I'm trying (on 3.16): >>> >>> # cd /sys/kernel/debug/tracing >>> # echo 'r:getname getname $retval:string' > kprobe_events >>> -bash: echo: write error: Invalid argument >>> # echo 'p:myprobe do_sys_open %dx:string' > kprobe_events >>> -bash: echo: write error: Invalid argument >>> >>> I'm following the syntax in Documentation/trace/kprobetrace.txt by >>> Masami Hiramatsu, and it is recognizing "string". But I'm getting >>> these errors: >>> >>> # dmesg | tail -4 >>> [98021.813560] string type has no corresponding fetch method. >>> [98021.813564] Parse error at argument[0]. (-22) >>> [98705.956199] string type has no corresponding fetch method. >>> [98705.956203] Parse error at argument[0]. (-22) >>> >>> Anyone seen this work? I'm checking the source... Thanks in advance, >> >> >> Have you seen this: >> https://lkml.org/lkml/2014/6/19/698 >> > > Thanks! (I'm trying to keep up with all the perf messages on lkml, but > missed this.) > > So, perf can already do this?? And it's the same syntax (":string") as > kprobes? > > I just tried it out (on 3.14.5): > > # perf probe 'getname filename:string' > Added new event: > [...] > # perf record -e probe:getname -a sleep 5 > [ perf record: Woken up 1 times to write data ] > [ perf record: Captured and wrote 0.234 MB perf.data (~10241 samples) ] > # perf script > perf 13587 [000] 3576718.127120: probe:getname: > (ffffffff811bbd20) > filename_string="/home/bgregg-testtest/libexec/perf-core/sleep" > perf 13587 [000] 3576718.127142: probe:getname: > (ffffffff811bbd20) filename_string="/usr/local/sbin/sleep" > perf 13587 [000] 3576718.127151: probe:getname: > (ffffffff811bbd20) filename_string="/usr/local/bin/sleep" > perf 13587 [000] 3576718.127159: probe:getname: > (ffffffff811bbd20) filename_string="/usr/sbin/sleep" > perf 13587 [000] 3576718.127170: probe:getname: > (ffffffff811bbd20) filename_string="/usr/bin/sleep" > perf 13587 [000] 3576718.127180: probe:getname: > (ffffffff811bbd20) filename_string="/sbin/sleep" > perf 13587 [000] 3576718.127189: probe:getname: > (ffffffff811bbd20) filename_string="/bin/sleep" > sleep 13587 [000] 3576718.162025: probe:getname: > (ffffffff811bbd20) filename_string="/etc/ld.so.cache" > sleep 13587 [000] 3576718.162057: probe:getname: > (ffffffff811bbd20) filename_string="/lib/x86_64-linux-gnu/libc.so.6" > [...] > > Wow. > > Ok, so that's one solution! :-) > > It does need debuginfo, which is a bit of a problem here (the Netflix > cloud, where instances are created and destroyed quickly, so they are > optimized to be small). > > I did try just using the return value, which does work without > debuginfo, however, the :string modifier doesn't work. Eg, trying: > > # perf probe 'getname%return $retval:string' > Added new event: > Failed to write event: Invalid argument > Error: Failed to add events. (-1) > > $retval really is a string (well, it's a struct filename *, where the > first member is a char *, so close enough), so perhaps perf could > enhanced to allow this, and I'd be able to trace strings without > debuginfo. (Unless there's another workaround.) > > So tools/perf/Documentation/perf-probe.txt does at least explain when > :string won't work: "You can specify 'string' type only for the local > variable or structure member which is an array of or a pointer to > 'char' or 'unsigned char' type." I wonder if kprobes has this > restriction as well.
Currently, "retval" and "reg" (and some other) fetch methods don't support string type. But I guess it can be easily worked around by using "deref" method. Have you tried something like below (untested)? # perf probe 'getname%return +0($retval):string' Hmm.. maybe below (as you mentioned it's a char * in struct filename *). # perf probe 'getname%return +0(+0($retval)):string' Thanks, Namhyung -- To unsubscribe from this list: send the line "unsubscribe linux-perf-users" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
