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Message-ID: <545B1DDE.9000202@linux.vnet.ibm.com> Date: Thu, 06 Nov 2014 12:36:06 +0530 From: Hemant Kumar <hemant@...ux.vnet.ibm.com> To: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>, Namhyung Kim <namhyung@...nel.org> CC: linux-kernel@...r.kernel.org, srikar@...ux.vnet.ibm.com, peterz@...radead.org, oleg@...hat.com, hegdevasant@...ux.vnet.ibm.com, mingo@...hat.com, anton@...hat.com, systemtap@...rceware.org, aravinda@...ux.vnet.ibm.com, penberg@....fi, Arnaldo Carvalho de Melo <acme@...hat.com> Subject: Re: [PATCH v4 5/5] perf/sdt: Add support to perf record to trace SDT events On 11/05/2014 02:35 PM, Masami Hiramatsu wrote: > (2014/11/05 16:06), Namhyung Kim wrote: >> On Tue, 04 Nov 2014 21:56:53 +0900, Masami Hiramatsu wrote: >>> Hi, >>> >>> (2014/11/04 17:06), Hemant Kumar wrote: >>>> Hi Namhyung, >>>> >>>> On 11/04/2014 01:08 PM, Namhyung Kim wrote: >>>>> Hi Hemant, >>>>> >>>>> As you know, you need to keep an eye on how (kprobes) event cache >>>>> patchset from Masami settles down. For those who aren't CC'ed, please >>>>> see the link below: >>>>> >>>>> https://lkml.org/lkml/2014/10/31/207 >>>>> >>>>> On Sun, 02 Nov 2014 16:26:28 +0530, Hemant Kumar wrote: >>>>>> This patch adds support to perf to record SDT events. When invoked, >>>>>> the SDT event is looked up in the sdt-cache. If its found, an entry is >>>>>> made silently to uprobe_events file and then recording is invoked, and >>>>>> then the entry for the SDT event in uprobe_events is silently discarded. >>>>>> >>>>>> The SDT events are already stored in a cache file >>>>>> (/var/cache/perf/perf-sdt-file.cache). >>>>>> Although the file_hash table helps in addition or deletion of SDT events >>>>>> from the cache, its not of much use when it comes to probing the actual >>>>>> SDT event, because the key to this hash list is a file name and not the >>>>>> SDT event name (which is given as an argument to perf record). So, we >>>>>> won't be able to hash into it. >>>>> It likely to be ended up with per-file or per-buildid cache files under >>>>> ~/.debug directory. In this case we also need to have the (central) >>>>> event-to-cache table anyway IMHO. >>> What we are talking is to make a new caching file with buildid under >>> .debug/. >>> We already has ~/.debug/.build-id/<build-id> for string the binary >>> symbol maps. >> ?? >> >> The ~/.debug/.build-id/<build-id> (actually first 2 hexdigits are used >> for directory name) is a symlink to a cached binary. >> >> $ file .debug/.build-id/00/08a6c4028b3826f8905324c770e7aa450e5d3b >> .debug/.build-id/00/08a6c4028b3826f8905324c770e7aa450e5d3b: symbolic link to `../../usr/lib64/libgtk-3.so.0.400.4/0008a6c4028b3826f8905324c770e7aa450e5d3b' >> >> $ file .debug/usr/lib64/libgtk-3.so.0.400.4/0008a6c4028b3826f8905324c770e7aa450e5d3b >> .debug/usr/lib64/libgtk-3.so.0.400.4/0008a6c4028b3826f8905324c770e7aa450e5d3b: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, BuildID[sha1]=0xc4a6080026388b02245390f8aae770c73b5d0e45, stripped >> >> >> Hmm.. it seems the file utility prints the build-id as a sequence of 4 >> byte little-endian integers. > Ah, I saw the kernel's build-id file... > >>> I think there are 2 options, one is expanding the current >>> build-id file format to include sdt and probe-event caches. The other is >>> to add ~/.debug/.build-id/<build-id>.probe and >>> ~/.debug/.build-id/<build-id>.sdt for caching probe/sdt information. >> I think a single .probe file is enough for this, no? > I hope to be so, but SDT is a bit different, that may have a semaphore. Currently > we just ignore it. But in some application, SDT events never be hit without enabling > the semaphores. I'm not sure how we can enable it. Systemtap enables it by modifying > application memory(data) on the fly. Maybe perftools can be done it by using ptrace, > but it's not for ftrace. > > [Off topic] I really don't like that the current SDT's semaphore. If the user apps > see the instruction at the probe point, it is easy to check whether the event is > enabled or not. Thus I recommend to change its implementation and update version > instead of supporting current semaphore by perftools. > > >>> And also, user interface is a discussion point. This series defines new >>> sdt-cache command, and we already have buildid-cache command. We should >>> have probe-cache command too? or consolidate those cache managing commands? >>> This question should be involving your series too. >>> >>>>>> To avoid this problem, we can create another hash list "event_hash" list >>>>>> which will be maintained along with the file_hash list. >>>>>> Whenever a user invokes 'perf record -e %provider:event, perf should >>>>>> initialize the event_hash list and the file_hash list. >>>>>> The key to event_hash list is calculated from the event name and its >>>>>> provider name. >>>>> Isn't it enough just to use provide name? I guess the provider names >>>>> are (should be?) unique among a system although there's no absolute >>>>> guarantee for that. >>>>> >>>> Yes, there is no guarantee for the provider names to be unique. >>>> If we use only provider name with "perf record", then, what if a user >>>> wants to trace >>>> only a specific SDT event (not all the events for that provider)? >>>> What do you think? >> What I'm saying is for managing cache not the usage of the cached >> events. IIUC you keep hash entry for all events to find matching file, >> but I think it can be managed in provider level as events in a single >> provider will live in a single binary. > Usually, the SDT provider names are managed by hand, as it is unique. > >> Btw, I think we should support such multiple events to like >> >> # perf record -e %provider_xxx:* -e %provider_yyy:prefix_* >> > what will be placed in the xxx and yyy ? > > Thank you, > >>> How about failing if the provider name is not unique unless user >>> gives the actual binary path? >> It looks like a possible option. :) >> >> Thanks, >> Namhyung >> > So, what should be our way forward here in case of SDT patchset wrt event_cache patchset? Shall we wait for event_cache patchset to be merged and then redesign the sdt_cache patchset according to new event_cache? Or, we can go ahead with the current sdt patchset (implementing the latest review comments) and we can change the sdt_cache according to the new event_cache design as and when required? What do you think? -- Thanks, Hemant Kumar -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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