Messages in this thread | | | Date | Wed, 13 Jun 2012 15:18:54 +0200 | Subject | Re: [PATCH 02/19] perf: Add ability to attach user level registers dump to sample | From | Stephane Eranian <> |
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On Wed, Jun 13, 2012 at 3:12 PM, Jiri Olsa <jolsa@redhat.com> wrote: > On Wed, Jun 13, 2012 at 01:16:44PM +0200, Stephane Eranian wrote: >> On Mon, Jun 11, 2012 at 3:19 PM, Jiri Olsa <jolsa@redhat.com> wrote: >> > Introducing sample_regs_user bitmask into perf_event_attr >> > struct to define the user level registers we want to attach >> > to the sample. The dump itself is triggered once the >> > sample_regs_user is not empty. >> > >> > Only user level registers are dump at the moment. Meaning the >> > register values of the user space context as it was before the >> > user entered the kernel for whatever reason (syscall, irq, >> > exception, or a PMI happening in userspace). >> > >> > The layout of the sample_regs_user bitmap is described in >> > asm/perf_regs.h for archs that support register dump. >> > >> > This is going to be useful to bring Dwarf CFI based stack >> > unwinding on top of samples. >> > >> > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> >> > Signed-off-by: Jiri Olsa <jolsa@redhat.com> >> > --- >> > include/linux/perf_event.h | 10 ++++++- >> > kernel/events/core.c | 61 ++++++++++++++++++++++++++++++++++++++++++++ >> > 2 files changed, 70 insertions(+), 1 deletions(-) >> > >> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h >> > index 1ce887a..d66cbeb 100644 >> > --- a/include/linux/perf_event.h >> > +++ b/include/linux/perf_event.h >> > @@ -271,7 +271,13 @@ struct perf_event_attr { >> > __u64 bp_len; >> > __u64 config2; /* extension of config1 */ >> > }; >> > - __u64 branch_sample_type; /* enum branch_sample_type */ >> > + __u64 branch_sample_type; /* enum perf_branch_sample_type */ >> > + >> > + /* >> > + * Defines set of user regs to dump on samples. >> > + * See asm/perf_regs.h for details. >> > + */ >> > + __u64 sample_regs_user; >> > }; >> That's not enough. You also need to define PERF_SAMPLE_USER_REGS >> for sample_type. Although the sample_regs_users might look like it's enough >> to capture regs, there is a problem when it comes to parsing the record. You >> need an ordering guarantee that is explicitly spelled out in the API (the header >> file). In your current patch, I have no way of knowing that sample_regs_users >> are saved after BRANCH_STACK (should you have that enabled). Remember >> that you can turn on/off sampled infos at will in sample_type. Yet to find the >> infos when parsing, you need to know the order. > > Well, the sample_regs_user != 0 substitute the PERF_SAMPLE_USER_REGS bit. > The behaviour is the same as if there was that bit defined.. > No it's not the same. Looking at sample_regs_user != 0, do you know in which order the regs array is going to appear RELATIVE to the other captured information?
Take sample_type = IP|CPU|PERIOD, sample_regs_users = EAX
Now, I get the raw record, want to parse it. Which comes first the user_regs or the IP, CPU, PERIOD?
Worst, I add more entries to PERF_SAMPLE_*, are they laid out before or after the regs?
If you look carefully at perf_output_sample(), you will notice that data is written in the exact order of the enum perf_event_sample_format. Otherwise there is no way to parse this in the right order without looking at the kernel source code, which is not the right way....
> After last discussion the idea was to keep this just with sample_regs_user != 0. > > I dont see any limitation except for being incosistent with the rest of > the sample dumps. I'm all for having that PERF_SAMPLE_USER_REGS bit and > the user stack bit as well. > > jirka -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.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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