Messages in this thread |  | | | Date | Thu, 25 Sep 2008 22:31:46 -0700 | | From | Jeremy Fitzhardinge <> | | Subject | Re: [RFC PATCH 1/3] Unified trace buffer |
Steven Rostedt wrote: > [ > "When people ask me what language my mother tongue is, > I simply reply 'C'" - Steven Rostedt > ] > > This is exactly why I have that saying ;-) > > > On Thu, 25 Sep 2008, Jeremy Fitzhardinge wrote: > > >> Steven Rostedt wrote: >> >>> OK, let me rephrase my question. >>> >>> How and where do we record this? Do we keep this information in some >>> global variable that we must compare to every time we add a new item in >>> the trace? >>> >>> Do we have the buffer register a call back to record this information? >>> >>> >> Something like (total pseudocode): >> > > Good enough. > > >> struct tsc_time_parameters { >> int version; /* even - values OK; odd - values being updated */ >> u64 tsc; >> u32 tsc_freq; >> u64 gtod; >> }; >> >> DEFINE_PERCPU(struct tsc_time_parameters, tsc_params); >> > > These are all global I presume (No "static" in front) >
No, they could probably be static, depending where everything ends up. It would only need to get accessed from a couple of places.
>> /* To be called after a tsc frequency change, before any new >> trace records are being emitted, in a context where we can call get_GTOD() */ >> void update_tsc_params(void) >> > > So this needs to be called by the cpu freq code? >
Yes, and any other place the tsc might get affected, like going into a C-state which stops the tsc, and things like suspend/resume.
>> { >> struct tsc_time_parameters *p = __get_percpu_var(tsc_params); >> >> p->version |= 1; >> wmb(); >> >> p->tsc = get_tsc(); >> p->tsc_freq = get_tsc_freq(); >> p->gtod = get_GTOD(); >> >> wmb(); >> p->version++; >> wmb(); >> } >> >> DEFINE_PERCPU(unsigned, current_tsc_version); >> DEFINE_PERCPU(u64, prev_tsc); >> >> /* may be called in any context */ >> u64 get_trace_timestamp_delta(void) >> { >> const struct tsc_time_parameters *p = &__get_percpu_var(tsc_params); >> unsigned *current_version = &__get_cpu_var(current_tsc_version); >> u64 prev = __get_cpu_var(prev_tsc); >> u64 now, ret; >> >> /* check the current tsc_params version against the last one we emitted; >> if the version is odd, then we interrupted the parameters as they were >> being updated, so just emit a new delta with the old parameters */ >> if (unlikely(*current_version != p->version && !(p->version & 1))) { >> /* XXX probably need a loop to deal with p->version changing under our feet */ >> emit_tsc_freq_record(p); >> > > I take it the above is your record to the tracer? >
Yeah. No doubt it needs a few more parameters.
>> prev = p->tsc; >> __get_cpu_var(current_tsc_version) = p->version; >> } >> >> now = read_tsc(); >> > > We probably wont to check here that p didn't change again. > and try again if it did. >
Yeah, you may want to put the whole thing in a loop to make sure that the version is consistent. You might end up emitting multiple redundant tsc parameters, but that should be very rare.
>> ret = now - prev; >> __get_cpu_var(prev_tsc) = now; >> >> return ret; >> } >> > > > Hmm, the beginning of each patch will need to record the global tsc, as > well as this information. Simply because in overwrite mode, we do not want > to lose it if the producer is faster than te consumer. >
Each patch? Page?
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