Messages in this thread | | | Date | Thu, 17 Sep 2009 14:34:47 +0800 | From | Zhaolei <> | Subject | Re: Re: [PATCH 2/3] ftrace: add tracepoint for xtime |
| |
john stultz wrote: > On Wed, 2009-09-16 at 16:32 -0400, Steven Rostedt wrote: >> On Wed, 2009-09-16 at 12:58 -0700, john stultz wrote: >>> On Wed, Sep 16, 2009 at 12:56 PM, john stultz <johnstul@us.ibm.com> wrote: >>>> On Tue, Aug 25, 2009 at 1:14 AM, Zhaolei <zhaolei@cn.fujitsu.com> wrote: >>>>> From: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com> >>>>> /* Structure holding internal timekeeping values. */ >>>>> struct timekeeper { >>>>> /* Current clocksource used for timekeeping. */ >>>>> @@ -338,6 +341,8 @@ int do_settimeofday(struct timespec *tv) >>>>> >>>>> update_vsyscall(&xtime, timekeeper.clock); >>>>> >>>>> + trace_gtod_update_xtime(&xtime, &wall_to_monotonic); >>>>> + >>>> So the only thing to watch out on here is that xtime doesn't hold the >>>> current time, but the accumulated time. There is some unaccumulated >>>> time still kept in the clocksource structure. >>>> >>>> You probably want (assuming you only need tick granularity time) to >>>> use current_kernel_time(). >>>> >>>> As an aside, is there a reason you have to have update callbacks and >>>> duplicate the time storage instead of using the existing interfaces? >>>> (ie: Is this due to locking or something else?) >>> Doh. Sorry, you're actually tracing the timekeeping code. Not using >>> this to assist tracing. Got this confused. >>> >>> So yea, I think this should be ok, plus or minus determining if you >>> really want xtime or xtime_cache. >> Well this may be a real concern. It's not about tracing timekeeping >> (although it adds that functionality). His second patch (I didn't Cc you >> on that one) hooks to these tracepoints to update time accordingly. >> >> What is being done is a way to have a "wall time" value being added to >> the ring buffer. But this needs to be very carefully done, because the >> all tracers use this, including the function tracer in NMI code. So the >> clock source can not take locks or do anything fancy. >> >> What the idea is, is to have a semi clock that is read with some kind of >> fast increment, and then at clock ticks, this clock is synced up. > > Hmm.. Yea, if that's the case, then I'm not a big fan of this approach. > > It sounds like what's really needed is a lock-free variant of > current_kernel_time() or something close to the CLOCK_MONOTONIC_COARSE > functionality currently queued.
Yes.
And it will be better if I can get current walltime fast and with high resolution.
I see your patch of __current_kernel_time() in Aug 19, and I think it can be used to get walltime in ftrace. But IMHO, use tracepoint to do it maybe better for its high resolution.
Thanks Zhaolei
> Doing it without locks might have some downsides, and I guess that's the > point of the callback method (updates happen at prescribed times and > likely under locks that the trace code understands so it avoids races > and deadlocks). > > -john >
| |