On Tue, Nov 22, 2016 at 4:23 PM, Joel Fernandes <joe...@google.com> wrote:
> Hi Thomas,
>
> On Tue, Nov 22, 2016 at 2:29 PM, Thomas Gleixner <t...@linutronix.de> wrote:
>>
>> On Mon, 21 Nov 2016, Joel Fernandes wrote:
>> > @@ -56,6 +56,12 @@ static struct timekeeper shadow_timekeeper;
>> > struct tk_fast {
>> > seqcount_t seq;
>> > struct tk_read_base base[2];
>> > +
>> > + /*
>> > + * first dimension is based on lower seq bit,
>> > + * second dimension is for offset type (real, boot, tai)
>> > + */
>>
>> Can you figure out why there are comments above the struct which explain
>> the members in Kernel Doc format and not randomly formatted comments inside
>> the struct definition?
>
> Ok sorry. I can move the comments before the function in the prescribed
> format.
>
>> > + ktime_t offsets[2][TK_OFFS_MAX];
>>
>> This bloats fast_tk_raw for no value, along with the extra stores in the
>> update function for fast_tk_raw which will never use that offset stuff.
>>
>> Aside of that, I really have to ask the question whether it's really
>> necessary to add this extra bloat in storage, update and readout code for
>> something which is not used by most people.
>>
>> What's wrong with adding a tracepoint into the boot offset update function
>> and let perf or the tracer inject the value of the boot offset into the
>> trace data when starting. The time adjustment can be done in
>> postprocessing.
>
> I agree we're bloating this and probably not very acceptable.
> tracepoint adds additional complexity and out of tree patches which
> we'd like to avoid. Would you be Ok if we added a relatively simple
> function like below that could do the job and not bloat the optimal
> case?
>
> /*
> * Fast and NMI safe access to boot time. It may be slightly out of date
> * as we're accessing offset without seqcount writes, but is safe to access.
s/writes/reads/
> */
> u64 ktime_get_boot_fast_ns(void)
> {
> struct timekeeper *tk = &tk_core.timekeeper;
> return __ktime_get_fast_ns(&tk_fast_mono) + tk->offs_boot;
I meant, __ktime_get_fast_ns(&tk_fast_mono) +
ktime_to_ns(tk->offs_boot). Was just showing the idea...
Joel
> }
> EXPORT_SYMBOL_GPL(ktime_get_boot_fast_ns);
>
>
>> That should be sufficient for tracing suspend/resume behaviour. If there is
>> not a really convincing reason for having that as a real trace clock then I
>> prefer to avoid that extra stuff.
>
> Several clocks are accessible just by simple writing of clock name to
> trace_clock in debugfs. This is really cool and doesn't require any
> out of tree patches or post processing complexity.
>
> Thanks,
> Joel
