Re: [PATCH] perf: POSIX CLOCK_PERF to report current time value

[John Stultz]

On 12/11/2013 04:07 AM, Ingo Molnar wrote:
> * John Stultz <john.stultz@xxxxxxxxxx> wrote:
>
>> [...]
>>
>> I'd much rather see perf export CLOCK_MONOTONIC_RAW timestamps, 
>> since that clockid is well defined. [...]
> So the problem with that clock is that it does the following for every 
> timestamp:
>
>         cycle_now = clock->read(clock);
>
> ... which is impossibly slow if something like the HPET is used, which 
> is rather common - so this is a non-starter to timestamp perf events 
> with. We use the scheduler clock as a reasonable compromise between 
> scalability and clock globality.
>
> I can see two solutions:
>
> 1)
>
> One approach is what I described in my other reply a few minutes ago: 
> track the flow of GTOD, timestamped with the fast perf timestamps, so 
> that GTOD can be correlated to the perf clock, if user-space so 
> wishes. The correlation is simple so this gets close to the ease of 
> use of being able to timestamp GTOD directly.
>
> (That would be useful for other purposes as well, such as 
> instrumenting GTOD updates.)
>
> 2)
>
> An alternate, rather interesting approach would be to change the 
> scheduler clock offset to be influenced by the above events, so that 
> it quasi-approximates GTOD and emits natural time of day timestamps.
>
> This already happens partially in the sched-clock slow path, 
> kernel/sched/clock.c's sched_clock_local(), it uses scd->tick_gtod 
> timestamps to correlate to the monotonic clock. This could be changed 
> over to use not get_ktime() but getnstimeofday(), to get true TOD 
> timestamps.
>
> The trickier bit is the x86 fast-path, in arch/x86/kernel/tsc.c's 
> native_sched_clock(). That relies on __cycles_2_ns() to transform a 
> CPU cycles timestamp into (boot time offset) nanoseconds. For that it 
> uses the cyc2ns_offset percpu variable. That variable could be updated 
> periodically so that it's TOD offset.
>
> My (strong!) preference would be #2, for the simple reason that it 
> would make perf timestamps instantly usable and tooling wouldn't have 
> to do anything to get true timestamps.

Right. #2 is basically what I was (probably poorly) trying to describe
as my ideal solution, making perf export what is essentially
CLOCK_MONOTONIC_RAW time (using CLOCK_MONOTONIC_RAW is more likely to be
easier to match then CLOCK_REALTIME/getnstimeofday(), since you don't
have to deal with anyone setting the clock, or frequency adjustments
from NTP).

thanks
-john

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