Re: [PATCH] trace: extend trace_clock to support arch_arm clock counter

From: Srinivas Ramana
Date: Mon Dec 12 2016 - 00:02:21 EST

On 12/06/2016 05:43 PM, Will Deacon wrote:
On Sun, Dec 04, 2016 at 02:06:23PM +0530, Srinivas Ramana wrote:
On 12/02/2016 04:38 PM, Will Deacon wrote:
On Fri, Dec 02, 2016 at 01:44:55PM +0530, Srinivas Ramana wrote:
Extend the trace_clock to support the arch timer cycle
counter so that we can get the monotonic cycle count
in the traces. This will help in correlating the traces with the
timestamps/events in other subsystems in the soc which share
this common counter for driving their timers.

I'm not sure I follow this reasoning. What's wrong with nanoseconds? In
particular, the "perf" trace_clock hangs off sched_clock, which should
be backed by the architected counter anyway. What does the cycle counter in
isolation tell you, given that the frequency isn't architected?

I think I'm missing something here.

Having cycle counter would help in the cases where we want to correlate the
time with other subsystems which are outside cpu subsystem.

Do you have an example of these subsystems? Can they be used to generate
trace data with mainline?

Some of the subsystems i can list are Modem(on a mobilephone), GPU or video subsystem, or a DSP among others.

local_clock or even the perf track_clock uses sched_clock which gets
suspended during system suspend. Yes, they are backed up by the
architected counter but they ignore the cycles spent in suspend.i

Does mono_raw solve this (also hangs off the architected counter and is
supported in the vdso)?

Doesn't seem like. Any of the existing clock sources are designed not show the jump, when there is a suspend and resume. Even though they run out of architected counter they just cane give exact correlation with the counter. Furthermore, during the initial kernel boot, these just run out of jiffies clock source. They also not account for the time spent in boot loaders.

so, when comparing with monotonically increasing cycle counter, other
clocks doesn't help. It seems X86 uses the TSC counter to help such cases.

Does this mean we need a way to expose the frequency to userspace, too?

Not really. The CNTFRQ_EL0 of timer subsystem holds the clock frequency of system timer and is available to EL0.


-- Srinivas R

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