linux-kernel - Re: [PATCH] trace: extend trace_clock to support arch

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Date:   Tue, 6 Dec 2016 12:13:47 +0000
From:   Will Deacon <will.deacon@....com>
To:     Srinivas Ramana <sramana@...eaurora.org>
Cc:     catalin.marinas@....com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] trace: extend trace_clock to support arch_arm clock
 counter

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?

> 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)?

> 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?

Will