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

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Date:   Sun, 04 Dec 2016 14:06:23 +0530
From:   Srinivas Ramana <sramana@...eaurora.org>
To:     Will Deacon <will.deacon@....com>
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 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.
>
> Will
>

Having cycle counter would help in the cases where we want to correlate 
the time with other subsystems which are outside cpu subsystem. 
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. so, 
when comparing with monotonically increasing cycle counter, other clocks 
doesn't help. It seems X86 uses the TSC counter to help such cases.

Thanks,
-- Srinivas R

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