Re: [PATCHv2 4/6] sched_clock: Add support for >32 bit sched_clock

[Stephen Boyd]

On 06/03/13 15:12, Russell King - ARM Linux wrote:
> On Mon, Jun 03, 2013 at 02:11:59PM -0700, Stephen Boyd wrote:
>> On 06/03/13 02:39, Russell King - ARM Linux wrote:
>>> On Sat, Jun 01, 2013 at 11:39:41PM -0700, Stephen Boyd wrote:
>>>> +}
>>>> +
>>>> +void __init
>>>> +setup_sched_clock_64(u64 (*read)(void), int bits, unsigned long rate)
>>>> +{
>>>> +	if (cd.rate > rate)
>>>> +		return;
>>>> +
>>>> +	BUG_ON(bits <= 32);
>>>> +	WARN_ON(!irqs_disabled());
>>>> +	read_sched_clock_64 = read;
>>>> +	sched_clock_func = sched_clock_64;
>>>> +	cd.rate = rate;
>>>> +	cd.mult = NSEC_PER_SEC / rate;
>>> Here, you don't check that the (2^bits) * mult results in a wrap of the
>>> resulting 64-bit number, which is a _basic_ requirement for sched_clock
>>> (hence all the code for <=32bit clocks, otherwise we wouldn't need this
>>> complexity in the first place.)
>> Ok I will use clocks_calc_mult_shift() here.
> No, that's not the problem.
>
> If you have a 56-bit clock which ticks at a period of 1ns, then
> cd.rate = 1, and your sched_clock() values will be truncated to 56-bits.
> The scheduler always _requires_ 64-bits from sched_clock.  That's why we
> have the complicated code to extend the 32-bits-or-less to a _full_
> 64-bit value.
>
> Let me make this clearer: sched_clock() return values _must_ without
> exception monotonically increment from zero to 2^64-1 and then wrap
> back to zero.  No other behaviour is acceptable for sched_clock().

Ok so you're saying if we have less than 64 bits of useable information
we _must_ do something to find where the wraparound will occur and
adjust for it so that epoch_ns is always incrementing until 2^64-1. Fair
enough. I was trying to avoid more work because on arm architected timer
platforms it takes many years for that to happen.

I'll see what I can do.

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