Messages in this thread | | | Date | Mon, 3 Jun 2013 23:12:11 +0100 | From | Russell King - ARM Linux <> | Subject | Re: [PATCHv2 4/6] sched_clock: Add support for >32 bit sched_clock |
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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().
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