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Date: Mon, 10 Jun 2013 20:46:36 +0530 From: anish singh <anish198519851985@...il.com> To: Russell King - ARM Linux <linux@....linux.org.uk> Cc: Stephen Boyd <sboyd@...eaurora.org>, linux-arm-msm@...r.kernel.org, Will Deacon <will.deacon@....com>, linux-kernel-mail <linux-kernel@...r.kernel.org>, arm@...nel.org, John Stultz <john.stultz@...aro.org>, Catalin Marinas <catalin.marinas@....com>, Thomas Gleixner <tglx@...utronix.de>, linux-arm-kernel@...ts.infradead.org Subject: Re: [PATCHv2 4/6] sched_clock: Add support for >32 bit sched_clock On Tue, Jun 4, 2013 at 3:42 AM, Russell King - ARM Linux <linux@....linux.org.uk> 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(). Probably a trivial question.I was wondering why this particular requirement exists in the first place.I looked into this commit 112f38a4a3 but couldn't gather the reason. > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@...r.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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