linux-kernel - Re: [PATCHv2 4/6] sched_clock: Add support for >32 bit sched

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Fri, 14 Jun 2013 18:17:05 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Rob Herring <robherring2@...il.com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Stephen Boyd <sboyd@...eaurora.org>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
	Will Deacon <Will.Deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"arm@...nel.org" <arm@...nel.org>,
	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCHv2 4/6] sched_clock: Add support for >32 bit sched_clock

On Mon, Jun 10, 2013 at 05:12:08AM +0100, Rob Herring wrote:
> On 06/04/2013 05:21 AM, Russell King - ARM Linux wrote:
> > On Mon, Jun 03, 2013 at 06:51:59PM -0700, Stephen Boyd wrote:
> >> On 06/03/13 15:12, Russell King - ARM Linux wrote:
> >>> 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.
> > 
> > Well, 56 bits at 1ns intervals is 833 days (2^56 / (1000000000*60*60*24)).
> > We used to say that 497 days was enough several years ago, and that got
> > fixed.  We used to say 640K was enough memory for anything, and that
> > got fixed.
> 
> The ARM ARM states a minimum resolution of 40 years AND at least 56-bits
> of resolution. So a 1Gz counter would have to have more that 56 bits.

At a quick calculation, with a full 64-bit counter and 40-year roll-over
we can have maximum 14.6GHz clock. So we shouldn't just mask the top
8-bit of the counter as the bottom 56 could roll over in much less time.

-- 
Catalin
--
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/