Messages in this thread | ![/](/images/icornerl.gif) | | Subject | Re: [PATCH] make tsc=reliable override boot time stability checks | From | john stultz <> | Date | Tue, 18 Aug 2009 18:04:31 -0700 |
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On Mon, 2009-08-17 at 18:39 -0700, Alok Kataria wrote: > Hi John, > > On Mon, 2009-08-17 at 16:40 -0700, john stultz wrote: > > On Mon, 2009-08-17 at 16:27 -0700, Stephen Hemminger wrote: > > > On Mon, 17 Aug 2009 16:17:54 -0700 > > > john stultz <johnstul@us.ibm.com> wrote: > > > > > > > On Mon, 2009-08-17 at 16:02 -0700, Stephen Hemminger wrote: > > > > > This adds tsc, but makes it first? it is reliable, but do I want > > > > > to make it most important? > > > > > > > > > > $ cat /sys/devices/system/clocksource/clocksource0/available_clocksource > > > > > tsc acpi_pm > > > > > > > > > > > > Well, if you're overriding the system saying that its safe, then sure, > > > > its better then anything else, why wouldn't we? > > > > > > > > > > That's acceptable, maybe add change to Documentation/kernel-parameters.txt > > > > > > tsc= Disable clocksource-must-verify flag for TSC. > > > Format: <string> > > > [x86] reliable: mark tsc clocksource as reliable and > > > makes tsc the default clocksource; this > > > disables clocksource verification at runtime. > > > Used to enable high-resolution timer mode on older > > > hardware, and in virtualized environment. > > > > > > > Sounds good. Thanks so much for the bug report and testing! > > > > > > This patch makes the tsc=reliable option disable the boot time stability > > checks. Currently the option only disables the runtime watchdog checks. > > This change allows folks who want to override the boot time TSC > > stability checks and use the TSC when the system would otherwise > > disqualify it. > > > > There still are some situations that the TSC will be disqualified, such > > as cpufreq scaling. But these are situations where the box will hang if > > allowed. > > > > I had purposefully kept the tsc=reliable separate from the TSC > synchronous checks. > With this patch TSC is marked as usable though the hardware below > doesn't export a CONSTANT_TSC, it might not be a problem generally, but > since TSC has the highest rating, don't you think that timekeeping might > be wayward on such systems ?
Oh yea, there's a risk of that, but we are telling the kernel to override its runtime checking of the clocksource, so it seems reasonable to also include the boot time checks. I worry otherwise the option becomes too subtle to be really useful to users.
> Having said that, I don't think I have a particular problem with the > patch as far as we are explicitly mentioning the fact that TSC=reliable > means TSC is blindly trusted on this system, and time might be little > off on some systems.
I think the explicit boot option, along with the kernel-parameters text makes it clear enough, but if you have a specific wording in mind that works better, please send a patch and I'll ack it.
thanks -john
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