linux-kernel - Re: [PATCH] Re: x86_32 tsc/pit and hrtimers

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Message-Id: <1223589220.32639.24.camel@alok-dev1>
Date:	Thu, 09 Oct 2008 14:53:40 -0700
From:	Alok Kataria <akataria@...are.com>
To:	Chris Snook <csnook@...hat.com>
Cc:	Alok kataria <alokkataria1@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jeff Hansen <x@...fhansen.com>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>
Subject: Re: [PATCH] Re: x86_32 tsc/pit and hrtimers

On Thu, 2008-10-09 at 14:03 -0700, Chris Snook wrote:
> Alok kataria wrote:
> > On Thu, Oct 9, 2008 at 12:53 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> >> On Thu, 9 Oct 2008, Jeff Hansen wrote:
> >>
> >>> OK, so are we all agreed that something like clocksource_trust=tsc would be
> >>> the best?
> >> No, it's per affected device: tsc=trust or tsc=stable or whatever
> >> unintuitive name we want to come up. And it is a modification to TSC
> >> not to the clocksource layer.
> >
> > Yep, this is cool. I too have a patch in my local tree which does a
> > similar thing i have a tsc_reliable flag which is set right now only
> > when we are running under a VMware hypervisor.
> > Along with marking the no_verify flag for TSC, this patch of mine also
> > skips the TSC synchornization checks.
> >
> >         The TSC synchronization loop which is run whenever a new cpu is
> > brought up is not actually needed on systems which are known to have a
> > reliable TSC. TSC between 2 cpus can be off by a marginal value on such
> > systems and thats okay for timekeeping, since we do check for tsc going
> > back in read_tsc.
> >
> > Can this reasoning be included and synchronization skipped for all
> > these systems with reliable aka trustworthy TSC's  ?
> 
> In general, no.  Not all hardware/hypervisors behave this way, even when the TSC
> is otherwise stable once synchronized.

I agree that in general this should be no, but since this is a
commandline variable it will be normally set for only those systems
which have only TSC as a option or know that the TSC is reliable.
wouldn't doing this be ok for such systems ? 

Thanks,
Alok

> 
> -- Chris

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