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

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Message-ID: <35f686220810091345h253d71e8s4fe9d7ea8e636ccc@mail.gmail.com>
Date:	Thu, 9 Oct 2008 13:45:52 -0700
From:	"Alok kataria" <alokkataria1@...il.com>
To:	"Thomas Gleixner" <tglx@...utronix.de>
Cc:	"Jeff Hansen" <x@...fhansen.com>,
	"Chris Snook" <csnook@...hat.com>, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org, mingo@...e.hu, akataria@...are.com
Subject: Re: [PATCH] Re: x86_32 tsc/pit and hrtimers

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  ?

Thanks,
Alok
>
> Thanks,
>
>        tglx
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