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

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Date:	Thu, 09 Oct 2008 17:03:37 -0400
From:	Chris Snook <csnook@...hat.com>
To:	Alok kataria <alokkataria1@...il.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Jeff Hansen <x@...fhansen.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

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.

-- Chris
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