linux-kernel - Re: [patch] x86: unify/rewrite SMP TSC sync code

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Message-Id: <200611242206.36681.ak@suse.de>
Date:	Fri, 24 Nov 2006 22:06:36 +0100
From:	Andi Kleen <ak@...e.de>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org, Andrew Morton <akpm@...l.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch] x86: unify/rewrite SMP TSC sync code

On Friday 24 November 2006 21:46, Ingo Molnar wrote:
> 
> * Andi Kleen <ak@...e.de> wrote:
> 
> > 
> > > yeah - the main new bit for x86-64 is the unconditional check for time 
> > > warps. We shouldnt (and cannot) really trust the CPU and the board/BIOS 
> > > getting it right. There were always some motherboards using Intel CPUs 
> > > that had the TSCs wrong.
> > 
> > In the 64bit capable generation I don't know of any run in spec 
> > (except for multinode systems and there was one overclocked system 
> > where the cores got unsync, but overclocking is an operator error)
> 
> i have one (Intel based), 64-bit, fully in spec, which is off by 
> ~3000-4000 cycles. So it happens.

More details?

> I was in fact surprised when i noticed that you removed the 
> unconditional TSC check that i put there years ago 

I removed it because you pointed out that it usually caused
trouble on Intel systems: we would always detect errors due to measurement errors
and then make things worse by trying to fix it.

But you're right it might have been better to keep 
a check with a threshold to catch totally broken cases.

> but which apps are using RDTSC natively? Trapping isnt too good i agree

The only sure way would be to trap+printk -- but from previous
user complaints it's a substantial number.

> - if then we should remove it from the CPU features and hence apps wont 
> (or shouldnt) use it.

I doubt the majority checks any cpu features first ...

> 
> > > nor can the TSC really be synced up properly in the hotplug CPU 
> > > case, after the fact - what if the app already read out an older TSC 
> > > value and a new CPU is added. If the TSC isnt sync on SMP then it 
> > > quickly gets pretty messy, and we should rather take a look at /why/ 
> > > these apps are using RDTSC.
> > 
> > Because gettimeofday is too slow.
> 
> as i indicated it in another discussion, i can fix that. Next patch will 
> be that.

Well I hope it's not making it HZ resolution. As noted earlier we tried
that already and it didn't work (it violates the "forward monotonity"
that is commonly expected) 

Ok I could imagine it making sense as a new CLOCK_FASTBUTLOUSYRESOLUTION timer in 
clock_gettime() [together with the new vdso fastpath], but not as default.

-Andi
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