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Date: Sat, 7 Nov 2015 07:50:16 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: "H. Peter Anvin" <hpa@zytor.com>
cc: Dan Williams <dan.j.williams@intel.com>,
        Ross Zwisler <ross.zwisler@linux.intel.com>,
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Subject: Re: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness
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On Fri, 6 Nov 2015, H. Peter Anvin wrote:
> On 11/06/15 15:17, Dan Williams wrote:
> >>
> >> Is it really required to do that on all cpus?
> > 
> > I believe it is, but I'll double check.
> > 
> 
> It's required on all CPUs on which the DAX memory may have been dirtied.
>  This is similar to the way we flush TLBs.

Right. And that's exactly the problem: "may have been dirtied"

If DAX is used on 50% of the CPUs and the other 50% are plumming away
happily in user space or run low latency RT tasks w/o ever touching
it, then having an unconditional flush on ALL CPUs is just wrong
because you penalize the uninvolved cores with a completely pointless
SMP function call and drain their caches.

Thanks,

	tglx