RE: [PATCH 3/3] x86: add local_tlb_flush_kernel_range()

[Dan Magenheimer <dan.magenheimer@oracle.com>]

> From: Minchan Kim [mailto:minchan@kernel.org]
> Subject: Re: [PATCH 3/3] x86: add local_tlb_flush_kernel_range()
> 
> Hello,
> 
> On 06/27/2012 03:14 PM, Alex Shi wrote:
> 
> > On 06/27/2012 01:53 PM, Minchan Kim wrote:
> >
> >> On 06/26/2012 01:14 AM, Seth Jennings wrote:
> >>
> >>> This patch adds support for a local_tlb_flush_kernel_range()
> >>> function for the x86 arch.  This function allows for CPU-local
> >>> TLB flushing, potentially using invlpg for single entry flushing,
> >>> using an arch independent function name.
> >>>
> >>> Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
> >>
> >>
> >> Anyway, we don't matter INVLPG_BREAK_EVEN_PAGES's optimization point is 8 or something.
> >
> >
> > Different CPU type has different balance point on the invlpg replacing
> > flush all. and some CPU never get benefit from invlpg, So, it's better
> > to use different value for different CPU, not a fixed
> > INVLPG_BREAK_EVEN_PAGES.
> 
> I think it could be another patch as further step and someone who are
> very familiar with architecture could do better than.
> So I hope it could be merged if it doesn't have real big problem.
> 
> Thanks for the comment, Alex.

Just my opinion, but I have to agree with Alex.  Hardcoding
behavior that is VERY processor-specific is a bad idea.  TLBs should
only be messed with when absolutely necessary, not for the
convenience of defending an abstraction that is nice-to-have
but, in current OS kernel code, unnecessary.

IIUC, zsmalloc only cares that the breakeven point is greater
than two.  An arch-specific choice of (A) two page flushes
vs (B) one all-TLB flush should be all that is necessary right
now.  (And, per separate discussion, even this isn't really
necessary either.)

If zsmalloc _ever_ gets extended to support items that might
span three or more pages, a more generic TLB flush-pages-vs-flush-all
approach may be warranted and, by then, may already exist in some
future kernel.  Until then, IMHO, keep it simple.

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