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

[Konrad Rzeszutek Wilk]

On Wed, Jun 27, 2012 at 08:12:56AM -0700, Dan Magenheimer wrote:
> > From: Minchan Kim [mailto:minchan@xxxxxxxxxx]
> > 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@xxxxxxxxxxxxxxxxxx>
> > >>
> > >>
> > >> 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.

At least put a big fat comment in the patch saying:
"This is based on research done by Alex, where ...


This needs to be redone where it is automatically figured
out based on the CPUID, but ." [include what Dan just
said about breakeven point]


> 
> 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.

Comments are simple :-)
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