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:

quoted

On 06/27/2012 01:53 PM, Minchan Kim wrote:

quoted

On 06/26/2012 01:14 AM, Seth Jennings wrote:

quoted

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 <redacted>

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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