From: Seth Jennings [mailto:sjenning@linux.vnet.ibm.com]
Sent: Friday, June 15, 2012 9:13 AM
To: Peter Zijlstra
Cc: Minchan Kim; Greg Kroah-Hartman; Nitin Gupta; Dan Magenheimer; linux-kernel@vger.kernel.org;
linux-mm@kvack.org; Thomas Gleixner; Ingo Molnar; Tejun Heo; David Howells; x86@kernel.org; Nick
Piggin
Subject: Re: [PATCH v2 3/3] x86: Support local_flush_tlb_kernel_range

On 05/17/2012 09:51 AM, Peter Zijlstra wrote:

quoted

On Thu, 2012-05-17 at 17:11 +0900, Minchan Kim wrote:

quoted

quoted

+++ b/arch/x86/include/asm/tlbflush.h

@@ -172,4 +172,16 @@ static inline void flush_tlb_kernel_range(unsigned long start,
      flush_tlb_all();
 }

+static inline void local_flush_tlb_kernel_range(unsigned long start,
+             unsigned long end)
+{
+     if (cpu_has_invlpg) {
+             while (start < end) {
+                     __flush_tlb_single(start);
+                     start += PAGE_SIZE;
+             }
+     } else
+             local_flush_tlb();
+}

It would be much better if you wait for Alex Shi's patch to mature.
doing the invlpg thing for ranges is not an unconditional win.

From what I can tell Alex's patches have stalled.  The last post was v6
on 5/17 and there wasn't a single reply to them afaict.

According to Alex's investigation of this "tipping point", it seems that
a good generic value is 8.  In other words, on most x86 hardware, it is
cheaper to flush up to 8 tlb entries one by one rather than doing a
complete flush.

So we can do something like:

     if (cpu_has_invlpg && (end - start)/PAGE_SIZE <= 8) {
             while (start < end) {

Would this be acceptable?

Hey Seth, Nitin --

After more work digging around zsmalloc and zbud, I really think
this TLB flushing, as well as the "page pair mapping" code can be
completely eliminated IFF zsmalloc is limited to items PAGE_SIZE or
less.  Since this is already true of zram (and in-tree zcache), and
zsmalloc currently has no other users, I think you should seriously
consider limiting zsmalloc in that way, or possibly splitting out
one version of zsmalloc which handles items PAGE_SIZE or less,
and a second version that can handle larger items but has (AFAIK)
no users.

If you consider it an option to have (a version of) zsmalloc
limited to items PAGE_SIZE or less, let me know and we can
get into the details.

Thanks,
Dan

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