Re: [PATCH 4/10] alternate 4-level page tables patches

[Nick Piggin <nickpiggin@yahoo.com.au>]

William Lee Irwin III wrote:
> On Sat, 18 Dec 2004, William Lee Irwin III wrote:
> 
>>>For clear_page_tables() you want to scan as little as possible. The
>>>exit()-time performance issue is tlb_finish_mmu().
> 
> 
> On Sat, Dec 18, 2004 at 05:31:48PM -0800, Linus Torvalds wrote:
> 
>>Note that the fact that we share lots of code between "unmap" and "exit" 
>>is likely a (performance) bug.
>>The exit case is really a lot simpler, not just because we get rid of the 
>>whole VM, but because nobody else can be reading the page tables at the 
>>same time, and in particular we do not have a lot of the races that a 
>>simple unmap can have. The whole "gather/flush" thing is overkill, I 
>>think.
> 
> 
> For x86-style MMU's you could literally not bother flushing the TLB at
> all, since you'll just switch to another set of pagetables.
> 
> 

I think you're right. At exit-time, I think the flush should just amount
to __flush_tlb() anyway, in which case it is only another cr3 reload, so
it is probably not a big saving?

> On Sat, Dec 18, 2004 at 05:31:48PM -0800, Linus Torvalds wrote:
> 
>>Actually, looking at the code, I wonder why we haven't marked the exit 
>>case to be "fast". We have this special optimization for single-CPU which 
>>doesn't bunch pages up and free them in chunks, and we should probably 
>>mark the exit case to use the fast-case where we can flush the TLB's 
>>early. Hmm?
>>Ingo, is there any reason we don't do this:
>>--- 1.24/include/asm-generic/tlb.h	2004-07-10 17:14:00 -07:00
>>+++ edited/include/asm-generic/tlb.h	2004-12-18 17:30:43 -08:00
>>@@ -58,7 +58,7 @@
>> 	tlb->mm = mm;
>> 
>> 	/* Use fast mode if only one CPU is online */
>>-	tlb->nr = num_online_cpus() > 1 ? 0U : ~0U;
>>+	tlb->nr = num_online_cpus() > 1 && !full_mm_flush ? 0U : ~0U;
>> 
>> 	tlb->fullmm = full_mm_flush;
>> 	tlb->freed = 0;
>>which should make the exit case TLB handling go much faster. Was there 
>>some race in that too? Nobody should be using the VM any more at that 
>>point, so it _should_ be safe, no?
> 
> 
> The stale translations can't be left around for ASID-tagged TLB's, lest
> the next user of the ASID inherit them.
> 

I think they should still be flushed before the ASID is reused though,
right? (not that I know how Linux uses ASID TLBs)

It looks to me like Linus' patch should fly (albiet with my limited
knowledge of !i386 architectures).
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