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

[William Lee Irwin III <wli@holomorphy.com>]

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.


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.


-- wli
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