Re: [PATCH -mm] mm: Clear to access sub-page last when clearing huge page

[Andrew Morton]

On Mon,  7 Aug 2017 15:21:31 +0800 "Huang, Ying" <ying.huang@xxxxxxxxx> wrote:

> From: Huang Ying <ying.huang@xxxxxxxxx>
> 
> Huge page helps to reduce TLB miss rate, but it has higher cache
> footprint, sometimes this may cause some issue.  For example, when
> clearing huge page on x86_64 platform, the cache footprint is 2M.  But
> on a Xeon E5 v3 2699 CPU, there are 18 cores, 36 threads, and only 45M
> LLC (last level cache).  That is, in average, there are 2.5M LLC for
> each core and 1.25M LLC for each thread.  If the cache pressure is
> heavy when clearing the huge page, and we clear the huge page from the
> begin to the end, it is possible that the begin of huge page is
> evicted from the cache after we finishing clearing the end of the huge
> page.  And it is possible for the application to access the begin of
> the huge page after clearing the huge page.
> 
> To help the above situation, in this patch, when we clear a huge page,
> the order to clear sub-pages is changed.  In quite some situation, we
> can get the address that the application will access after we clear
> the huge page, for example, in a page fault handler.  Instead of
> clearing the huge page from begin to end, we will clear the sub-pages
> farthest from the the sub-page to access firstly, and clear the
> sub-page to access last.  This will make the sub-page to access most
> cache-hot and sub-pages around it more cache-hot too.  If we cannot
> know the address the application will access, the begin of the huge
> page is assumed to be the the address the application will access.
> 
> With this patch, the throughput increases ~28.3% in vm-scalability
> anon-w-seq test case with 72 processes on a 2 socket Xeon E5 v3 2699
> system (36 cores, 72 threads).  The test case creates 72 processes,
> each process mmap a big anonymous memory area and writes to it from
> the begin to the end.  For each process, other processes could be seen
> as other workload which generates heavy cache pressure.  At the same
> time, the cache miss rate reduced from ~33.4% to ~31.7%, the
> IPC (instruction per cycle) increased from 0.56 to 0.74, and the time
> spent in user space is reduced ~7.9%
> 
> Thanks Andi Kleen to propose to use address to access to determine the
> order of sub-pages to clear.
> 
> The hugetlbfs access address could be improved, will do that in
> another patch.

I agree with what others said, plus...

> @@ -4374,9 +4374,31 @@ void clear_huge_page(struct page *page,
>  	}
>  
>  	might_sleep();
> -	for (i = 0; i < pages_per_huge_page; i++) {
> +	VM_BUG_ON(clamp(addr_hint, addr, addr +
> +			(pages_per_huge_page << PAGE_SHIFT)) != addr_hint);
> +	n = (addr_hint - addr) / PAGE_SIZE;
> +	if (2 * n <= pages_per_huge_page) {
> +		base = 0;
> +		l = n;
> +		for (i = pages_per_huge_page - 1; i >= 2 * n; i--) {
> +			cond_resched();
> +			clear_user_highpage(page + i, addr + i * PAGE_SIZE);
> +		}
> +	} else {
> +		base = 2 * n - pages_per_huge_page;
> +		l = pages_per_huge_page - n;
> +		for (i = 0; i < base; i++) {
> +			cond_resched();
> +			clear_user_highpage(page + i, addr + i * PAGE_SIZE);
> +		}
> +	}
> +	for (i = 0; i < l; i++) {
> +		cond_resched();
> +		clear_user_highpage(page + base + i,
> +				    addr + (base + i) * PAGE_SIZE);
>  		cond_resched();
> -		clear_user_highpage(page + i, addr + i * PAGE_SIZE);
> +		clear_user_highpage(page + base + 2 * l - 1 - i,
> +				    addr + (base + 2 * l - 1 - i) * PAGE_SIZE);

Please document this design with a carefully written code comment.
For example, why was "2 * n" chosen?  What is it trying to achieve?

Also, the final clearing loop "for (i = 0; i < l; i++)" might cause
eviction of data which was cached in the previous loop.  Perhaps some
additional gains will be made by clearing the hugepage in a
left-right-left-right "start from the ends and work inwards" manner, if
you see what I mean.  So the 4k pages immediately surrounding addr_hint
are the most-recently-cleared.  Although accesses to the data at lower
addresses than addr_hint are probably somewhat rare (and may be
nonexistent in your synthetic test case).