Re: [Xen-devel] [PATCH v2 1/3] xen: delay page scrubbing to allocation path

[Bob Liu <bob.liu@xxxxxxxxxx>]

On 06/30/2014 11:56 PM, Jan Beulich wrote:
>>>> On 30.06.14 at 15:39, <lliubbo@xxxxxxxxx> wrote:
>> --- a/xen/common/page_alloc.c
>> +++ b/xen/common/page_alloc.c
>> @@ -711,6 +711,12 @@ static struct page_info *alloc_heap_pages(
>>  
>>      for ( i = 0; i < (1 << order); i++ )
>>      {
>> +        if ( test_bit(_PGC_need_scrub, &pg[i].count_info) )
>> +        {
>> +            scrub_one_page(&pg[i]);
>> +            pg[i].count_info &= ~PGC_need_scrub;
>> +        }
>> +
> 
> heap_lock is still being held here - scrubbing should be done after it
> was dropped (or else you re-introduce the same latency problem to
> other paths now needing to wait for the scrubbing to complete).
> 

I see, now it only avoids this case e.g don't scrub all 4Tb when a 4Tb
chunk found for a request of a single page.

Anyway I will move the scrubbing out of spinlock in next version.

>> @@ -876,6 +882,15 @@ static void free_heap_pages(
>>          midsize_alloc_zone_pages = max(
>>              midsize_alloc_zone_pages, total_avail_pages / 
>> MIDSIZE_ALLOC_FRAC);
>>  
>> +    if ( need_scrub )
>> +    {
>> +        if ( !tainted )
>> +        {
>> +            for ( i = 0; i < (1 << order); i++ )
>> +                pg[i].count_info |= PGC_need_scrub;
>> +        }
>> +    }
> 
> Two if()s like these should be folded into one.
> 

Will be fixed.

>> @@ -889,6 +904,17 @@ static void free_heap_pages(
>>                   (PFN_ORDER(pg-mask) != order) ||
>>                   (phys_to_nid(page_to_maddr(pg-mask)) != node) )
>>                  break;
>> +            /* If we need scrub, only merge with PGC_need_scrub pages */
>> +            if ( need_scrub )
>> +            {
>> +                if ( !test_bit(_PGC_need_scrub, &(pg-mask)->count_info) )
>> +                    break;
>> +            }
>> +            else
>> +            {
>> +                if ( test_bit(_PGC_need_scrub, &(pg-mask)->count_info) )
>> +                    break;
>> +            }
> 
> You're setting PGC_need_scrub on each 4k page anyway (which
> is debatable), hence there's no need to look at the passed in
> need_scrub flag here: Just check whether both chunks have the
> flag set the same. Same below.
> 

Right, thanks for your suggestion.

>> @@ -1535,7 +1571,7 @@ void free_xenheap_pages(void *v, unsigned int order)
>>  
>>      memguard_guard_range(v, 1 << (order + PAGE_SHIFT));
>>  
>> -    free_heap_pages(virt_to_page(v), order);
>> +    free_heap_pages(virt_to_page(v), order, 1);
> 
> Why?
> 

Sorry, a mistake here and will be fixed.

>> @@ -1588,11 +1624,10 @@ void free_xenheap_pages(void *v, unsigned int order)
>>  
>>      for ( i = 0; i < (1u << order); i++ )
>>      {
>> -        scrub_one_page(&pg[i]);
>>          pg[i].count_info &= ~PGC_xen_heap;
>>      }
>>  
>> -    free_heap_pages(pg, order);
>> +    free_heap_pages(pg, order, 1);
> 
> The flags needs to be 1 here, but I don't see why you also pass 1 in
> the other free_xenheap_pages() incarnation above.
> 
>> @@ -1745,24 +1780,20 @@ void free_domheap_pages(struct page_info *pg, 
>> unsigned int order)
>>           * domain has died we assume responsibility for erasure.
>>           */
>>          if ( unlikely(d->is_dying) )
>> -            for ( i = 0; i < (1 << order); i++ )
>> -                scrub_one_page(&pg[i]);
>> -
>> -        free_heap_pages(pg, order);
>> +            free_heap_pages(pg, order, 1);
>> +        else
>> +            free_heap_pages(pg, order, 0);
>>      }
>>      else if ( unlikely(d == dom_cow) )
>>      {
>>          ASSERT(order == 0); 
>> -        scrub_one_page(pg);
>> -        free_heap_pages(pg, 0);
>> +        free_heap_pages(pg, 0, 1);
>>          drop_dom_ref = 0;
>>      }
>>      else
>>      {
>>          /* Freeing anonymous domain-heap pages. */
>> -        for ( i = 0; i < (1 << order); i++ )
>> -            scrub_one_page(&pg[i]);
>> -        free_heap_pages(pg, order);
>> +        free_heap_pages(pg, order, 1);
>>          drop_dom_ref = 0;
>>      }
>>  
> 
> This hunk is patching no longer existing code (see commit daa4b800
> "slightly consolidate code in free_domheap_pages()").
> 

I'll rebase this patch to an newer version after that commit.
Thanks again.

-- 
Regards,
-Bob

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