Re: [Xen-devel] [PATCH v2] Xen: Spread boot time page scrubbing across all available CPU's

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Mon, Sep 30, 2013 at 01:35:17PM +0100, Malcolm Crossley wrote:
> The page scrubbing is done in 128MB chunks in lockstep across all the CPU's.
> This allows for the boot CPU to hold the heap_lock whilst each chunk is being
> scrubbed and then release the heap_lock when all CPU's are finished scrubing
> their individual chunk. This allows for the heap_lock to not be held
> continously and for pending softirqs are to be serviced periodically across
> all CPU's.
> 
> The page scrub memory chunks are allocated to the CPU's in a NUMA aware
> fashion to reduce Socket interconnect overhead and improve performance.
> 
> This patch reduces the boot page scrub time on a 128GB 64 core AMD Opteron
> 6386 machine from 49 seconds to 3 seconds.

A bit older version of this one cut down the 1TB machine scrubbing from minutes
(I think it was 5 or 10 - I gave up on counting) down to less than a minute.

> 
> Changes in v2
>  - Reduced default chunk size to 128MB
>  - Added code to scrub NUMA nodes with no active CPU linked to them
>  - Be robust to boot CPU not being linked to a NUMA node
> 
> diff -r a03cc3136759 -r ee1108d26fc5 docs/misc/xen-command-line.markdown
> --- a/docs/misc/xen-command-line.markdown
> +++ b/docs/misc/xen-command-line.markdown
> @@ -188,6 +188,16 @@ Scrub free RAM during boot.  This is a s
>  accidentally leaking sensitive VM data into other VMs if Xen crashes
>  and reboots.
>  
> +### bootscrub_blocksize
> +> `= <size>`
> +
> +> Default: `128MiB`
> +
> +Maximum RAM block size to be scrubbed whilst holding the page heap lock and 
> not
> +running softirqs. Reduce this if softirqs are not being run frequently 
> enough.
> +Setting this to a high value may cause cause boot failure, particularly if 
> the
> +NMI watchdog is also enabled.
> +
>  ### cachesize
>  > `= <size>`
>  
> diff -r a03cc3136759 -r ee1108d26fc5 xen/common/page_alloc.c
> --- a/xen/common/page_alloc.c
> +++ b/xen/common/page_alloc.c
> @@ -65,6 +65,12 @@ static bool_t opt_bootscrub __initdata =
>  boolean_param("bootscrub", opt_bootscrub);
>  
>  /*
> + * bootscrub_blocksize -> Size (bytes) of mem block to scrub with heaplock 
> held
> + */
> +static unsigned int __initdata opt_bootscrub_blocksize = 128 * 1024 * 1024;
> +size_param("bootscrub_blocksize", opt_bootscrub_blocksize);
> +
> +/*
>   * Bit width of the DMA heap -- used to override NUMA-node-first.
>   * allocation strategy, which can otherwise exhaust low memory.
>   */
> @@ -90,6 +96,16 @@ static struct bootmem_region {
>  } *__initdata bootmem_region_list;
>  static unsigned int __initdata nr_bootmem_regions;
>  
> +static atomic_t __initdata bootscrub_count = ATOMIC_INIT(0);
> +
> +struct scrub_region {
> +    u64 offset;
> +    u64 start;
> +    u64 chunk_size;
> +    u64 cpu_block_size;
> +};
> +static struct scrub_region __initdata region[MAX_NUMNODES];
> +
>  static void __init boot_bug(int line)
>  {
>      panic("Boot BUG at %s:%d\n", __FILE__, line);
> @@ -1254,28 +1270,44 @@ void __init end_boot_allocator(void)
>      printk("\n");
>  }
>  
> -/*
> - * Scrub all unallocated pages in all heap zones. This function is more
> - * convoluted than appears necessary because we do not want to continuously
> - * hold the lock while scrubbing very large memory areas.
> - */
> -void __init scrub_heap_pages(void)
> +void __init smp_scrub_heap_pages(void *data)
>  {
> -    unsigned long mfn;
> +    unsigned long mfn, start_mfn, end_mfn;
>      struct page_info *pg;
> +    struct scrub_region *region = data;
> +    unsigned int temp_cpu, local_node, local_cpu_index = 0;
> +    unsigned int cpu = smp_processor_id();
>  
> -    if ( !opt_bootscrub )
> -        return;
> +    ASSERT(region != NULL);
>  
> -    printk("Scrubbing Free RAM: ");
> +    local_node = cpu_to_node(cpu);
> +    /* Determine if we are scrubbing using the boot CPU */
> +    if ( region->cpu_block_size != ~0ULL )
> +        /* Determine the current CPU's index into CPU's linked to this node*/
> +        for_each_cpu( temp_cpu, &node_to_cpumask(local_node) )
> +        {
> +            if ( cpu == temp_cpu )
> +                break;
> +            local_cpu_index++;
> +        }
>  
> -    for ( mfn = first_valid_mfn; mfn < max_page; mfn++ )
> +    /* Calculate the starting mfn for this CPU's memory block */
> +    start_mfn = region->start + (region->cpu_block_size * local_cpu_index)
> +                + region->offset;
> +
> +    /* Calculate the end mfn into this CPU's memory block for this iteration 
> */
> +    if ( region->offset + region->chunk_size > region->cpu_block_size )
> +        end_mfn = region->start + (region->cpu_block_size * local_cpu_index)
> +                  + region->cpu_block_size;
> +    else
> +        end_mfn = start_mfn + region->chunk_size;
> +
> +
> +    for ( mfn = start_mfn; mfn < end_mfn; mfn++ )
>      {
> -        process_pending_softirqs();
> -
>          pg = mfn_to_page(mfn);
>  
> -        /* Quick lock-free check. */
> +        /* Check the mfn is valid and page is free. */
>          if ( !mfn_valid(mfn) || !page_state_is(pg, free) )
>              continue;
>  
> @@ -1283,15 +1315,124 @@ void __init scrub_heap_pages(void)
>          if ( (mfn % ((100*1024*1024)/PAGE_SIZE)) == 0 )
>              printk(".");
>  
> +        /* Do the scrub if possible */
> +        if ( page_state_is(pg, free) )
> +            scrub_one_page(pg);
> +    }
> +    /* Increment count to indicate scrubbing complete on this CPU */
> +    atomic_dec(&bootscrub_count);
> +}
> +
> +/*
> + * Scrub all unallocated pages in all heap zones. This function uses all
> + * online cpu's to scrub the memory in parallel.
> + */
> +void __init scrub_heap_pages(void)
> +{
> +    cpumask_t node_cpus, total_node_cpus_mask = {{ 0 }};
> +    unsigned int i, boot_cpu_node, total_node_cpus, cpu = smp_processor_id();
> +    unsigned long mfn, mfn_off, chunk_size, max_cpu_blk_size = 0;
> +    unsigned long mem_start, mem_end;
> +
> +    if ( !opt_bootscrub )
> +        return;
> +
> +    boot_cpu_node = cpu_to_node(cpu);
> +
> +    printk("Scrubbing Free RAM: ");
> +
> +    /* Scrub block size */
> +    chunk_size = opt_bootscrub_blocksize >> PAGE_SHIFT;
> +    if ( chunk_size == 0 )
> +        chunk_size = 1;
> +
> +    /* Determine the amount of memory to scrub, per CPU on each Node */
> +    for_each_online_node ( i )
> +    {
> +        /* Calculate Node memory start and end address */
> +        mem_start = max(node_start_pfn(i), first_valid_mfn);
> +        mem_end = min(mem_start + node_spanned_pages(i), max_page);
> +        /* Divide by number of CPU's for this node */
> +        node_cpus = node_to_cpumask(i);
> +        /* It's possible a node has no CPU's */
> +        if ( cpumask_empty(&node_cpus) )
> +            continue;
> +        cpumask_or(&total_node_cpus_mask, &total_node_cpus_mask, &node_cpus);
> +
> +        region[i].cpu_block_size = (mem_end - mem_start) /
> +                                    cpumask_weight(&node_cpus);
> +        region[i].start = mem_start;
> +
> +        if ( region[i].cpu_block_size > max_cpu_blk_size )
> +            max_cpu_blk_size = region[i].cpu_block_size;
> +    }
> +
> +    /* Round default chunk size down if required */
> +    if ( max_cpu_blk_size && chunk_size > max_cpu_blk_size )
> +        chunk_size = max_cpu_blk_size;
> +
> +    total_node_cpus = cpumask_weight(&total_node_cpus_mask);
> +    /* Start all CPU's scrubbing memory, chunk_size at a time */
> +    for ( mfn_off = 0; mfn_off < max_cpu_blk_size; mfn_off += chunk_size )
> +    {
> +        process_pending_softirqs();
> +
> +        atomic_set(&bootscrub_count, total_node_cpus);
> +
>          spin_lock(&heap_lock);
>  
> -        /* Re-check page status with lock held. */
> -        if ( page_state_is(pg, free) )
> -            scrub_one_page(pg);
> +        /* Start all other CPU's on all nodes */
> +        for_each_online_node ( i )
> +        {
> +            region[i].chunk_size = chunk_size;
> +            region[i].offset = mfn_off;
> +            node_cpus = node_to_cpumask(i);
> +            /* Clear local cpu ID */
> +            cpumask_clear_cpu(cpu, &node_cpus);
> +            /* Start page scrubbing on all other CPU's */
> +            on_selected_cpus(&node_cpus, smp_scrub_heap_pages, &region[i], 
> 0);
> +        }
> +
> +        /* Start scrub on local CPU if CPU linked to a memory node */
> +        if ( boot_cpu_node != NUMA_NO_NODE )
> +            smp_scrub_heap_pages(&region[boot_cpu_node]);
> +
> +        /* Wait for page scrubbing to complete on all other CPU's */
> +        while ( atomic_read(&bootscrub_count) > 0 )
> +            cpu_relax();
>  
>          spin_unlock(&heap_lock);
>      }
>  
> +    /* Use the boot CPU to scrub any nodes which have no CPU's linked to 
> them */
> +    for_each_online_node ( i )
> +    {
> +        node_cpus = node_to_cpumask(i);
> +
> +        if ( !cpumask_empty(&node_cpus) )
> +            continue;
> +
> +        mem_start = max(node_start_pfn(i), first_valid_mfn);
> +        mem_end = min(mem_start + node_spanned_pages(i), max_page);
> +
> +        region[0].offset = 0;
> +        region[0].cpu_block_size = ~0ULL;
> +
> +        for ( mfn = mem_start; mfn < mem_end; mfn += chunk_size )
> +        {
> +            spin_lock(&heap_lock);
> +            if ( mfn + chunk_size > mem_end )
> +                region[0].chunk_size = mem_end - mfn;
> +            else
> +                region[0].chunk_size = chunk_size;
> +
> +            region[0].start = mfn;
> +
> +            smp_scrub_heap_pages(&region[0]);
> +            spin_unlock(&heap_lock);
> +            process_pending_softirqs();
> +        }
> +    }
>      printk("done.\n");
>  
>      /* Now that the heap is initialized, run checks and set bounds

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