Re: [Xen-devel] [PATCH] Xen: Spread boot time page scrubbing across all available CPU's (v5)

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

On Thu, Jun 05, 2014 at 12:22:41PM +0100, Jan Beulich wrote:
> >>> On 04.06.14 at 15:29, <konrad.wilk@xxxxxxxxxx> wrote:
> > --- a/docs/misc/xen-command-line.markdown
> > +++ b/docs/misc/xen-command-line.markdown
> > @@ -198,6 +198,16 @@ Scrub free RAM during boot.  This is a safety feature 
> > to prevent
> >  accidentally leaking sensitive VM data into other VMs if Xen crashes
> >  and reboots.
> >  
> > +### bootscrub_chunk_
> 
> Looking at other examples in that file, underscores appear to need
> backslash escaping here. And I don't think the trailing one should
> be there.
> 
> > +> `= <size>`
> > +
> > +> Default: `128MB`
> > +
> > +Maximum RAM block size chunks 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
> 
> Double "cause".
> 
> > +static void __init smp_scrub_heap_pages(void *data)
> > +{
> > +    unsigned long mfn, start, end;
> > +    struct page_info *pg;
> > +    struct scrub_region *r;
> > +    unsigned int temp_cpu, node, cpu_idx = 0;
> > +    unsigned int cpu = smp_processor_id();
> > +
> > +    if ( data )
> > +        r = data;
> > +    else
> > +    {
> > +        node = cpu_to_node(cpu);
> > +        if ( node == NUMA_NO_NODE )
> > +            return;
> > +        r = &region[node];
> > +    }
> > +
> > +    /* Determine the current CPU's index into CPU's linked to this node. */
> > +    for_each_cpu ( temp_cpu, &r->cpus )
> > +    {
> > +        if ( cpu == temp_cpu )
> > +            break;
> > +        cpu_idx++;
> > +    }
> > +
> > +    /* Calculate the starting mfn for this CPU's memory block. */
> > +    start = r->start + (r->per_cpu_sz * cpu_idx) + r->offset;
> > +
> > +    /* Calculate the end mfn into this CPU's memory block for this 
> > iteration. */
> > +    if ( r->offset + chunk_size >= r->per_cpu_sz )
> > +    {
> > +        end = r->start + (r->per_cpu_sz * cpu_idx) + r->per_cpu_sz;
> > +
> > +        if ( r->rem && ((cpumask_weight(&r->cpus) - 1 == cpu_idx )) )
> > +            end += r->rem;
> > +    }
> > +    else
> > +        end = start + chunk_size;
> > +
> > +    for ( mfn = start; mfn < end; mfn++ )
> > +    {
> > +        pg = mfn_to_page(mfn);
> > +
> > +        /* Check the mfn is valid and page is free. */
> > +        if ( !mfn_valid(mfn) || !page_state_is(pg, free) )
> > +            continue;
> > +
> > +        scrub_one_page(pg);
> > +    }
> > +}
> > +
> > +static int __init find_non_smt(unsigned int node, cpumask_t *dest)
> > +{
> > +    cpumask_t node_cpus;
> > +    unsigned int i, cpu;
> > +
> > +    cpumask_and(&node_cpus, &node_to_cpumask(node), &cpu_online_map);
> > +    cpumask_clear(dest);
> > +    for_each_cpu ( i, &node_cpus )
> > +    {
> > +        if ( cpumask_intersects(dest, per_cpu(cpu_sibling_mask, i)) )
> > +            continue;
> > +        cpu = cpumask_first(per_cpu(cpu_sibling_mask, i));
> > +        cpumask_set_cpu(cpu, dest);
> > +    }
> > +    return cpumask_weight(dest);
> > +}
> > +
> >  /*
> > - * 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.
> > + * 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)
> >  {
> > -    unsigned long mfn;
> > -    struct page_info *pg;
> > +    cpumask_t node_cpus, all_worker_cpus;
> > +    unsigned int i, j;
> > +    unsigned long offset, max_per_cpu_sz = 0;
> > +    unsigned long start, end;
> > +    unsigned long rem = 0;
> > +    int last_distance, best_node;
> >  
> >      if ( !opt_bootscrub )
> >          return;
> >  
> > -    printk("Scrubbing Free RAM: ");
> > +    cpumask_clear(&all_worker_cpus);
> > +    /* Scrub block size. */
> > +    chunk_size = opt_bootscrub_chunk >> PAGE_SHIFT;
> > +    if ( chunk_size == 0 )
> > +        chunk_size = MB(128);
> > +
> > +    /* Round #0 - figure out amounts and which CPUs to use. */
> > +    for_each_online_node ( i )
> > +    {
> > +        if ( !node_spanned_pages(i) )
> > +            continue;
> > +        /* Calculate Node memory start and end address. */
> > +        start = max(node_start_pfn(i), first_valid_mfn);
> 
> This implies you're aware that possibly node_start_pfn(i) <
> first_valid_mfn.
> 
> > +        end = min(node_start_pfn(i) + node_spanned_pages(i), max_page);
> 
> Which in turn means that this may yield end < start. Is all the rest
> of the code prepared to deal with this? At least the divide and
> modulo operations on the difference further down doesn't look like
> so.

It will loop forever. I think adding

        end = max(end, start);

Would fix it.
> 
> > +        /* CPUs that are online and on this node (if none, that it is OK). 
> > */
> > +        find_non_smt(i, &node_cpus);
> 
> Here you could latch the weight, avoiding ... 
> 
> > +        cpumask_or(&all_worker_cpus, &all_worker_cpus, &node_cpus);
> > +        if ( cpumask_empty(&node_cpus) )
> 
> ... the extra operation on the mask here and the explicit use of
> cpumask_weight() on node_cpus in the else path.

<nods>
> 
> > +        {
> > +            /* No CPUs on this node. Round #2 will take of it. */
> > +            rem = 0;
> > +            region[i].per_cpu_sz = (end - start);
> > +        } else {
> 
> Coding style - this takes 3 lines.
> 
> > +            rem = (end - start) % cpumask_weight(&node_cpus);
> > +            region[i].per_cpu_sz = (end - start) / 
> > cpumask_weight(&node_cpus);
> > +            if ( region[i].per_cpu_sz > max_per_cpu_sz )
> > +                max_per_cpu_sz = region[i].per_cpu_sz;
> > +        }
> > +        region[i].start = start;
> > +        region[i].rem = rem;
> > +        cpumask_copy(&region[i].cpus, &node_cpus);
> > +
> > +    }
> > +
> > +    printk("Scrubbing Free RAM on %u nodes using %u CPUs\n", 
> > num_online_nodes(),
> > +           cpumask_weight(&all_worker_cpus));
> >  
> > -    for ( mfn = first_valid_mfn; mfn < max_page; mfn++ )
> > +    /* Round: #1 - do NUMA nodes with CPUs. */
> > +    for ( offset = 0; offset < max_per_cpu_sz; offset += chunk_size )
> >      {
> > +        for_each_online_node ( i )
> > +            region[i].offset = offset;
> > +
> >          process_pending_softirqs();
> >  
> > -        pg = mfn_to_page(mfn);
> > +        spin_lock(&heap_lock);
> > +        on_selected_cpus(&all_worker_cpus, smp_scrub_heap_pages, NULL, 1);
> > +        spin_unlock(&heap_lock);
> >  
> > -        /* Quick lock-free check. */
> > -        if ( !mfn_valid(mfn) || !page_state_is(pg, free) )
> > +        printk(".");
> > +    }
> > +
> > +    /*
> > +     * Round #2: NUMA nodes with no CPUs get scrubbed with CPUs on the node
> > +     * closest to us and with CPUs.
> > +     */
> > +    for_each_online_node ( i )
> > +    {
> > +        node_cpus = node_to_cpumask(i);
> > +
> > +        if ( !cpumask_empty(&node_cpus) )
> >              continue;
> >  
> > -        /* Every 100MB, print a progress dot. */
> > -        if ( (mfn % ((100*1024*1024)/PAGE_SIZE)) == 0 )
> > -            printk(".");
> > +        last_distance = INT_MAX;
> > +        best_node = first_node(node_online_map);
> > +        /* Figure out which NODE CPUs are close. */
> > +        for_each_online_node ( j )
> > +        {
> > +            int distance;
> >  
> > -        spin_lock(&heap_lock);
> > +            if ( i == j )
> > +                continue;
> 
> This could be replaced with cpumask_empty(&node_to_cpumask(j)),
> allowing ...
> 
> > +            distance = __node_distance(i, j);
> > +            if ( distance < last_distance )
> > +            {
> > +                if ( cpumask_empty(&node_to_cpumask(j)) )
> > +                    continue;
> 
> this check to be dropped.

Clever! Will do.
> 
> Jan

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