Re: [Xen-devel] [PATCH 1 of 2] Global virq for low memory situations

["Jan Beulich" <JBeulich@xxxxxxxx>]

 >>> On 28.02.12 at 22:56, Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx> wrote:
> @@ -300,6 +301,91 @@ static unsigned long init_node_heap(int 
>      return needed;
>  }
>  
> +/* Default to 64 MiB */
> +#define DEFAULT_LOW_MEM_VIRQ    (((paddr_t) 64)   << 20)
> +#define MAX_LOW_MEM_VIRQ        (((paddr_t) 1024) << 20)
> +
> +static paddr_t __read_mostly opt_low_mem_virq = DEFAULT_LOW_MEM_VIRQ;
> +size_param("low_mem_virq_limit", opt_low_mem_virq);
> +
> +/* Thresholds to control hysteresis. In pages */
> +/* When memory grows above this threshold, reset hysteresis.
> + * -1 initially to not reset until at least one virq issued. */
> +static unsigned long low_mem_virq_high      = -1UL;
> +/* Threshold at which we issue virq */
> +static unsigned long low_mem_virq_th        = 0;
> +/* Original threshold after all checks completed */
> +static unsigned long low_mem_virq_orig      = 0;
> +/* Order for current threshold */
> +static unsigned int  low_mem_virq_th_order  = 0;
> +
> +/* Perform bootstrapping checks and set bounds */
> +static void __init setup_low_mem_virq(void)
> +{
> +    unsigned int order;
> +    paddr_t threshold;
> +
> +    /* Dom0 has already been allocated by now. So check we won't 
> +     * be complaining immediately with whatever's left of the heap. */
> +    threshold = min(opt_low_mem_virq, 
> +                    ((paddr_t) total_avail_pages) << PAGE_SHIFT);
> +
> +    /* Then, cap to some predefined maximum */
> +    threshold = min(threshold, MAX_LOW_MEM_VIRQ);
> +
> +    /* If the user specified no knob, and we are at the current available 
> +     * level, halve the threshold. */
> +    if ( (opt_low_mem_virq == DEFAULT_LOW_MEM_VIRQ)

This condition being true is not an indication that there was nothing
specified on the command line.

> +            && (threshold == (((paddr_t) total_avail_pages) << PAGE_SHIFT)) )
> +        threshold >>= 1;
> +
> +    /* Threshold bytes -> pages */
> +    low_mem_virq_th = threshold >> PAGE_SHIFT;
> +
> +    /* Next, round the threshold down to the next order */
> +    order = get_order_from_pages(low_mem_virq_th);

This has undefined (and unexpected by you) behavior if
low_mem_virq_th is zero (which I would think ought to mean "disable
this whole logic").

> +    if ( (1UL << order) > low_mem_virq_th ) 
> +        order--;
> +
> +    /* Set bounds, ready to go */
> +    low_mem_virq_th = low_mem_virq_orig = 1UL << order;
> +    low_mem_virq_th_order = order;
> +
> +    printk("Initial low memory virq threshold set at 0x%lx pages.\n",
> +            low_mem_virq_th);
> +}
> +
> +static void check_low_mem_virq(void)
> +{
> +    if ( total_avail_pages <= low_mem_virq_th )
> +    {
> +        send_global_virq(VIRQ_ENOMEM);
> +
> +        /* Update thresholds. Next warning will be when we drop below
> +         * next order. However, we wait until we grow beyond one
> +         * order above us to complain again at the current order */
> +        low_mem_virq_high   = 1UL << (low_mem_virq_th_order + 1);
> +        if ( low_mem_virq_th_order > 0 )
> +            low_mem_virq_th_order--;
> +        low_mem_virq_th     = 1UL << low_mem_virq_th_order;
> +        return;
> +    }
> +
> +    if ( total_avail_pages >= low_mem_virq_high )
> +    {
> +        /* Reset hysteresis. Bring threshold up one order.
> +         * If we are back where originally set, set high
> +         * threshold to -1 to avoid further growth of 
> +         * virq threshold. */
> +        low_mem_virq_th_order++;
> +        low_mem_virq_th = 1UL << low_mem_virq_th_order;
> +        if ( low_mem_virq_th == low_mem_virq_orig )
> +            low_mem_virq_high = -1UL;
> +        else
> +            low_mem_virq_high = 1UL << (low_mem_virq_th_order + 2);
> +    }
> +}
> +
>  /* Allocate 2^@order contiguous pages. */
>  static struct page_info *alloc_heap_pages(
>      unsigned int zone_lo, unsigned int zone_hi,

Also, could you please check your patches for not introducing trailing
whitespace?

Jan

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