Re: [PATCH v2 04/17] powerpc/msi: Improve IRQ bitmap allocator

Michael Ellerman
2014-10-01T07:13:34Z

On Tue, 2014-30-09 at 10:34:53 UTC, Michael Neuling wrote:
> From: Ian Munsie <imunsie@au1.ibm.com>
>
> Currently msi_bitmap_alloc_hwirqs() will round up any IRQ allocation requests
request
> to the nearest power of 2. eg. ask for 5 IRQs and you'll get 8. This wastes a
^ one space after a period, or die!

> lot of IRQs which can be a scarce resource.
>
> For cxl we can require multiple IRQs for every contexts that is attached to the
context
> accelerator. For AFU directed accelerators, there may be 1000s of contexts

What is an AFU directed accelerator?

> attached, hence we can easily run out of IRQs, especially if we are needlessly
> wasting them.
>
> This changes the msi_bitmap_alloc_hwirqs() to allocate only the required number
x
> of IRQs, hence avoiding this wastage.

The crucial detail you failed to mention is that you maintain the behaviour that
allocations are naturally aligned.

Can you add a check in the test code at the bottom of the file to confirm that
please?

> diff --git a/arch/powerpc/sysdev/msi_bitmap.c b/arch/powerpc/sysdev/msi_bitmap.c
> index 2ff6302..961a358 100644
> --- a/arch/powerpc/sysdev/msi_bitmap.c
> +++ b/arch/powerpc/sysdev/msi_bitmap.c
> @@ -20,32 +20,37 @@ int msi_bitmap_alloc_hwirqs(struct msi_bitmap *bmp, int num)
> int offset, order = get_count_order(num);
>
> spin_lock_irqsave(&bmp->lock, flags);
> - /*
> - * This is fast, but stricter than we need. We might want to add
> - * a fallback routine which does a linear search with no alignment.
> - */
> - offset = bitmap_find_free_region(bmp->bitmap, bmp->irq_count, order);
> +
> + offset = bitmap_find_next_zero_area(bmp->bitmap, bmp->irq_count, 0,
> + num, (1 << order) - 1);
> + if (offset > bmp->irq_count)
> + goto err;

Can we get a newline here :)

> + bitmap_set(bmp->bitmap, offset, num);
> spin_unlock_irqrestore(&bmp->lock, flags);
>
> pr_debug("msi_bitmap: allocated 0x%x (2^%d) at offset 0x%x\n",
> num, order, offset);

This print out is a bit confusing now, should probably just drop the order.

cheers