Re: [PATCH] staging: gasket: Convert get_user_pages*() --> pin_user_pages*()

[John Hubbard <jhubbard@nvidia.com>]

On 2020-05-29 04:53, Dan Carpenter wrote:
...
> What are the runtime implications of this patch?  I'm still not clear on
> that honestly.

Instead of incrementing each page's refcount by 1 (with get_user_pages()),
pin_user_pages*() will increment by GUP_PIN_COUNTING_BIAS, which is 1024.
That by itself should not have any performance impact, of course, but
there's a couple more things:

For compound pages of more than 2 page size, it will also increment
a separate struct page's field, via hpage_pincount_add().

And finally, it will update /proc/vmstat counters on pin and unpin, via
the optimized mod_node_page_state() call.

So it's expected to be very light. And, for DMA (as opposed to DIO)
situations, the DMA setup time is inevitably much greater than any of
the above overheads, so I expect that this patch will be completely
invisible from a performance point of view.

It would be a "nice to have", though, if anyone were able to do a
performance comparison on the gasket driver for this patch, and/or
basic runtime verification, since I'm sure it's a specialized setup.


thanks,
-- 
John Hubbard
NVIDIA