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SubjectRe: [RFC 1/3] lib: copy_{from,to}_user using gup & kmap_atomic()

On Sat, Jun 13, 2020 at 10:45:33PM +0200, Arnd Bergmann wrote:

> 4% boot time increase sounds like a lot, especially if that is only for
> copy_from_user/copy_to_user. In the end it really depends on how well
> get_user()/put_user() and small copies can be optimized in the end.

i mentioned the worst case(happened only once), normally it was in
the range 2-3%

> From the numbers you
> measured, it seems the beaglebone currently needs an extra ~6µs or
> 3µs per copy_to/from_user() call with your patch, depending on what
> your benchmark was (MB/s for just reading or writing vs MB/s for
> copying from one file to another through a user space buffer).

It is MB/s for copying one file to another via user space buffer, i.e.
the value coreutils 'dd' shows w/ status=progress (here it was busybox
'dd', so instead it was enabling a compile time option)

> but if you want to test what the overhead is, you could try changing
> /dev/zero (or a different chardev like it) to use a series of
> put_user(0, u32uptr++) in place of whatever it has, and then replace the
> 'str' instruction with dummy writes to ttbr0 using the value it already
> has, like:
> mcr p15, 0, %0, c2, c0, 0 /* set_ttbr0() */
> isb /* prevent speculative access to kernel table */
> str %1, [%2],0 /* write 32 bit to user space */
> mcr p15, 0, %0, c2, c0, 0 /* set_ttbr0() */
> isb /* prevent speculative access to user table */

> It would be interesting to compare it to the overhead of a
> get_user_page_fast() based implementation.

i have to relocate & be on quarantine couple of weeks, so i will
temporarily stop here, otherwise might end up in roadside.

Reading feedbacks from everyone, some of it i could grasp only bits &
pieces, familiarizing more w/ mm & vfs would help me add value better
to the goal/discussion. Linus Walleij, if you wish to explore things,
feel free, right now don't know how my connectivity would be for next
3 weeks.


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