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

Hi,

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.

Regards
afzal

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