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Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/ data=journal

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On Mon, Oct 5, 2015 at 9:48 PM, Dave Hansen <dave.hansen@linux.intel.com> wrote:
>
> Although I was probably wrong about the source of the overhead, the
> point still remains that the prefaulting is eating cycles for no
> practical benefit.

Yeah, no, I'm not disagreeing with that part, I'm just more of a "at
this point in the rc series we are probably better off reverting".

Your ext4 patch may well fix the issue, and be the right thing to do
(_regardless_ of the revert, in fact - while it might make the revert
unnecessary, it might also be a good idea even if we do revert).

The subtlety of this just worries me, and the reason I'd still be
inclined to revert is simply "it's been that way a long time, the safe
thing is to go back and take this slow".

> With "-e cycles:pp":
>>        │      sub    $0x8,%rsp
>>  24.57 │      stac
>>  15.49 │      mov    (%rcx),%sil
>>  29.06 │      clac
>>   2.24 │      test   %eax,%eax
>>   8.77 │      mov    %sil,-0x1(%rbp)
>>   2.22 │    ↓ jne    66
>>        │      movslq %edx,%rdx

Ok, so it really is the stac/clac that is the bulk of the cost. Hmm.

You're right that the loop there will only be executed once for your
case, so moving the stac/clac outside probably doesn't help. It
*might* still make a difference just for microarchitectural reasons
(ie they may cause more trouble just because they are close to an
instruction that depends on them), but it's questionable.

It is a bit worrisome to see that those things are so expensive. Right
now almost all user accesses will cause *lots* of clac/stac stuff.

I originally asked Intel to do SMAP using a segment prefix, but that
was not to be..

              Linus
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[REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/  data=journal Theodore Ts'o <tytso@mit.edu> - 2015-10-05 17:30 +0200
  Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/ data=journal Linus Torvalds <torvalds@linux-foundation.org> - 2015-10-05 18:00 +0200
    Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/  data=journal Dave Hansen <dave.hansen@linux.intel.com> - 2015-10-05 18:30 +0200
      Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/ data=journal Linus Torvalds <torvalds@linux-foundation.org> - 2015-10-05 22:30 +0200
        Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/  data=journal Dave Hansen <dave.hansen@linux.intel.com> - 2015-10-05 22:50 +0200
          Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/ data=journal Linus Torvalds <torvalds@linux-foundation.org> - 2015-10-05 23:20 +0200
            Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/ data=journal Linus Torvalds <torvalds@linux-foundation.org> - 2015-10-06 00:00 +0200
              Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/  data=journal Dave Hansen <dave.hansen@linux.intel.com> - 2015-10-06 01:40 +0200
                Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/ data=journal Linus Torvalds <torvalds@linux-foundation.org> - 2015-10-06 11:10 +0200
        Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/ data=journal "H. Peter Anvin" <hpa@zytor.com> - 2015-10-05 22:50 +0200
          Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/  data=journal Ingo Molnar <mingo@kernel.org> - 2015-10-06 10:00 +0200
            Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/ data=journal Linus Torvalds <torvalds@linux-foundation.org> - 2015-10-06 11:20 +0200
              Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/  data=journal Ingo Molnar <mingo@kernel.org> - 2015-10-06 11:30 +0200
                Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/ data=journal Linus Torvalds <torvalds@linux-foundation.org> - 2015-10-06 15:30 +0200
                Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/  data=journal Ingo Molnar <mingo@kernel.org> - 2015-10-06 15:50 +0200
  Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/  data=journal Dave Hansen <dave.hansen@linux.intel.com> - 2015-10-05 18:10 +0200
  Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/  data=journal Dave Hansen <dave.hansen@intel.com> - 2015-10-05 20:10 +0200
    Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/  data=journal Theodore Ts'o <tytso@mit.edu> - 2015-10-07 05:40 +0200
      Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/ data=journal Linus Torvalds <torvalds@linux-foundation.org> - 2015-10-07 09:40 +0200
        Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/  data=journal Theodore Ts'o <tytso@mit.edu> - 2015-10-07 17:50 +0200
          [PATCH] ext4: use private version of page_zero_new_buffers() for data=journal mode Theodore Ts'o <tytso@mit.edu> - 2015-10-09 06:10 +0200
            Re: [PATCH] ext4: use private version of page_zero_new_buffers()  for data=journal mode "Leonid V. Fedorenchik" <Leonid.Fedorenchik@paragon-software.com> - 2015-10-13 08:40 +0200
            Re: [PATCH] ext4: use private version of page_zero_new_buffers() for  data=journal mode Jan Kara <jack@suse.cz> - 2015-10-15 13:20 +0200

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