Re: [PATCH -tip] x86/locking/atomic: Use asm_inline for atomic locking insns

Uros Bizjak
2025-03-08T07:22:43Z

On Thu, Mar 6, 2025 at 11:19 PM Ingo Molnar <mingo@kernel.org> wrote:
>
>
> * Uros Bizjak <ubizjak@gmail.com> wrote:
>
> > On Thu, Mar 6, 2025 at 11:45 AM Uros Bizjak <ubizjak@gmail.com> wrote:
> > >
> > > On Wed, Mar 5, 2025 at 9:14 PM David Laight
> > > <david.laight.linux@gmail.com> wrote:
> > > >
> > > > On Wed, 5 Mar 2025 07:04:08 -1000
> > > > Linus Torvalds <torvalds@linuxfoundation.org> wrote:
> > > >
> > > > > On Tue, 4 Mar 2025 at 22:54, Uros Bizjak <ubizjak@gmail.com> wrote:
> > > > > >
> > > > > > Even to my surprise, the patch has some noticeable effects on the
> > > > > > performance, please see the attachment in [1] for LMBench data or [2]
> > > > > > for some excerpts from the data. So, I think the patch has potential
> > > > > > to improve the performance.
> > > > >
> > > > > I suspect some of the performance difference - which looks
> > > > > unexpectedly large - is due to having run them on a CPU with the
> > > > > horrendous indirect return costs, and then inlining can make a huge
> > > > > difference.
> > > > ...
> > > >
> > > > Another possibility is that the processes are getting bounced around
> > > > cpu in a slightly different way.
> > > > An idle cpu might be running at 800MHz, run something that spins on it
> > > > and the clock speed will soon jump to 4GHz.
> > > > But if your 'spinning' process is migrated to a different cpu it starts
> > > > again at 800MHz.
> > > >
> > > > (I had something where a fpga compile when from 12 mins to over 20 because
> > > > the kernel RSB stuffing caused the scheduler to behave differently even
> > > > though nothing was doing a lot of system calls.)
> > > >
> > > > All sorts of things can affect that - possibly even making some code faster!
> > > >
> > > > The (IIRC) 30k increase in code size will be a few functions being inlined.
> > > > The bloat-o-meter might show which, and forcing a few inlines the same way
> > > > should reduce that difference.
> > >
> > > bloat-o-meter is an excellent idea, I'll analyse binaries some more
> > > and report my findings.
> >
> > Please find attached bloat.txt where:
> >
> > a) some functions now include once-called functions. These are:
> >
> > copy_process 6465 10191 +3726
> > balance_dirty_pages_ratelimited_flags 237 2949 +2712
> > icl_plane_update_noarm 5800 7969 +2169
> > samsung_input_mapping 3375 5170 +1795
> > ext4_do_update_inode.isra - 1526 +1526
> >
> > that now include:
> >
> > ext4_mark_iloc_dirty 1735 106 -1629
> > samsung_gamepad_input_mapping.isra 2046 - -2046
> > icl_program_input_csc 2203 - -2203
> > copy_mm 2242 - -2242
> > balance_dirty_pages 2657 - -2657
> >
> > b) ISRA [interprocedural scalar replacement of aggregates,
> > interprocedural pass that removes unused function return values
> > (turning functions returning a value which is never used into void
> > functions) and removes unused function parameters. It can also
> > replace an aggregate parameter by a set of other parameters
> > representing part of the original, turning those passed by reference
> > into new ones which pass the value directly.]
> >
> > ext4_do_update_inode.isra - 1526 +1526
> > nfs4_begin_drain_session.isra - 249 +249
> > nfs4_end_drain_session.isra - 168 +168
> > __guc_action_register_multi_lrc_v70.isra 335 500 +165
> > __i915_gem_free_objects.isra - 144 +144
> > ...
> > membarrier_register_private_expedited.isra 108 - -108
> > syncobj_eventfd_entry_func.isra 445 314 -131
> > __ext4_sb_bread_gfp.isra 140 - -140
> > class_preempt_notrace_destructor.isra 145 - -145
> > p9_fid_put.isra 151 - -151
> > __mm_cid_try_get.isra 238 - -238
> > membarrier_global_expedited.isra 294 - -294
> > mm_cid_get.isra 295 - -295
> > samsung_gamepad_input_mapping.isra.cold 604 - -604
> > samsung_gamepad_input_mapping.isra 2046 - -2046
> >
> > c) different split points of hot/cold split that just move code around:
> >
> > samsung_input_mapping.cold 900 1500 +600
> > __i915_request_reset.cold 311 389 +78
> > nfs_update_inode.cold 77 153 +76
> > __do_sys_swapon.cold 404 455 +51
> > copy_process.cold - 45 +45
> > tg3_get_invariants.cold 73 115 +42
> > ...
> > hibernate.cold 671 643 -28
> > copy_mm.cold 31 - -31
> > software_resume.cold 249 207 -42
> > io_poll_wake.cold 106 54 -52
> > samsung_gamepad_input_mapping.isra.cold 604 - -604
> >
> > c) full inline of small functions with locking insn (~150 cases).
> > These bring in most of the performance increase because there is no
> > call setup. E.g.:
> >
> > 0000000000a50e10 <release_devnum>:
> > a50e10: 48 63 07 movslq (%rdi),%rax
> > a50e13: 85 c0 test %eax,%eax
> > a50e15: 7e 10 jle a50e27 <release_devnum+0x17>
> > a50e17: 48 8b 4f 50 mov 0x50(%rdi),%rcx
> > a50e1b: f0 48 0f b3 41 50 lock btr %rax,0x50(%rcx)
> > a50e21: c7 07 ff ff ff ff movl $0xffffffff,(%rdi)
> > a50e27: e9 00 00 00 00 jmp a50e2c <release_devnum+0x1c>
> > a50e28: R_X86_64_PLT32 __x86_return_thunk-0x4
> > a50e2c: 0f 1f 40 00 nopl 0x0(%rax)
> >
> > IMO, for 0.14% code increase, these changes are desirable.
>
> I concur, and it's extra desirable IMHO due to the per function
> overhead of CPU bug mitigations like retpolines.

Thanks!

I will submit a v2 patch next week that will summarize all the
measurements in its commit message.

BR,
Uros.