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

Ingo Molnar
2025-03-06T22:19:18Z

* 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.

The number of function calls executed in a workload can be measured via
perf on most modern x86 CPUs as well. For example on Zen5 CPUs the
number of RET instructions can be counted:

{
EventName: ex_ret_near_ret,
EventCode: 0xc8,
BriefDescription: Retired near returns (RET or RET Iw).
},

Which ought to be a good proxy for function calls (modulo
tail-optimized jumps).

Thanks,

Ingo