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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Thu, 13 May 2021 11:58:38 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Alexey Dobriyan <adobriyan@gmail.com>, mingo@redhat.com,
        Borislav Petkov <bp@alien8.de>, linux-kernel@vger.kernel.org,
        x86@kernel.org, Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 1/4] sched: make nr_running() return 32-bit
Message-ID: <YJz4TmZ7fmKFchRe@gmail.com>
References: <20210422200228.1423391-1-adobriyan@gmail.com>
 <87fsyr5wtj.ffs@nanos.tec.linutronix.de>
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* Thomas Gleixner <tglx@linutronix.de> wrote:

> Alexey,
> 
> On Thu, Apr 22 2021 at 23:02, Alexey Dobriyan wrote:
> > Creating 2**32 tasks is impossible due to futex pid limits and wasteful
> > anyway. Nobody has done it.
> >
> 
> this whole pile lacks useful numbers. What's the actual benefit of that
> churn?

I applied the 4 patch series from Alexey Dobriyan because they offer four 
distinct technical advantages to the scheduler code:

 - Shorter instructions generated in nr_running(), nr_iowait(), 
   nr_iowait_cpu() due to losing the REX prefix.

 - Shorter instructions generated by users of these 3 functions as well.

 - Tighter data packing and structure size reduction in 'struct rt_rq', 
   'struct dl_rq' and 'struct rq', due to 8-byte 'long' fields shrinking to 
   4-byte 'int' based fields.

 - Together these 4 patches clean up all derivative uses of the 
   rq::nr_running base type, which is already 'unsigned int' and that's 
   pretty fundamental given our PID ABI limits. Having type mismatch where 
   we use 64-bit data types for certain APIs while 32-bit data types for 
   others is inconsistent crap I wouldn't accept if it was submitted as new 
   code.

As to the numbers:

The data structure size improvements are IMO obvious, and they are also 
measurable, here's the before/after Pahole comparison of 'struct rt_rq':

--- pahole.struct.rt_rq.before	2021-05-13 11:40:53.207077908 +0200
+++ pahole.struct.rt_rq.after	2021-05-13 11:41:42.257385897 +0200
@@ -7,22 +7,22 @@ struct rt_rq {
 		int                curr;                 /*  1624     4 */
 		int                next;                 /*  1628     4 */
 	} highest_prio;                                  /*  1624     8 */
-	long unsigned int          rt_nr_migratory;      /*  1632     8 */
-	long unsigned int          rt_nr_total;          /*  1640     8 */
-	int                        overloaded;           /*  1648     4 */
+	unsigned int               rt_nr_migratory;      /*  1632     4 */
+	unsigned int               rt_nr_total;          /*  1636     4 */
+	int                        overloaded;           /*  1640     4 */
 
 	/* XXX 4 bytes hole, try to pack */
 
-	struct plist_head          pushable_tasks;       /*  1656    16 */
-	/* --- cacheline 26 boundary (1664 bytes) was 8 bytes ago --- */
-	int                        rt_queued;            /*  1672     4 */
-	int                        rt_throttled;         /*  1676     4 */
-	u64                        rt_time;              /*  1680     8 */
-	u64                        rt_runtime;           /*  1688     8 */
-	raw_spinlock_t             rt_runtime_lock;      /*  1696     4 */
+	struct plist_head          pushable_tasks;       /*  1648    16 */
+	/* --- cacheline 26 boundary (1664 bytes) --- */
+	int                        rt_queued;            /*  1664     4 */
+	int                        rt_throttled;         /*  1668     4 */
+	u64                        rt_time;              /*  1672     8 */
+	u64                        rt_runtime;           /*  1680     8 */
+	raw_spinlock_t             rt_runtime_lock;      /*  1688     4 */
 
-	/* size: 1704, cachelines: 27, members: 13 */
-	/* sum members: 1696, holes: 1, sum holes: 4 */
+	/* size: 1696, cachelines: 27, members: 13 */
+	/* sum members: 1688, holes: 1, sum holes: 4 */
 	/* padding: 4 */
-	/* last cacheline: 40 bytes */
+	/* last cacheline: 32 bytes */

'struct rt_rq' got shrunk from 1704 bytes to 1696 bytes, an 8 byte 
reduction.

'struct dl_rq' shrunk by 8 bytes:

-	/* size: 104, cachelines: 2, members: 10 */
-	/* sum members: 100, holes: 1, sum holes: 4 */
-	/* last cacheline: 40 bytes */
+	/* size: 96, cachelines: 2, members: 10 */
+	/* sum members: 92, holes: 1, sum holes: 4 */
+	/* last cacheline: 32 bytes */

'struct rq', which embedds both rt_rq and dl_rq, got 20 bytes smaller:

-	/* sum members: 2967, holes: 10, sum holes: 137 */
+	/* sum members: 2947, holes: 11, sum holes: 157 */

Side note: there's now 20 bytes more new padding holes which could possibly 
be coalesced a bit more by reordering members sensibly - resulting in even 
more data footprint savings. (But those should be separate changes and only 
for fields we truly care about from a performance POV.)

As to code generation improvements:

> Just with the default config for one of my reference machines:
> 
>    text		data	bss	dec	 hex	 filename
> 16679864	6627950	1671296	24979110 17d26a6 ../build/vmlinux-before
> 16679894	6627950	1671296	24979140 17d26c4 ../build/vmlinux-after
> ------------------------------------------------------------------------
>      +30

Using '/usr/bin/size' to compare before/after generated code is the wrong 
way to measure code generation improvements for smaller changes due to 
default alignment creating a reserve of 'padding' bytes at the end of most 
functions. You have to look at the low level generated assembly directly.

For example, the nr_iowait_cpu() commit, with before/after generated code 
of the callers of the function:

  9745516841a5: ("sched: Make nr_iowait() return 32-bit value")

  ffffffffxxxxxxxx:      e8 49 8e fb ff          call   ffffffffxxxxxxxx <nr_iowait_cpu>
- ffffffffxxxxxxxx:      48 85 c0                test   %rax,%rax

  ffffffffxxxxxxxx:      e8 64 8e fb ff          call   ffffffffxxxxxxxx <nr_iowait_cpu>
+ ffffffffxxxxxxxx:      85 c0                   test   %eax,%eax

Note how the 'test %rax,%rax' lost the 0x48 64-bit REX prefix and the 
generated code got one byte shorter from "48 85 c0" to "85 c0".

( Note, my asm generation scripts filter out some of the noise to make it 
  easier to diff generated asm, hence the ffffffffxxxxxxxx placeholder. )

The nr_iowait() function itself got shorter by two bytes as well, due to:

Before:

ffffffffxxxxxxxx <nr_iowait>:
ffffffffxxxxxxxx:       41 54                   push   %r12
ffffffffxxxxxxxx:       48 c7 c7 ff ff ff ff    mov    $0xffffffffxxxxxxxx,%rdi
ffffffffxxxxxxxx:       45 31 e4                xor    %r12d,%r12d
ffffffffxxxxxxxx:       55                      push   %rbp
ffffffffxxxxxxxx:       8b 2d 01 ea 5d 01       mov    0x15dea01(%rip),%ebp        # ffffffffxxxxxxxx <nr_cpu_ids>
ffffffffxxxxxxxx:       53                      push   %rbx
ffffffffxxxxxxxx:       48 c7 c3 c0 95 02 00    mov    $0x295c0,%rbx
ffffffffxxxxxxxx:       eb 17                   jmp    ffffffffxxxxxxxx <nr_iowait+0x34>
ffffffffxxxxxxxx:       48 98                   cltq   
ffffffffxxxxxxxx:       48 8b 14 c5 a0 c6 2e    mov    -0x7dd13960(,%rax,8),%rdx
ffffffffxxxxxxxx:       82 
ffffffffxxxxxxxx:       48 01 da                add    %rbx,%rdx
ffffffffxxxxxxxx:       48 63 82 98 09 00 00    movslq 0x998(%rdx),%rax
ffffffffxxxxxxxx:       49 01 c4                add    %rax,%r12
ffffffffxxxxxxxx:       48 c7 c6 18 72 67 82    mov    $0xffffffffxxxxxxxx,%rsi
ffffffffxxxxxxxx:       e8 80 46 39 00          call   ffffffffxxxxxxxx <cpumask_next>
ffffffffxxxxxxxx:       89 c7                   mov    %eax,%edi
ffffffffxxxxxxxx:       39 e8                   cmp    %ebp,%eax
ffffffffxxxxxxxx:       72 d7                   jb     ffffffffxxxxxxxx <nr_iowait+0x1d>
ffffffffxxxxxxxx:       4c 89 e0                mov    %r12,%rax
ffffffffxxxxxxxx:       5b                      pop    %rbx
ffffffffxxxxxxxx:       5d                      pop    %rbp
ffffffffxxxxxxxx:       41 5c                   pop    %r12
ffffffffxxxxxxxx:       c3                      ret    

After:

ffffffffxxxxxxxx <nr_iowait>:
ffffffffxxxxxxxx:       41 54                   push   %r12
ffffffffxxxxxxxx:       bf ff ff ff ff          mov    $0xffffffff,%edi
ffffffffxxxxxxxx:       45 31 e4                xor    %r12d,%r12d
ffffffffxxxxxxxx:       55                      push   %rbp
ffffffffxxxxxxxx:       8b 2d 03 ea 5d 01       mov    0x15dea03(%rip),%ebp        # ffffffffxxxxxxxx <nr_cpu_ids>
ffffffffxxxxxxxx:       53                      push   %rbx
ffffffffxxxxxxxx:       48 c7 c3 c0 95 02 00    mov    $0x295c0,%rbx
ffffffffxxxxxxxx:       eb 17                   jmp    ffffffffxxxxxxxx <nr_iowait+0x32>
ffffffffxxxxxxxx:       48 63 c7                movslq %edi,%rax
ffffffffxxxxxxxx:       48 8b 14 c5 a0 c6 2e    mov    -0x7dd13960(,%rax,8),%rdx
ffffffffxxxxxxxx:       82 
ffffffffxxxxxxxx:       48 01 da                add    %rbx,%rdx
ffffffffxxxxxxxx:       8b 82 98 09 00 00       mov    0x998(%rdx),%eax
ffffffffxxxxxxxx:       41 01 c4                add    %eax,%r12d
ffffffffxxxxxxxx:       48 c7 c6 18 72 67 82    mov    $0xffffffffxxxxxxxx,%rsi
ffffffffxxxxxxxx:       e8 a2 46 39 00          call   ffffffffxxxxxxxx <cpumask_next>
ffffffffxxxxxxxx:       89 c7                   mov    %eax,%edi
ffffffffxxxxxxxx:       39 c5                   cmp    %eax,%ebp
ffffffffxxxxxxxx:       77 d7                   ja     ffffffffxxxxxxxx <nr_iowait+0x1b>
ffffffffxxxxxxxx:       44 89 e0                mov    %r12d,%eax
ffffffffxxxxxxxx:       5b                      pop    %rbx
ffffffffxxxxxxxx:       5d                      pop    %rbp
ffffffffxxxxxxxx:       41 5c                   pop    %r12
ffffffffxxxxxxxx:       c3                      ret    

The size of nr_iowait() shrunk from 78 bytes to 76 bytes.

Or the other commit:

  01aee8fd7fb2: ("sched: Make nr_running() return 32-bit value")

  ffffffffxxxxxxxx:       e8 d9 24 e8 ff          call   ffffffffxxxxxxxx <nr_running>
  ffffffffxxxxxxxx:       4c 8b 05 cc 92 70 01    mov    0x17092cc(%rip),%r8        # ffffffffxxxxxxxx <total_forks>
  ffffffffxxxxxxxx:       4c 8b 64 24 50          mov    0x50(%rsp),%r12
- ffffffffxxxxxxxx:       48 89 44 24 08          mov    %rax,0x8(%rsp)
  ffffffffxxxxxxxx:       4c 89 04 24             mov    %r8,(%rsp)

  ffffffffxxxxxxxx:       e8 f9 24 e8 ff          call   ffffffffxxxxxxxx <nr_running>
  ffffffffxxxxxxxx:       4c 8b 05 ed 92 70 01    mov    0x17092ed(%rip),%r8        # ffffffffxxxxxxxx <total_forks>
  ffffffffxxxxxxxx:       4c 8b 64 24 50          mov    0x50(%rsp),%r12
+ ffffffffxxxxxxxx:       89 44 24 08             mov    %eax,0x8(%rsp)
  ffffffffxxxxxxxx:       4c 89 04 24             mov    %r8,(%rsp)

Note how "mov %rax,0x8(%rsp)" got shortened by one byte to "mov %eax,0x8(%rsp)":

- ffffffffxxxxxxxx:       48 89 44 24 08          mov    %rax,0x8(%rsp)
+ ffffffffxxxxxxxx:       89 44 24 08             mov    %eax,0x8(%rsp)

The nr_running() function itself got shorter by 2 bytes, due to shorter 
instruction sequences.

The third commit improved code generation too:

  8fc2858e572c: ("sched: Make nr_iowait_cpu() return 32-bit value")

  ffffffffxxxxxxxx <nr_iowait_cpu>:
  ffffffffxxxxxxxx:       48 63 ff                movslq %edi,%rdi
  ffffffffxxxxxxxx:       48 c7 c0 c0 95 02 00    mov    $0x295c0,%rax
  ffffffffxxxxxxxx:       48 03 04 fd a0 46 0f    add    -0x7df0b960(,%rdi,8),%rax
  ffffffffxxxxxxxx:       82 
- ffffffffxxxxxxxx:       48 63 80 98 09 00 00    movslq 0x998(%rax),%rax
  ffffffffxxxxxxxx:       c3                      ret    

  ffffffffxxxxxxxx <nr_iowait_cpu>:
  ffffffffxxxxxxxx:       48 63 ff                movslq %edi,%rdi
  ffffffffxxxxxxxx:       48 c7 c0 c0 95 02 00    mov    $0x295c0,%rax
  ffffffffxxxxxxxx:       48 03 04 fd a0 46 0f    add    -0x7df0b960(,%rdi,8),%rax
  ffffffffxxxxxxxx:       82 
- ffffffffxxxxxxxx:       8b 80 98 09 00 00       mov    0x998(%rax),%eax
  ffffffffxxxxxxxx:       c3                      ret    

Note how the 'movslq 0x998(%rax),%rax' lost the REX prefix and got one byte 
shorter. Call sites got shorter too:

  ffffffffxxxxxxxx:       e8 e8 73 fa ff          call   ffffffffxxxxxxxx <nr_iowait_cpu>
- ffffffffxxxxxxxx:       48 85 c0                test   %rax,%rax

  ffffffffxxxxxxxx:       e8 c8 73 fa ff          call   ffffffffxxxxxxxx <nr_iowait_cpu>
- ffffffffxxxxxxxx:       85 c0                   test   %eax,%eax

You often won't see these effects in the 'size vmlinux' output, because 
function alignment padding reserves usually hide 1-2 byte size improvements 
in generated code.

> I'm truly impressed by the massive savings of this change and I'm even 
> more impressed by the justification:
> 
> > Bring nr_running() into 32-bit world to save on REX prefixes.
> 
> Aside of the obvious useless churn, REX prefixes are universaly true for 
> all architectures, right? There is a world outside x86 ...

Even architectures that have the same instruction length for 32-bit and 
64-bit data types benefit:

 - smaller data structures benefit all 64-bit architectures

 - the cleaner, more coherent nr_running type definitions are now more 
   consistently 'int' based, not the previous nonsensical 'int/long' mix 
   that also made the generated code larger on x86.

More importantly, the maintenance benchmark in these cases is not whether a 
change actively helps every architecture we care about - but whether the 
change is a *disadvantage* for them - and it isn't here.

Changes that primarily benefit one common architecture, while not others, 
are still eligible for upstream merging if they otherwise meet the quality 
threshold and don't hurt the other architectures.

TL;DR:

This benefits from this series are small, but are far from 'useless churn', 
unless we want to arbitrarily cut off technically valid contributions that 
improve generated code, data structure size and code readability, submitted 
by a long-time contributor who has contributed over 1,300 patches to the 
kernel already, just because we don't think these add up a significant 
enough benefit?

No doubt the quality barrier must be and is higher for smaller changes - 
but this series IMO passed that barrier.

Anyway, I've Cc:-ed Linus and Greg, if you are advocating for some sort of 
cut-off threshold for small but measurable improvements from long-time 
contributors, it should probably be clearly specified & documented in 
Documentation/SubmittingPatches ...

Thanks,

	Ingo