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Message-ID: <20250906125058.1139346d@pumpkin>
Date: Sat, 6 Sep 2025 12:50:58 +0100
From: David Laight <david.laight.linux@...il.com>
To: Eero Tamminen <oak@...sinkinet.fi>
Cc: Geert Uytterhoeven <geert@...ux-m68k.org>, Finn Thain
<fthain@...ux-m68k.org>, Andrew Morton <akpm@...ux-foundation.org>, Lance
Yang <lance.yang@...ux.dev>, Masami Hiramatsu <mhiramat@...nel.org>, Peter
Zijlstra <peterz@...radead.org>, Will Deacon <will@...nel.org>,
stable@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-m68k@...ts.linux-m68k.org
Subject: Re: [PATCH] atomic: Specify natural alignment for atomic_t
On Mon, 1 Sep 2025 18:12:53 +0300
Eero Tamminen <oak@...sinkinet.fi> wrote:
> Hi Geert,
>
> On 1.9.2025 11.51, Geert Uytterhoeven wrote:
> >> On 23.8.2025 10.49, Lance Yang wrote:
> >> > Anyway, I've prepared two patches for discussion, either of which should
> >> > fix the alignment issue :)
> >> >
> >> > Patch A[1] adjusts the runtime checks to handle unaligned pointers.
> >> > Patch B[2] enforces 4-byte alignment on the core lock structures.
> >> >
> >> > Both tested on x86-64.
> >> >
> >> > [1]
> >> https://lore.kernel.org/lkml/20250823050036.7748-1-lance.yang@linux.dev
> >> > [2] https://lore.kernel.org/lkml/20250823074048.92498-1-
> >> > lance.yang@...ux.dev
> >>
> >> Same goes for both of these, except that removing warnings makes minimal
> >> kernel boot 1-2% faster than 4-aligning the whole struct.
>
> Note that above result was from (emulated) 68030 Falcon, i.e. something
> that has really small caches (256-byte i-/d-cache), *and* a kernel
> config using CONFIG_CC_OPTIMIZE_FOR_SIZE=y (with GCC 12.2).
If you are emulating it on x86 the misaligned memory accesses are
likely to be zero cost.
On a real '030 I'd expect them to be implemented as two memory accesses.
I also doubt (but a guess) that the emulator even attempts to emulate
the '030 caches. If they are like the '020 ones the i-cache really
only helps short loops.
It is more likely that the cost of WARN_ON_ONCE() is far more than
you might expect.
Especially since it will affect register allocation in the function(s).
David
>
>
> > That is an interesting outcome! So the gain of naturally-aligning the
> > lock is more than offset by the increased cache pressure due to wasting
> > (a bit?) more memory.
>
> Another reason could be those extra inlined warning checks in:
> -----------------------------------------------------
> $ git grep -e hung_task_set_blocker -e hung_task_clear_blocker kernel/
> kernel/locking/mutex.c: hung_task_set_blocker(lock, BLOCKER_TYPE_MUTEX);
> kernel/locking/mutex.c: hung_task_clear_blocker();
> kernel/locking/rwsem.c: hung_task_set_blocker(sem,
> BLOCKER_TYPE_RWSEM_READER);
> kernel/locking/rwsem.c: hung_task_clear_blocker();
> kernel/locking/rwsem.c: hung_task_set_blocker(sem,
> BLOCKER_TYPE_RWSEM_WRITER);
> kernel/locking/rwsem.c: hung_task_clear_blocker();
> kernel/locking/semaphore.c: hung_task_set_blocker(sem,
> BLOCKER_TYPE_SEM);
> kernel/locking/semaphore.c: hung_task_clear_blocker();
> -----------------------------------------------------
>
>
> > Do you know what was the impact on total kernel size?
>
> As expected, kernel code size is smaller with the static inlined warn
> checks removed:
> -----------------------------------------------------
> $ size vmlinux-m68k-6.16-fix1 vmlinux-m68k-6.16-fix2
> text data bss dec hex filename
> 3088520 953532 84224 4126276 3ef644 vmlinux-m68k-6.16-fix1 [1]
> 3088730 953564 84192 4126486 3ef716 vmlinux-m68k-6.16-fix2 [2]
> -----------------------------------------------------
>
> But could aligning of structs have caused 32 bytes moving from BSS to
> DATA section?
>
>
> - Eero
>
> PS. I profiled these 3 kernels on emulated Falcon. According to (Hatari)
> profiler, main difference in the kernel with the warnings removed, is it
> doing less than half of the calls to NCR5380_read() /
> atari_scsi_reg_read(), compared to the other 2 versions.
>
> These additional 2x calls in the other two versions, seem to mostly come
> through chain originating from process_scheduled_works(),
> NCR5380_poll_politely*() functions and bus probing.
>
> After quick look at the WARN_ON_ONCE()s and SCSI code, I have no idea
> how having those checks being inlined to locking functions, or not,
> would cause a difference like that. I've tried patching & building
> kernels again, and repeating profiling, but result is same.
>
> While Hatari call (graph) tracking might have some issue (due to kernel
> stack return address manipulation), I don't see how there could be a
> problem with the profiler instruction counts. Kernel code at given
> address does not change during boot in monolithic kernel, (emulator)
> profiler tracks _every_ executed instruction/address, and it's clearly
> correct function:
> ------------------------------------
> # disassembly with profile data: <instructions percentage>% (<sum of
> instructions>, <sum of cycles>, <sum of i-cache misses>, <sum of d-cache
> hits>)
> ...
> atari_scsi_falcon_reg_read:
> $001dd826 link.w a6,#$0 0.43% (414942, 1578432, 44701, 0)
> $001dd82a move.w sr,d1 0.43% (414942, 224, 8, 0)
> $001dd82c ori.w #$700,sr 0.43% (414942, 414368, 44705, 0)
> $001dd830 move.l $8(a6),d0 0.43% (414942, 357922, 44705, 414911)
> $001dd834 addi.l #$88,d0 0.43% (414942, 1014804, 133917, 0)
> $001dd83a move.w d0,$8606.w 0.43% (414942, 3618352, 89169, 0)
> $001dd83e move.w $8604.w,d0 0.43% (414942, 3620646, 89162, 0)
> $001dd842 move.w d1,sr 0.43% (414942, 2148, 142, 0)
> $001dd844 unlk a6 0.43% (414942, 436, 0, 414893)
> $001dd846 rts 0.43% (414942, 1073934, 134123, 414942)
> atari_scsi_falcon_reg_write:
> $001dd848 link.w a6,#$0 0.00% (81, 484, 29, 0)
> $001dd84c move.l $c(a6),d0 0.00% (81, 326, 29, 73)
> ...
> ------------------------------------
>
> Maybe those WARN_ON_ONCE() checks just happen to slow down something
> marginally so that things get interrupted & re-started more for the SCSI
> code?
>
> PPS. emulated machine has no SCSI drives, only one IDE drive (with 4MB
> Busybox partition):
> ----------------------------------------------------
> scsi host0: Atari native SCSI, irq 15, io_port 0x0, base 0x0, can_queue
> 1, cmd_per_lun 2, sg_tablesize 1, this_id 7, flags { }
> atari-falcon-ide atari-falcon-ide: Atari Falcon and Q40/Q60 PATA controller
> scsi host1: pata_falcon
> ata1: PATA max PIO4 cmd fff00000 ctl fff00038 data fff00000 no IRQ,
> using PIO polling
> ...
> ata1: found unknown device (class 0)
> ata1.00: ATA-7: Hatari IDE disk 4M, 1.0, max UDMA/100
> ata1.00: 8192 sectors, multi 16: LBA48
> ata1.00: configured for PIO
> ...
> scsi 1:0:0:0: Direct-Access ATA Hatari IDE disk 1.0 PQ: 0 ANSI: 5
> sd 1:0:0:0: [sda] 8192 512-byte logical blocks: (4.19 MB/4.00 MiB)
> sd 1:0:0:0: [sda] Write Protect is off
> sd 1:0:0:0: [sda] Mode Sense: 00 3a 00 00
> sd 1:0:0:0: [sda] Write cache: disabled, read cache: enabled, doesn't
> support DPO or FUA
> sd 1:0:0:0: [sda] Preferred minimum I/O size 512 bytes
> sd 1:0:0:0: [sda] Attached SCSI disk
> VFS: Mounted root (ext2 filesystem) readonly on device 8:0.
> ---------------------------------------------------
>
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