| Message ID | 20201205165406.108990-1-yury.norov@gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | arm64: enable GENERIC_FIND_FIRST_BIT | expand |
On Sat, Dec 05, 2020 at 08:54:06AM -0800, Yury Norov wrote: > ARM64 doesn't implement find_first_{zero}_bit in arch code and doesn't > enable it in config. It leads to using find_next_bit() which is less > efficient: [...] > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig > index 1515f6f153a0..2b90ef1f548e 100644 > --- a/arch/arm64/Kconfig > +++ b/arch/arm64/Kconfig > @@ -106,6 +106,7 @@ config ARM64 > select GENERIC_CPU_AUTOPROBE > select GENERIC_CPU_VULNERABILITIES > select GENERIC_EARLY_IOREMAP > + select GENERIC_FIND_FIRST_BIT Does this actually make any measurable difference? The disassembly with or without this is _very_ similar for me (clang 11). Will
(CC: Alexey Klimov) On Mon, Dec 7, 2020 at 3:25 AM Will Deacon <will@kernel.org> wrote: > > On Sat, Dec 05, 2020 at 08:54:06AM -0800, Yury Norov wrote: > > ARM64 doesn't implement find_first_{zero}_bit in arch code and doesn't > > enable it in config. It leads to using find_next_bit() which is less > > efficient: > > [...] > > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig > > index 1515f6f153a0..2b90ef1f548e 100644 > > --- a/arch/arm64/Kconfig > > +++ b/arch/arm64/Kconfig > > @@ -106,6 +106,7 @@ config ARM64 > > select GENERIC_CPU_AUTOPROBE > > select GENERIC_CPU_VULNERABILITIES > > select GENERIC_EARLY_IOREMAP > > + select GENERIC_FIND_FIRST_BIT > > Does this actually make any measurable difference? The disassembly with > or without this is _very_ similar for me (clang 11). > > Will On A-53 find_first_bit() is almost twice faster than find_next_bit(), according to lib/find_bit_benchmark. (Thanks to Alexey for testing.) Yury --- Tested-by: Alexey Klimov <aklimov@redhat.com> Start testing find_bit() with random-filled bitmap [7126084.864616] find_next_bit: 9653351 ns, 164280 iterations [7126084.881146] find_next_zero_bit: 9591974 ns, 163401 iterations [7126084.893859] find_last_bit: 5778627 ns, 164280 iterations [7126084.948181] find_first_bit: 47389224 ns, 16357 iterations [7126084.958975] find_next_and_bit: 3875849 ns, 73487 iterations [7126084.965884] Start testing find_bit() with sparse bitmap [7126084.973474] find_next_bit: 109879 ns, 655 iterations [7126084.999365] find_next_zero_bit: 18968440 ns, 327026 iterations [7126085.006351] find_last_bit: 80503 ns, 655 iterations [7126085.032315] find_first_bit: 19048193 ns, 655 iterations [7126085.039303] find_next_and_bit: 82628 ns, 1 iterations with enabled GENERIC_FIND_FIRST_BIT: Start testing find_bit() with random-filled bitmap [ 84.095335] find_next_bit: 9600970 ns, 163770 iterations [ 84.111695] find_next_zero_bit: 9613137 ns, 163911 iterations [ 84.124143] find_last_bit: 5713907 ns, 163770 iterations [ 84.158068] find_first_bit: 27193319 ns, 16406 iterations [ 84.168663] find_next_and_bit: 3863814 ns, 73671 iterations [ 84.175392] Start testing find_bit() with sparse bitmap [ 84.182660] find_next_bit: 112334 ns, 656 iterations [ 84.208375] find_next_zero_bit: 18976981 ns, 327025 iterations [ 84.215184] find_last_bit: 79584 ns, 656 iterations [ 84.233005] find_first_bit: 11082437 ns, 656 iterations [ 84.239821] find_next_and_bit: 82209 ns, 1 iterations root@pine:~# cpupower -c all frequency-info | grep asserted current CPU frequency: 648 MHz (asserted by call to hardware) current CPU frequency: 648 MHz (asserted by call to hardware) current CPU frequency: 648 MHz (asserted by call to hardware) current CPU frequency: 648 MHz (asserted by call to hardware) root@pine:~# lscpu Architecture: aarch64 CPU op-mode(s): 32-bit, 64-bit Byte Order: Little Endian CPU(s): 4 On-line CPU(s) list: 0-3 Thread(s) per core: 1 Core(s) per socket: 4 Socket(s): 1 Vendor ID: ARM Model: 4 Model name: Cortex-A53 Stepping: r0p4 CPU max MHz: 1152.0000 CPU min MHz: 648.0000 BogoMIPS: 48.00 Vulnerability Itlb multihit: Not affected Vulnerability L1tf: Not affected Vulnerability Mds: Not affected Vulnerability Meltdown: Not affected Vulnerability Spec store bypass: Not affected Vulnerability Spectre v1: Mitigation; __user pointer sanitization Vulnerability Spectre v2: Not affected Vulnerability Srbds: Not affected Vulnerability Tsx async abort: Not affected Flags: fp asimd evtstrm aes pmull sha1 sha2 crc32 cpuid
On Mon, Dec 07, 2020 at 05:59:16PM -0800, Yury Norov wrote: > (CC: Alexey Klimov) > > On Mon, Dec 7, 2020 at 3:25 AM Will Deacon <will@kernel.org> wrote: > > > > On Sat, Dec 05, 2020 at 08:54:06AM -0800, Yury Norov wrote: > > > ARM64 doesn't implement find_first_{zero}_bit in arch code and doesn't > > > enable it in config. It leads to using find_next_bit() which is less > > > efficient: > > > > [...] > > > > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig > > > index 1515f6f153a0..2b90ef1f548e 100644 > > > --- a/arch/arm64/Kconfig > > > +++ b/arch/arm64/Kconfig > > > @@ -106,6 +106,7 @@ config ARM64 > > > select GENERIC_CPU_AUTOPROBE > > > select GENERIC_CPU_VULNERABILITIES > > > select GENERIC_EARLY_IOREMAP > > > + select GENERIC_FIND_FIRST_BIT > > > > Does this actually make any measurable difference? The disassembly with > > or without this is _very_ similar for me (clang 11). > > > > Will > > On A-53 find_first_bit() is almost twice faster than find_next_bit(), > according to > lib/find_bit_benchmark. (Thanks to Alexey for testing.) I guess it's more compiler dependent than anything else, and it's a pity that find_next_bit() isn't implemented in terms of the generic find_first_bit() tbh, but if the numbers are as you suggest then I don't have a problem selecting this on arm64. Will
On Tue, Dec 08, 2020 at 10:35:50AM +0000, Will Deacon wrote: > On Mon, Dec 07, 2020 at 05:59:16PM -0800, Yury Norov wrote: > > (CC: Alexey Klimov) > > > > On Mon, Dec 7, 2020 at 3:25 AM Will Deacon <will@kernel.org> wrote: > > > > > > On Sat, Dec 05, 2020 at 08:54:06AM -0800, Yury Norov wrote: > > > > ARM64 doesn't implement find_first_{zero}_bit in arch code and doesn't > > > > enable it in config. It leads to using find_next_bit() which is less > > > > efficient: > > > > > > [...] > > > > > > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig > > > > index 1515f6f153a0..2b90ef1f548e 100644 > > > > --- a/arch/arm64/Kconfig > > > > +++ b/arch/arm64/Kconfig > > > > @@ -106,6 +106,7 @@ config ARM64 > > > > select GENERIC_CPU_AUTOPROBE > > > > select GENERIC_CPU_VULNERABILITIES > > > > select GENERIC_EARLY_IOREMAP > > > > + select GENERIC_FIND_FIRST_BIT > > > > > > Does this actually make any measurable difference? The disassembly with > > > or without this is _very_ similar for me (clang 11). > > > > > > Will > > > > On A-53 find_first_bit() is almost twice faster than find_next_bit(), > > according to > > lib/find_bit_benchmark. (Thanks to Alexey for testing.) > > I guess it's more compiler dependent than anything else, and it's a pity > that find_next_bit() isn't implemented in terms of the generic > find_first_bit() tbh, but if the numbers are as you suggest then I don't > have a problem selecting this on arm64. Ping?
On Tue, Feb 23, 2021 at 09:27:44PM -0800, Yury Norov wrote: > On Tue, Dec 08, 2020 at 10:35:50AM +0000, Will Deacon wrote: > > On Mon, Dec 07, 2020 at 05:59:16PM -0800, Yury Norov wrote: > > > (CC: Alexey Klimov) > > > > > > On Mon, Dec 7, 2020 at 3:25 AM Will Deacon <will@kernel.org> wrote: > > > > > > > > On Sat, Dec 05, 2020 at 08:54:06AM -0800, Yury Norov wrote: > > > > > ARM64 doesn't implement find_first_{zero}_bit in arch code and doesn't > > > > > enable it in config. It leads to using find_next_bit() which is less > > > > > efficient: > > > > > > > > [...] > > > > > > > > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig > > > > > index 1515f6f153a0..2b90ef1f548e 100644 > > > > > --- a/arch/arm64/Kconfig > > > > > +++ b/arch/arm64/Kconfig > > > > > @@ -106,6 +106,7 @@ config ARM64 > > > > > select GENERIC_CPU_AUTOPROBE > > > > > select GENERIC_CPU_VULNERABILITIES > > > > > select GENERIC_EARLY_IOREMAP > > > > > + select GENERIC_FIND_FIRST_BIT > > > > > > > > Does this actually make any measurable difference? The disassembly with > > > > or without this is _very_ similar for me (clang 11). > > > > > > > > Will > > > > > > On A-53 find_first_bit() is almost twice faster than find_next_bit(), > > > according to > > > lib/find_bit_benchmark. (Thanks to Alexey for testing.) > > > > I guess it's more compiler dependent than anything else, and it's a pity > > that find_next_bit() isn't implemented in terms of the generic > > find_first_bit() tbh, but if the numbers are as you suggest then I don't > > have a problem selecting this on arm64. > > Ping? Not sure what happened to this. Maybe resend at -rc1? Will
From: Yury Norov <yury.norov@gmail.com> Date: Sat, 5 Dec 2020 08:54:06 -0800 Hi, > ARM64 doesn't implement find_first_{zero}_bit in arch code and doesn't > enable it in config. It leads to using find_next_bit() which is less > efficient: > > 0000000000000000 <find_first_bit>: > 0: aa0003e4 mov x4, x0 > 4: aa0103e0 mov x0, x1 > 8: b4000181 cbz x1, 38 <find_first_bit+0x38> > c: f9400083 ldr x3, [x4] > 10: d2800802 mov x2, #0x40 // #64 > 14: 91002084 add x4, x4, #0x8 > 18: b40000c3 cbz x3, 30 <find_first_bit+0x30> > 1c: 14000008 b 3c <find_first_bit+0x3c> > 20: f8408483 ldr x3, [x4], #8 > 24: 91010045 add x5, x2, #0x40 > 28: b50000c3 cbnz x3, 40 <find_first_bit+0x40> > 2c: aa0503e2 mov x2, x5 > 30: eb02001f cmp x0, x2 > 34: 54ffff68 b.hi 20 <find_first_bit+0x20> // b.pmore > 38: d65f03c0 ret > 3c: d2800002 mov x2, #0x0 // #0 > 40: dac00063 rbit x3, x3 > 44: dac01063 clz x3, x3 > 48: 8b020062 add x2, x3, x2 > 4c: eb02001f cmp x0, x2 > 50: 9a829000 csel x0, x0, x2, ls // ls = plast > 54: d65f03c0 ret > > ... > > 0000000000000118 <_find_next_bit.constprop.1>: > 118: eb02007f cmp x3, x2 > 11c: 540002e2 b.cs 178 <_find_next_bit.constprop.1+0x60> // b.hs, b.nlast > 120: d346fc66 lsr x6, x3, #6 > 124: f8667805 ldr x5, [x0, x6, lsl #3] > 128: b4000061 cbz x1, 134 <_find_next_bit.constprop.1+0x1c> > 12c: f8667826 ldr x6, [x1, x6, lsl #3] > 130: 8a0600a5 and x5, x5, x6 > 134: ca0400a6 eor x6, x5, x4 > 138: 92800005 mov x5, #0xffffffffffffffff // #-1 > 13c: 9ac320a5 lsl x5, x5, x3 > 140: 927ae463 and x3, x3, #0xffffffffffffffc0 > 144: ea0600a5 ands x5, x5, x6 > 148: 54000120 b.eq 16c <_find_next_bit.constprop.1+0x54> // b.none > 14c: 1400000e b 184 <_find_next_bit.constprop.1+0x6c> > 150: d346fc66 lsr x6, x3, #6 > 154: f8667805 ldr x5, [x0, x6, lsl #3] > 158: b4000061 cbz x1, 164 <_find_next_bit.constprop.1+0x4c> > 15c: f8667826 ldr x6, [x1, x6, lsl #3] > 160: 8a0600a5 and x5, x5, x6 > 164: eb05009f cmp x4, x5 > 168: 540000c1 b.ne 180 <_find_next_bit.constprop.1+0x68> // b.any > 16c: 91010063 add x3, x3, #0x40 > 170: eb03005f cmp x2, x3 > 174: 54fffee8 b.hi 150 <_find_next_bit.constprop.1+0x38> // b.pmore > 178: aa0203e0 mov x0, x2 > 17c: d65f03c0 ret > 180: ca050085 eor x5, x4, x5 > 184: dac000a5 rbit x5, x5 > 188: dac010a5 clz x5, x5 > 18c: 8b0300a3 add x3, x5, x3 > 190: eb03005f cmp x2, x3 > 194: 9a839042 csel x2, x2, x3, ls // ls = plast > 198: aa0203e0 mov x0, x2 > 19c: d65f03c0 ret > > ... > > 0000000000000238 <find_next_bit>: > 238: a9bf7bfd stp x29, x30, [sp, #-16]! > 23c: aa0203e3 mov x3, x2 > 240: d2800004 mov x4, #0x0 // #0 > 244: aa0103e2 mov x2, x1 > 248: 910003fd mov x29, sp > 24c: d2800001 mov x1, #0x0 // #0 > 250: 97ffffb2 bl 118 <_find_next_bit.constprop.1> > 254: a8c17bfd ldp x29, x30, [sp], #16 > 258: d65f03c0 ret > > Enabling this functions would also benefit for_each_{set,clear}_bit(). > Would it make sense to enable this config for all such architectures by > default? I confirm that GENERIC_FIND_FIRST_BIT also produces more optimized and fast code on MIPS (32 R2) where there is also no architecture-specific bitsearching routines. So, if it's okay for other folks, I'd suggest to go for it and enable for all similar arches. (otherwise, I'll publish a separate entry for mips-next after 5.12-rc1 release and mention you in "Suggested-by:") > Signed-off-by: Yury Norov <yury.norov@gmail.com> > > --- > arch/arm64/Kconfig | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig > index 1515f6f153a0..2b90ef1f548e 100644 > --- a/arch/arm64/Kconfig > +++ b/arch/arm64/Kconfig > @@ -106,6 +106,7 @@ config ARM64 > select GENERIC_CPU_AUTOPROBE > select GENERIC_CPU_VULNERABILITIES > select GENERIC_EARLY_IOREMAP > + select GENERIC_FIND_FIRST_BIT > select GENERIC_IDLE_POLL_SETUP > select GENERIC_IRQ_IPI > select GENERIC_IRQ_MULTI_HANDLER > -- > 2.25.1 Thanks, Al
On Wed, Feb 24, 2021 at 11:52:55AM +0000, Alexander Lobakin wrote: > From: Yury Norov <yury.norov@gmail.com> > Date: Sat, 5 Dec 2020 08:54:06 -0800 > > Hi, > > > ARM64 doesn't implement find_first_{zero}_bit in arch code and doesn't > > enable it in config. It leads to using find_next_bit() which is less > > efficient: > > > > 0000000000000000 <find_first_bit>: > > 0: aa0003e4 mov x4, x0 > > 4: aa0103e0 mov x0, x1 > > 8: b4000181 cbz x1, 38 <find_first_bit+0x38> > > c: f9400083 ldr x3, [x4] > > 10: d2800802 mov x2, #0x40 // #64 > > 14: 91002084 add x4, x4, #0x8 > > 18: b40000c3 cbz x3, 30 <find_first_bit+0x30> > > 1c: 14000008 b 3c <find_first_bit+0x3c> > > 20: f8408483 ldr x3, [x4], #8 > > 24: 91010045 add x5, x2, #0x40 > > 28: b50000c3 cbnz x3, 40 <find_first_bit+0x40> > > 2c: aa0503e2 mov x2, x5 > > 30: eb02001f cmp x0, x2 > > 34: 54ffff68 b.hi 20 <find_first_bit+0x20> // b.pmore > > 38: d65f03c0 ret > > 3c: d2800002 mov x2, #0x0 // #0 > > 40: dac00063 rbit x3, x3 > > 44: dac01063 clz x3, x3 > > 48: 8b020062 add x2, x3, x2 > > 4c: eb02001f cmp x0, x2 > > 50: 9a829000 csel x0, x0, x2, ls // ls = plast > > 54: d65f03c0 ret > > > > ... > > > > 0000000000000118 <_find_next_bit.constprop.1>: > > 118: eb02007f cmp x3, x2 > > 11c: 540002e2 b.cs 178 <_find_next_bit.constprop.1+0x60> // b.hs, b.nlast > > 120: d346fc66 lsr x6, x3, #6 > > 124: f8667805 ldr x5, [x0, x6, lsl #3] > > 128: b4000061 cbz x1, 134 <_find_next_bit.constprop.1+0x1c> > > 12c: f8667826 ldr x6, [x1, x6, lsl #3] > > 130: 8a0600a5 and x5, x5, x6 > > 134: ca0400a6 eor x6, x5, x4 > > 138: 92800005 mov x5, #0xffffffffffffffff // #-1 > > 13c: 9ac320a5 lsl x5, x5, x3 > > 140: 927ae463 and x3, x3, #0xffffffffffffffc0 > > 144: ea0600a5 ands x5, x5, x6 > > 148: 54000120 b.eq 16c <_find_next_bit.constprop.1+0x54> // b.none > > 14c: 1400000e b 184 <_find_next_bit.constprop.1+0x6c> > > 150: d346fc66 lsr x6, x3, #6 > > 154: f8667805 ldr x5, [x0, x6, lsl #3] > > 158: b4000061 cbz x1, 164 <_find_next_bit.constprop.1+0x4c> > > 15c: f8667826 ldr x6, [x1, x6, lsl #3] > > 160: 8a0600a5 and x5, x5, x6 > > 164: eb05009f cmp x4, x5 > > 168: 540000c1 b.ne 180 <_find_next_bit.constprop.1+0x68> // b.any > > 16c: 91010063 add x3, x3, #0x40 > > 170: eb03005f cmp x2, x3 > > 174: 54fffee8 b.hi 150 <_find_next_bit.constprop.1+0x38> // b.pmore > > 178: aa0203e0 mov x0, x2 > > 17c: d65f03c0 ret > > 180: ca050085 eor x5, x4, x5 > > 184: dac000a5 rbit x5, x5 > > 188: dac010a5 clz x5, x5 > > 18c: 8b0300a3 add x3, x5, x3 > > 190: eb03005f cmp x2, x3 > > 194: 9a839042 csel x2, x2, x3, ls // ls = plast > > 198: aa0203e0 mov x0, x2 > > 19c: d65f03c0 ret > > > > ... > > > > 0000000000000238 <find_next_bit>: > > 238: a9bf7bfd stp x29, x30, [sp, #-16]! > > 23c: aa0203e3 mov x3, x2 > > 240: d2800004 mov x4, #0x0 // #0 > > 244: aa0103e2 mov x2, x1 > > 248: 910003fd mov x29, sp > > 24c: d2800001 mov x1, #0x0 // #0 > > 250: 97ffffb2 bl 118 <_find_next_bit.constprop.1> > > 254: a8c17bfd ldp x29, x30, [sp], #16 > > 258: d65f03c0 ret > > > > Enabling this functions would also benefit for_each_{set,clear}_bit(). > > Would it make sense to enable this config for all such architectures by > > default? > > I confirm that GENERIC_FIND_FIRST_BIT also produces more optimized and > fast code on MIPS (32 R2) where there is also no architecture-specific > bitsearching routines. > So, if it's okay for other folks, I'd suggest to go for it and enable > for all similar arches. As far as I understand the idea of GENERIC_FIND_FIRST_BIT=n, it's intended to save some space in .text. But in fact it bloats the kernel: yury:linux$ scripts/bloat-o-meter vmlinux vmlinux.ffb add/remove: 4/1 grow/shrink: 19/251 up/down: 564/-1692 (-1128) ... For the next cycle, I'm going to submit a patch that removes the GENERIC_FIND_FIRST_BIT completely and forces all architectures to use find_first{_zero}_bit() > (otherwise, I'll publish a separate entry for mips-next after 5.12-rc1 > release and mention you in "Suggested-by:") I think it worth to enable GENERIC_FIND_FIRST_BIT for mips and arm now and see how it works for people. If there'll be no complains I'll remove the config entirely. I'm OK if you submit the patch for mips now, or we can make a series and submit together. Works either way.
From: Yury Norov <yury.norov@gmail.com> Date: Wed, 24 Feb 2021 07:44:16 -0800 > On Wed, Feb 24, 2021 at 11:52:55AM +0000, Alexander Lobakin wrote: > > From: Yury Norov <yury.norov@gmail.com> > > Date: Sat, 5 Dec 2020 08:54:06 -0800 > > > > Hi, > > > > > ARM64 doesn't implement find_first_{zero}_bit in arch code and doesn't > > > enable it in config. It leads to using find_next_bit() which is less > > > efficient: > > > > > > 0000000000000000 <find_first_bit>: > > > 0: aa0003e4 mov x4, x0 > > > 4: aa0103e0 mov x0, x1 > > > 8: b4000181 cbz x1, 38 <find_first_bit+0x38> > > > c: f9400083 ldr x3, [x4] > > > 10: d2800802 mov x2, #0x40 // #64 > > > 14: 91002084 add x4, x4, #0x8 > > > 18: b40000c3 cbz x3, 30 <find_first_bit+0x30> > > > 1c: 14000008 b 3c <find_first_bit+0x3c> > > > 20: f8408483 ldr x3, [x4], #8 > > > 24: 91010045 add x5, x2, #0x40 > > > 28: b50000c3 cbnz x3, 40 <find_first_bit+0x40> > > > 2c: aa0503e2 mov x2, x5 > > > 30: eb02001f cmp x0, x2 > > > 34: 54ffff68 b.hi 20 <find_first_bit+0x20> // b.pmore > > > 38: d65f03c0 ret > > > 3c: d2800002 mov x2, #0x0 // #0 > > > 40: dac00063 rbit x3, x3 > > > 44: dac01063 clz x3, x3 > > > 48: 8b020062 add x2, x3, x2 > > > 4c: eb02001f cmp x0, x2 > > > 50: 9a829000 csel x0, x0, x2, ls // ls = plast > > > 54: d65f03c0 ret > > > > > > ... > > > > > > 0000000000000118 <_find_next_bit.constprop.1>: > > > 118: eb02007f cmp x3, x2 > > > 11c: 540002e2 b.cs 178 <_find_next_bit.constprop.1+0x60> // b.hs, b.nlast > > > 120: d346fc66 lsr x6, x3, #6 > > > 124: f8667805 ldr x5, [x0, x6, lsl #3] > > > 128: b4000061 cbz x1, 134 <_find_next_bit.constprop.1+0x1c> > > > 12c: f8667826 ldr x6, [x1, x6, lsl #3] > > > 130: 8a0600a5 and x5, x5, x6 > > > 134: ca0400a6 eor x6, x5, x4 > > > 138: 92800005 mov x5, #0xffffffffffffffff // #-1 > > > 13c: 9ac320a5 lsl x5, x5, x3 > > > 140: 927ae463 and x3, x3, #0xffffffffffffffc0 > > > 144: ea0600a5 ands x5, x5, x6 > > > 148: 54000120 b.eq 16c <_find_next_bit.constprop.1+0x54> // b.none > > > 14c: 1400000e b 184 <_find_next_bit.constprop.1+0x6c> > > > 150: d346fc66 lsr x6, x3, #6 > > > 154: f8667805 ldr x5, [x0, x6, lsl #3] > > > 158: b4000061 cbz x1, 164 <_find_next_bit.constprop.1+0x4c> > > > 15c: f8667826 ldr x6, [x1, x6, lsl #3] > > > 160: 8a0600a5 and x5, x5, x6 > > > 164: eb05009f cmp x4, x5 > > > 168: 540000c1 b.ne 180 <_find_next_bit.constprop.1+0x68> // b.any > > > 16c: 91010063 add x3, x3, #0x40 > > > 170: eb03005f cmp x2, x3 > > > 174: 54fffee8 b.hi 150 <_find_next_bit.constprop.1+0x38> // b.pmore > > > 178: aa0203e0 mov x0, x2 > > > 17c: d65f03c0 ret > > > 180: ca050085 eor x5, x4, x5 > > > 184: dac000a5 rbit x5, x5 > > > 188: dac010a5 clz x5, x5 > > > 18c: 8b0300a3 add x3, x5, x3 > > > 190: eb03005f cmp x2, x3 > > > 194: 9a839042 csel x2, x2, x3, ls // ls = plast > > > 198: aa0203e0 mov x0, x2 > > > 19c: d65f03c0 ret > > > > > > ... > > > > > > 0000000000000238 <find_next_bit>: > > > 238: a9bf7bfd stp x29, x30, [sp, #-16]! > > > 23c: aa0203e3 mov x3, x2 > > > 240: d2800004 mov x4, #0x0 // #0 > > > 244: aa0103e2 mov x2, x1 > > > 248: 910003fd mov x29, sp > > > 24c: d2800001 mov x1, #0x0 // #0 > > > 250: 97ffffb2 bl 118 <_find_next_bit.constprop.1> > > > 254: a8c17bfd ldp x29, x30, [sp], #16 > > > 258: d65f03c0 ret > > > > > > Enabling this functions would also benefit for_each_{set,clear}_bit(). > > > Would it make sense to enable this config for all such architectures by > > > default? > > > > I confirm that GENERIC_FIND_FIRST_BIT also produces more optimized and > > fast code on MIPS (32 R2) where there is also no architecture-specific > > bitsearching routines. > > So, if it's okay for other folks, I'd suggest to go for it and enable > > for all similar arches. > > As far as I understand the idea of GENERIC_FIND_FIRST_BIT=n, it's > intended to save some space in .text. But in fact it bloats the > kernel: > > yury:linux$ scripts/bloat-o-meter vmlinux vmlinux.ffb > add/remove: 4/1 grow/shrink: 19/251 up/down: 564/-1692 (-1128) > ... Same for MIPS, enabling GENERIC_FIND_FIRST_BIT saves a bunch of .text memory despite that it introduces new entries. > For the next cycle, I'm going to submit a patch that removes the > GENERIC_FIND_FIRST_BIT completely and forces all architectures to > use find_first{_zero}_bit() I like that idea. I'm almost sure there'll be no arch that benefits from CONFIG_GENERIC_FIND_FIRST_BIT=n (and has no arch-optimized versions). > > (otherwise, I'll publish a separate entry for mips-next after 5.12-rc1 > > release and mention you in "Suggested-by:") > > I think it worth to enable GENERIC_FIND_FIRST_BIT for mips and arm now > and see how it works for people. If there'll be no complains I'll remove > the config entirely. I'm OK if you submit the patch for mips now, or we > can make a series and submit together. Works either way. Lez make a series and see how it goes. I'll send you MIPS part soon. Al
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig index 1515f6f153a0..2b90ef1f548e 100644 --- a/arch/arm64/Kconfig +++ b/arch/arm64/Kconfig @@ -106,6 +106,7 @@ config ARM64 select GENERIC_CPU_AUTOPROBE select GENERIC_CPU_VULNERABILITIES select GENERIC_EARLY_IOREMAP + select GENERIC_FIND_FIRST_BIT select GENERIC_IDLE_POLL_SETUP select GENERIC_IRQ_IPI select GENERIC_IRQ_MULTI_HANDLER
ARM64 doesn't implement find_first_{zero}_bit in arch code and doesn't enable it in config. It leads to using find_next_bit() which is less efficient: 0000000000000000 <find_first_bit>: 0: aa0003e4 mov x4, x0 4: aa0103e0 mov x0, x1 8: b4000181 cbz x1, 38 <find_first_bit+0x38> c: f9400083 ldr x3, [x4] 10: d2800802 mov x2, #0x40 // #64 14: 91002084 add x4, x4, #0x8 18: b40000c3 cbz x3, 30 <find_first_bit+0x30> 1c: 14000008 b 3c <find_first_bit+0x3c> 20: f8408483 ldr x3, [x4], #8 24: 91010045 add x5, x2, #0x40 28: b50000c3 cbnz x3, 40 <find_first_bit+0x40> 2c: aa0503e2 mov x2, x5 30: eb02001f cmp x0, x2 34: 54ffff68 b.hi 20 <find_first_bit+0x20> // b.pmore 38: d65f03c0 ret 3c: d2800002 mov x2, #0x0 // #0 40: dac00063 rbit x3, x3 44: dac01063 clz x3, x3 48: 8b020062 add x2, x3, x2 4c: eb02001f cmp x0, x2 50: 9a829000 csel x0, x0, x2, ls // ls = plast 54: d65f03c0 ret ... 0000000000000118 <_find_next_bit.constprop.1>: 118: eb02007f cmp x3, x2 11c: 540002e2 b.cs 178 <_find_next_bit.constprop.1+0x60> // b.hs, b.nlast 120: d346fc66 lsr x6, x3, #6 124: f8667805 ldr x5, [x0, x6, lsl #3] 128: b4000061 cbz x1, 134 <_find_next_bit.constprop.1+0x1c> 12c: f8667826 ldr x6, [x1, x6, lsl #3] 130: 8a0600a5 and x5, x5, x6 134: ca0400a6 eor x6, x5, x4 138: 92800005 mov x5, #0xffffffffffffffff // #-1 13c: 9ac320a5 lsl x5, x5, x3 140: 927ae463 and x3, x3, #0xffffffffffffffc0 144: ea0600a5 ands x5, x5, x6 148: 54000120 b.eq 16c <_find_next_bit.constprop.1+0x54> // b.none 14c: 1400000e b 184 <_find_next_bit.constprop.1+0x6c> 150: d346fc66 lsr x6, x3, #6 154: f8667805 ldr x5, [x0, x6, lsl #3] 158: b4000061 cbz x1, 164 <_find_next_bit.constprop.1+0x4c> 15c: f8667826 ldr x6, [x1, x6, lsl #3] 160: 8a0600a5 and x5, x5, x6 164: eb05009f cmp x4, x5 168: 540000c1 b.ne 180 <_find_next_bit.constprop.1+0x68> // b.any 16c: 91010063 add x3, x3, #0x40 170: eb03005f cmp x2, x3 174: 54fffee8 b.hi 150 <_find_next_bit.constprop.1+0x38> // b.pmore 178: aa0203e0 mov x0, x2 17c: d65f03c0 ret 180: ca050085 eor x5, x4, x5 184: dac000a5 rbit x5, x5 188: dac010a5 clz x5, x5 18c: 8b0300a3 add x3, x5, x3 190: eb03005f cmp x2, x3 194: 9a839042 csel x2, x2, x3, ls // ls = plast 198: aa0203e0 mov x0, x2 19c: d65f03c0 ret ... 0000000000000238 <find_next_bit>: 238: a9bf7bfd stp x29, x30, [sp, #-16]! 23c: aa0203e3 mov x3, x2 240: d2800004 mov x4, #0x0 // #0 244: aa0103e2 mov x2, x1 248: 910003fd mov x29, sp 24c: d2800001 mov x1, #0x0 // #0 250: 97ffffb2 bl 118 <_find_next_bit.constprop.1> 254: a8c17bfd ldp x29, x30, [sp], #16 258: d65f03c0 ret Enabling this functions would also benefit for_each_{set,clear}_bit(). Would it make sense to enable this config for all such architectures by default? Signed-off-by: Yury Norov <yury.norov@gmail.com> --- arch/arm64/Kconfig | 1 + 1 file changed, 1 insertion(+)