| Message ID | 20210218040512.709186-9-yury.norov@gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | lib/find_bit: fast path for small bitmaps | expand |
On Wed, Feb 17, 2021 at 08:05:06PM -0800, Yury Norov wrote: > This series introduces fast paths for find_bit() routines. It is > beneficial for typical systems, but those who limited in I-cache > may be concerned about increasing the .text size of the Image. > > To address this concern, one can disable FAST_PATH option in the config > and some save memory. > > The effect of this option on my arm64 next-20210217 build is: (Maybe bloat-o-meter will give better view on this, i.e. more human-readable) > Before: > Sections: > Idx Name Size VMA LMA File off Algn > 0 .head.text 00010000 ffff800010000000 ffff800010000000 00010000 2**16 > CONTENTS, ALLOC, LOAD, READONLY, CODE > 1 .text 0115e3a8 ffff800010010000 ffff800010010000 00020000 2**16 > CONTENTS, ALLOC, LOAD, READONLY, CODE > 2 .got.plt 00000018 ffff80001116e3a8 ffff80001116e3a8 0117e3a8 2**3 > CONTENTS, ALLOC, LOAD, DATA > 3 .rodata 007a72ca ffff800011170000 ffff800011170000 01180000 2**12 > CONTENTS, ALLOC, LOAD, DATA > ... > > After: > Sections: > Idx Name Size VMA LMA File off Algn > 0 .head.text 00010000 ffff800010000000 ffff800010000000 00010000 2**16 > CONTENTS, ALLOC, LOAD, READONLY, CODE > 1 .text 011623a8 ffff800010010000 ffff800010010000 00020000 2**16 > CONTENTS, ALLOC, LOAD, READONLY, CODE > 2 .got.plt 00000018 ffff8000111723a8 ffff8000111723a8 011823a8 2**3 > CONTENTS, ALLOC, LOAD, DATA > 3 .rodata 007a772a ffff800011180000 ffff800011180000 01190000 2**12 > CONTENTS, ALLOC, LOAD, DATA > ... > > Notice that this is the cumulive effect on already existing fast paths > controlled by SMALL_CONST() together with ones added by this series. ... > +config FAST_PATH I think the name is to broad for this cases, perhaps BITS_FAST_PATH? or BITMAP? > + bool "Enable fast path code generation" > + default y > + help > + This option enables fast path optimization with the cost of increasing > + the text section.
On Thu, Feb 18, 2021 at 05:15:43PM +0200, Andy Shevchenko wrote: > On Wed, Feb 17, 2021 at 08:05:06PM -0800, Yury Norov wrote: > > This series introduces fast paths for find_bit() routines. It is > > beneficial for typical systems, but those who limited in I-cache > > may be concerned about increasing the .text size of the Image. > > > > To address this concern, one can disable FAST_PATH option in the config > > and some save memory. > > > > The effect of this option on my arm64 next-20210217 build is: > > (Maybe bloat-o-meter will give better view on this, i.e. more human-readable) Never heard about this tool, thanks for the hint. scripts/bloat-o-meter vmlinux vmlinux.new add/remove: 16/13 grow/shrink: 111/439 up/down: 3616/-19352 (-15736) Function old new delta find_next_bit.constprop - 220 +220 apply_wqattrs_cleanup - 176 +176 memcg_free_shrinker_maps - 172 +172 ... cpuset_hotplug_workfn 2584 2288 -296 task_numa_fault 3640 3320 -320 kmem_cache_free_bulk 1684 1280 -404 Total: Before=26085140, After=26069404, chg -0.06% The complete output is here: https://pastebin.com/kBSdVJcK So if I understand the output correctly, the size of .text is decreased... Looks weird, but if it's true, we don't need the FAST_BIT config at all because there's no tradeoff, and I should drop the patch. Hmm... > > +config FAST_PATH > > I think the name is to broad for this cases, perhaps BITS_FAST_PATH? or BITMAP? My logic was that since SMALL_CONST() is global, and FAST_PATH controls the SMALL_CONST, it should also be global. I believe, Linux should have a global switch to control the behaviour in such cases, similarly to -Os compiler option. And I was surprized when I found nothing like FAST_PATH in the config. What about having FAST_PATH as a global option, and later if someone will request for granularity, we'll introduce nested configs? > > + bool "Enable fast path code generation" > > + default y > > + help > > + This option enables fast path optimization with the cost of increasing > > + the text section. > > -- > With Best Regards, > Andy Shevchenko >
On Thu, Feb 18, 2021 at 11:24:19AM -0800, Yury Norov wrote: > On Thu, Feb 18, 2021 at 05:15:43PM +0200, Andy Shevchenko wrote: > > On Wed, Feb 17, 2021 at 08:05:06PM -0800, Yury Norov wrote: > > > This series introduces fast paths for find_bit() routines. It is > > > beneficial for typical systems, but those who limited in I-cache > > > may be concerned about increasing the .text size of the Image. > > > > > > To address this concern, one can disable FAST_PATH option in the config > > > and some save memory. > > > > > > The effect of this option on my arm64 next-20210217 build is: > > > > (Maybe bloat-o-meter will give better view on this, i.e. more human-readable) > > Never heard about this tool, thanks for the hint. > > scripts/bloat-o-meter vmlinux vmlinux.new > add/remove: 16/13 grow/shrink: 111/439 up/down: 3616/-19352 (-15736) > Function old new delta > find_next_bit.constprop - 220 +220 > apply_wqattrs_cleanup - 176 +176 > memcg_free_shrinker_maps - 172 +172 > ... > cpuset_hotplug_workfn 2584 2288 -296 > task_numa_fault 3640 3320 -320 > kmem_cache_free_bulk 1684 1280 -404 > Total: Before=26085140, After=26069404, chg -0.06% > > The complete output is here: > https://pastebin.com/kBSdVJcK > > So if I understand the output correctly, the size of .text is decreased... > Looks weird, but if it's true, we don't need the FAST_BIT config at all > because there's no tradeoff, and I should drop the patch. I actually expected the text size decrease when it's about constants. I remember that in PCI case we discussed with Bjorn the use of for_each_set_bit() that brought entire function into the object file that increased it by ~300 bytes (or so). But the code is something like for_each_set_bit(i, &addr, 32) ... > > I think the name is to broad for this cases, perhaps BITS_FAST_PATH? or BITMAP? > > My logic was that since SMALL_CONST() is global, and FAST_PATH > controls the SMALL_CONST, it should also be global. I believe, > Linux should have a global switch to control the behaviour in > such cases, similarly to -Os compiler option. And I was surprized > when I found nothing like FAST_PATH in the config. > > What about having FAST_PATH as a global option, and later if someone > will request for granularity, we'll introduce nested configs? I think it is too far from now. Let's do one step at a time.
diff --git a/include/asm-generic/bitsperlong.h b/include/asm-generic/bitsperlong.h index 0eeb77544f1d..209e531074c1 100644 --- a/include/asm-generic/bitsperlong.h +++ b/include/asm-generic/bitsperlong.h @@ -23,6 +23,10 @@ #define BITS_PER_LONG_LONG 64 #endif +#ifdef CONFIG_FAST_PATH #define SMALL_CONST(n) (__builtin_constant_p(n) && (unsigned long)(n) < BITS_PER_LONG) +#else +#define SMALL_CONST(n) (0) +#endif #endif /* __ASM_GENERIC_BITS_PER_LONG */ diff --git a/lib/Kconfig b/lib/Kconfig index a38cc61256f1..7a1b9c8d2a32 100644 --- a/lib/Kconfig +++ b/lib/Kconfig @@ -39,6 +39,13 @@ config PACKING When in doubt, say N. +config FAST_PATH + bool "Enable fast path code generation" + default y + help + This option enables fast path optimization with the cost of increasing + the text section. + config BITREVERSE tristate
This series introduces fast paths for find_bit() routines. It is beneficial for typical systems, but those who limited in I-cache may be concerned about increasing the .text size of the Image. To address this concern, one can disable FAST_PATH option in the config and some save memory. The effect of this option on my arm64 next-20210217 build is: Before: Sections: Idx Name Size VMA LMA File off Algn 0 .head.text 00010000 ffff800010000000 ffff800010000000 00010000 2**16 CONTENTS, ALLOC, LOAD, READONLY, CODE 1 .text 0115e3a8 ffff800010010000 ffff800010010000 00020000 2**16 CONTENTS, ALLOC, LOAD, READONLY, CODE 2 .got.plt 00000018 ffff80001116e3a8 ffff80001116e3a8 0117e3a8 2**3 CONTENTS, ALLOC, LOAD, DATA 3 .rodata 007a72ca ffff800011170000 ffff800011170000 01180000 2**12 CONTENTS, ALLOC, LOAD, DATA ... After: Sections: Idx Name Size VMA LMA File off Algn 0 .head.text 00010000 ffff800010000000 ffff800010000000 00010000 2**16 CONTENTS, ALLOC, LOAD, READONLY, CODE 1 .text 011623a8 ffff800010010000 ffff800010010000 00020000 2**16 CONTENTS, ALLOC, LOAD, READONLY, CODE 2 .got.plt 00000018 ffff8000111723a8 ffff8000111723a8 011823a8 2**3 CONTENTS, ALLOC, LOAD, DATA 3 .rodata 007a772a ffff800011180000 ffff800011180000 01190000 2**12 CONTENTS, ALLOC, LOAD, DATA ... Notice that this is the cumulive effect on already existing fast paths controlled by SMALL_CONST() together with ones added by this series. Signed-off-by: Yury Norov <yury.norov@gmail.com> --- include/asm-generic/bitsperlong.h | 4 ++++ lib/Kconfig | 7 +++++++ 2 files changed, 11 insertions(+)