Messages in this thread | | | From | Uros Bizjak <> | Date | Wed, 18 Jan 2023 22:55:46 +0100 | Subject | Re: [PATCH] lib/genalloc: use try_cmpxchg in {set,clear}_bits_ll |
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On Wed, Jan 18, 2023 at 10:47 PM Uros Bizjak <ubizjak@gmail.com> wrote: > > On Wed, Jan 18, 2023 at 10:18 PM Andrew Morton > <akpm@linux-foundation.org> wrote: > > > > On Wed, 18 Jan 2023 16:07:03 +0100 Uros Bizjak <ubizjak@gmail.com> wrote: > > > > > Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in > > > {set,clear}_bits_ll. x86 CMPXCHG instruction returns success in ZF > > > flag, so this change saves a compare after cmpxchg (and related move > > > instruction in front of cmpxchg). > > > > > > Also, try_cmpxchg implicitly assigns old *ptr value to "old" > > > when cmpxchg fails. > > > > > > Note that the value from *ptr should be read using READ_ONCE to prevent > > > the compiler from merging, refetching or reordering the read. > > > > > > The patch also declares these two functions inline, to ensure inlining. > > > > But why is that better? This adds a few hundred bytes more text, which > > has a cost. > > Originally, both functions are inlined and the size of an object file > is (gcc version 12.2.1, x86_64): > > text data bss dec hex filename > 4661 480 0 5141 1415 genalloc-orig.o > > When try_cmpxchg is used, gcc chooses to not inline set_bits_ll (its > estimate of code size is not very precise when multi-line assembly is > involved), resulting in: > > text data bss dec hex filename > 4705 488 0 5193 1449 genalloc-noinline.o > > And with an inline added to avoid gcc's quirks: > > text data bss dec hex filename > 4629 480 0 5109 13f5 genalloc.o > > Considering that these two changed functions are used only in > genalloc.o, adding inline qualifier is a win, also when comparing to > the original size.
BTW: Recently, it was determined [1] that the usage of cpu_relax() inside the cmpxchg loop can be harmful for performance. We actually have the same situation here, so perhaps cpu_relax() should be removed in the same way it was removed from the lockref.
[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f5fe24ef17b5fbe6db49534163e77499fb10ae8c
Uros.
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