Yicong Yang <[email protected]> writes:
> From: Yicong Yang <[email protected]>
>
> Though ARM64 has the hardware to do tlb shootdown, the hardware
> broadcasting is not free.
> A simplest micro benchmark shows even on snapdragon 888 with only
> 8 cores, the overhead for ptep_clear_flush is huge even for paging
> out one page mapped by only one process:
> 5.36% a.out [kernel.kallsyms] [k] ptep_clear_flush
>
> While pages are mapped by multiple processes or HW has more CPUs,
> the cost should become even higher due to the bad scalability of
> tlb shootdown.
>
> The same benchmark can result in 16.99% CPU consumption on ARM64
> server with around 100 cores according to Yicong's test on patch
> 4/4.
>
> This patchset leverages the existing BATCHED_UNMAP_TLB_FLUSH by
> 1. only send tlbi instructions in the first stage -
> arch_tlbbatch_add_mm()
> 2. wait for the completion of tlbi by dsb while doing tlbbatch
> sync in arch_tlbbatch_flush()
> Testing on snapdragon shows the overhead of ptep_clear_flush
> is removed by the patchset. The micro benchmark becomes 5% faster
> even for one page mapped by single process on snapdragon 888.
>
> With this support we're possible to do more optimization for memory
> reclamation and migration[*].
I applied the patches on v6.1-rc4 and was able to see the drop in
ptep_clear_flush() in the perf report when running the test program from
Patch 2. The tests were done on a rk3399 based system with benefits
visible when running the tests on either of the clusters.
So, for the series,
Tested-by: Punit Agrawal <[email protected]>
Thanks,
Punit
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