Re: [PATCH -v6 2/2] arm64, tlbflush: don't TLBI broadcast if page reused in write fault

["Huang, Ying" <ying.huang@linux.alibaba.com>]

"David Hildenbrand (Red Hat)" <david@kernel.org> writes:

> On 14.11.25 09:54, Huang Ying wrote:
>> A multi-thread customer workload with large memory footprint uses
>> fork()/exec() to run some external programs every tens seconds.  When
>> running the workload on an arm64 server machine, it's observed that
>> quite some CPU cycles are spent in the TLB flushing functions.  While
>> running the workload on the x86_64 server machine, it's not.  This
>> causes the performance on arm64 to be much worse than that on x86_64.
>> During the workload running, after fork()/exec() write-protects all
>> pages in the parent process, memory writing in the parent process
>> will cause a write protection fault.  Then the page fault handler
>> will make the PTE/PDE writable if the page can be reused, which is
>> almost always true in the workload.  On arm64, to avoid the write
>> protection fault on other CPUs, the page fault handler flushes the TLB
>> globally with TLBI broadcast after changing the PTE/PDE.  However, this
>> isn't always necessary.  Firstly, it's safe to leave some stale
>> read-only TLB entries as long as they will be flushed finally.
>> Secondly, it's quite possible that the original read-only PTE/PDEs
>> aren't cached in remote TLB at all if the memory footprint is large.
>> In fact, on x86_64, the page fault handler doesn't flush the remote
>> TLB in this situation, which benefits the performance a lot.
>> To improve the performance on arm64, make the write protection fault
>> handler flush the TLB locally instead of globally via TLBI broadcast
>> after making the PTE/PDE writable.  If there are stale read-only TLB
>> entries in the remote CPUs, the page fault handler on these CPUs will
>> regard the page fault as spurious and flush the stale TLB entries.
>> To test the patchset, make the usemem.c from
>> vm-scalability (https://git.kernel.org/pub/scm/linux/kernel/git/wfg/vm-scalability.git).
>> support calling fork()/exec() periodically.  To mimic the behavior of
>> the customer workload, run usemem with 4 threads, access 100GB memory,
>> and call fork()/exec() every 40 seconds.  Test results show that with
>> the patchset the score of usemem improves ~40.6%.  The cycles% of TLB
>> flush functions reduces from ~50.5% to ~0.3% in perf profile.
>> Signed-off-by: Huang Ying <ying.huang@linux.alibaba.com>
>> Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
>> Reviewed-by: Barry Song <baohua@kernel.org>
>> Acked-by: Zi Yan <ziy@nvidia.com>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>> Cc: Vlastimil Babka <vbabka@suse.cz>
>> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
>> Cc: Yang Shi <yang@os.amperecomputing.com>
>> Cc: "Christoph Lameter (Ampere)" <cl@gentwo.org>
>> Cc: Dev Jain <dev.jain@arm.com>
>> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
>> Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
>> Cc: Kevin Brodsky <kevin.brodsky@arm.com>
>> Cc: Yin Fengwei <fengwei_yin@linux.alibaba.com>
>> Cc: linux-arm-kernel@lists.infradead.org
>> Cc: linux-kernel@vger.kernel.org
>> Cc: linux-mm@kvack.org
>> ---
>
> (no need to resend just for acks/rbs, maintainers can pick that up)

Sure.

> Reviewed-by: David Hildenbrand (Red Hat) <david@kernel.org>

Thank you David!

Hi, Maintainers,

I have collected some acks/rbs.  What do you think about the patchset?
What do I need to do for the next step?

---
Best Regards,
Huang, Ying