Messages in this thread | | | Subject | Re: [RFC][PATCH v2 00/21] x86/pti: Defer CR3 switch to C code | From | Alexandre Chartre <> | Date | Wed, 18 Nov 2020 11:29:52 +0100 |
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On 11/18/20 10:30 AM, David Laight wrote: > From: Alexandre Chartre >> Sent: 18 November 2020 07:42 >> >> >> On 11/17/20 10:26 PM, Borislav Petkov wrote: >>> On Tue, Nov 17, 2020 at 07:12:07PM +0100, Alexandre Chartre wrote: >>>> Some benchmarks are available, in particular from phoronix: >>> >>> What I was expecting was benchmarks *you* have run which show that >>> perf penalty, not something one can find quickly on the internet and >>> something one cannot always reproduce her-/himself. >>> >>> You do know that presenting convincing numbers with a patchset greatly >>> improves its chances of getting it upstreamed, right? >>> >> >> Well, it looks like I wrongfully assume that KPTI was a well known performance >> overhead since it was introduced (because it adds extra page-table switches), >> but you are right I should be presenting my own numbers. > > IIRC the penalty comes from the page table switch. > Doing it at a different time is unlikely to make much difference. >
Correct, this RFC is not changing the overhead. However, it is a step forward for being able to execute some selected syscalls or interrupt handlers without switching to the kernel page-table. The next step would be to identify and add the necessary mapping to the user page-table so that specified syscalls can be executed without switching the page-table.
> For some workloads the penalty is massive - getting on for 50%. > We are still using old kernels on AWS. >
Here are some micro benchmarks of the getppid and getpid syscalls which highlight the PTI overhead. This uses the kernel tools/perf command, and the getpid command from libMICRO (https://github.com/redhat-performance/libMicro):
system running 5.10-rc4 booted with nopti: ------------------------------------------
# perf bench syscall basic # Running 'syscall/basic' benchmark: # Executed 10000000 getppid() calls Total time: 0.792 [sec]
0.079223 usecs/op 12622549 ops/sec
# getpid -B 100000 prc thr usecs/call samples errors cnt/samp getpid 1 1 0.08029 102 0 100000
We can see that getpid and getppid syscall have the same execution time around 0.08 usecs. These syscalls are very small and just return a value, so the time is mostly spent entering/exiting the kernel.
same system booted with pti: ----------------------------
# perf bench syscall basic # Running 'syscall/basic' benchmark: # Executed 10000000 getppid() calls Total time: 2.025 [sec]
0.202527 usecs/op 4937605 ops/sec
# getpid -B 100000 prc thr usecs/call samples errors cnt/samp getpid 1 1 0.20241 102 0 100000
With PTI, the execution time jumps to 0.20 usecs (+0.12 usecs = +150%).
That's a very extreme case because these are very small syscalls, and in that case the overhead to switch page-tables is significant compared to the execution time of the syscall.
So with an overhead of +0.12 usecs per syscall, the PTI impact is significant with workload which uses a lot of short syscalls. But if you use longer syscalls, for example with an average execution time of 2.0 usecs per syscall then you have a lower overhead of 6%.
alex.
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