[PATCH v3 00/41] Optimize KVM/ARM for VHE systems

Christoffer Dall christoffer.dall at linaro.org
Thu Jan 18 05:32:24 PST 2018 

On Thu, Jan 18, 2018 at 03:18:21PM +0300, Yury Norov wrote:
> On Thu, Jan 18, 2018 at 12:16:32PM +0100, Christoffer Dall wrote:
> > Hi Yury,
> > 
> > [cc'ing Alex Bennee who had some thoughts on this]
> > 
> > On Mon, Jan 15, 2018 at 05:14:23PM +0300, Yury Norov wrote:
> > > On Fri, Jan 12, 2018 at 01:07:06PM +0100, Christoffer Dall wrote:
> > > > This series redesigns parts of KVM/ARM to optimize the performance on
> > > > VHE systems.  The general approach is to try to do as little work as
> > > > possible when transitioning between the VM and the hypervisor.  This has
> > > > the benefit of lower latency when waiting for interrupts and delivering
> > > > virtual interrupts, and reduces the overhead of emulating behavior and
> > > > I/O in the host kernel.
> > > > 
> > > > Patches 01 through 06 are not VHE specific, but rework parts of KVM/ARM
> > > > that can be generally improved.  We then add infrastructure to move more
> > > > logic into vcpu_load and vcpu_put, we improve handling of VFP and debug
> > > > registers.
> > > > 
> > > > We then introduce a new world-switch function for VHE systems, which we
> > > > can tweak and optimize for VHE systems.  To do that, we rework a lot of
> > > > the system register save/restore handling and emulation code that may
> > > > need access to system registers, so that we can defer as many system
> > > > register save/restore operations to vcpu_load and vcpu_put, and move
> > > > this logic out of the VHE world switch function.
> > > > 
> > > > We then optimize the configuration of traps.  On non-VHE systems, both
> > > > the host and VM kernels run in EL1, but because the host kernel should
> > > > have full access to the underlying hardware, but the VM kernel should
> > > > not, we essentially make the host kernel more privileged than the VM
> > > > kernel despite them both running at the same privilege level by enabling
> > > > VE traps when entering the VM and disabling those traps when exiting the
> > > > VM.  On VHE systems, the host kernel runs in EL2 and has full access to
> > > > the hardware (as much as allowed by secure side software), and is
> > > > unaffected by the trap configuration.  That means we can configure the
> > > > traps for VMs running in EL1 once, and don't have to switch them on and
> > > > off for every entry/exit to/from the VM.
> > > > 
> > > > Finally, we improve our VGIC handling by moving all save/restore logic
> > > > out of the VHE world-switch, and we make it possible to truly only
> > > > evaluate if the AP list is empty and not do *any* VGIC work if that is
> > > > the case, and only do the minimal amount of work required in the course
> > > > of the VGIC processing when we have virtual interrupts in flight.
> > > > 
> > > > The patches are based on v4.15-rc3, v9 of the level-triggered mapped
> > > > interrupts support series [1], and the first five patches of James' SDEI
> > > > series [2].
> > > > 
> > > > I've given the patches a fair amount of testing on Thunder-X, Mustang,
> > > > Seattle, and TC2 (32-bit) for non-VHE testing, and tested VHE
> > > > functionality on the Foundation model, running both 64-bit VMs and
> > > > 32-bit VMs side-by-side and using both GICv3-on-GICv3 and
> > > > GICv2-on-GICv3.
> > > > 
> > > > The patches are also available in the vhe-optimize-v3 branch on my
> > > > kernel.org repository [3].  The vhe-optimize-v3-base branch contains
> > > > prerequisites of this series.
> > > > 
> > > > Changes since v2:
> > > >  - Rebased on v4.15-rc3.
> > > >  - Includes two additional patches that only does vcpu_load after
> > > >    kvm_vcpu_first_run_init and only for KVM_RUN.
> > > >  - Addressed review comments from v2 (detailed changelogs are in the
> > > >    individual patches).
> > > > 
> > > > Thanks,
> > > > -Christoffer
> > > > 
> > > > [1]: git://git.kernel.org/pub/scm/linux/kernel/git/cdall/linux.git level-mapped-v9
> > > > [2]: git://linux-arm.org/linux-jm.git sdei/v5/base
> > > > [3]: git://git.kernel.org/pub/scm/linux/kernel/git/cdall/linux.git vhe-optimize-v3
> > > 
> > > I tested this v3 series on ThunderX2 with IPI benchmark:
> > > https://lkml.org/lkml/2017/12/11/364
> > > 
> > > I tried to address your comments in discussion to v2, like pinning
> > > the module to specific CPU (with taskset), increasing the number of
> > > iterations, tuning governor to max performance. Results didn't change
> > > much, and are pretty stable.
> > > 
> > > Comparing to vanilla guest, Norml IPI delivery for v3 is 20% slower.
> > > For v2 it was 27% slower, and for v1 - 42% faster. What's interesting,
> > > the acknowledge time is much faster for v3, so overall time to
> > > deliver and acknowledge IPI (2nd column) is less than vanilla
> > > 4.15-rc3 kernel.
> > > 
> > > Test setup is not changed since v2: ThunderX2, 112 online CPUs,
> > > guest is running under qemu-kvm, emulating gic version 3.
> > > 
> > > Below is test results for v1-3 normalized to host vanilla kernel
> > > dry-run time.
> > > 
> > > Yury
> > > 
> > > Host, v4.14:
> > > Dry-run:          0         1
> > > Self-IPI:         9        18
> > > Normal IPI:      81       110
> > > Broadcast IPI:    0      2106
> > > 
> > > Guest, v4.14:
> > > Dry-run:          0         1
> > > Self-IPI:        10        18
> > > Normal IPI:     305       525
> > > Broadcast IPI:    0      9729
> > > 
> > > Guest, v4.14 + VHE:
> > > Dry-run:          0         1
> > > Self-IPI:         9        18
> > > Normal IPI:     176       343
> > > Broadcast IPI:    0      9885
> > > 
> > > And for v2.
> > > 
> > > Host, v4.15:                   
> > > Dry-run:          0         1
> > > Self-IPI:         9        18
> > > Normal IPI:      79       108
> > > Broadcast IPI:    0      2102
> > >                         
> > > Guest, v4.15-rc:
> > > Dry-run:          0         1
> > > Self-IPI:         9        18
> > > Normal IPI:     291       526
> > > Broadcast IPI:    0     10439
> > > 
> > > Guest, v4.15-rc + VHE:
> > > Dry-run:          0         2
> > > Self-IPI:        14        28
> > > Normal IPI:     370       569
> > > Broadcast IPI:    0     11688
> > > 
> > > And for v3.
> > > 
> > > Host 4.15-rc3					
> > > Dry-run:	  0	    1
> > > Self-IPI:	  9	   18
> > > Normal IPI:	 80	  110
> > > Broadcast IPI:	  0	 2088
> > > 		
> > > Guest, 4.15-rc3	
> > > Dry-run:	  0	    1
> > > Self-IPI:	  9	   18
> > > Normal IPI:	289	  497
> > > Broadcast IPI:	  0	 9999
> > > 		
> > > Guest, 4.15-rc3	+ VHE
> > > Dry-run:	  0	    2
> > > Self-IPI:	 12	   24
> > > Normal IPI:	347	  490
> > > Broadcast IPI:	  0	11906
> > 
> > So, I had a look at your measurement code, and just want to make a
> > sanity check that I understand the measurements correctly.
> > 
> > Firstly, if we execute something 100,000 times and summarize the result
> > for each run, and get anything less than 100,000 (in this case ~300),
> > without scaling the value, doesn't that mean that in the vast majority
> > of cases, you are getting 0 as your measurement?
> 
> I cannot report absolute numbers so I posted normalized values to dry-run
> case. 300 for IPI delivery means that it 300 times slower than no-op
> (dry-run case). Absolute numbers looks quite reasonable, few useconds
> for normal IPI.

Ah, I see, you normalized it after the output from your benchmark.  I
thought you normalized it in the benchmark code originally, but then I
didn't see it in the patch you linked to, so wasn't sure what was going
on.

> 
> Let me know if you need absolute numbers.
> https://lkml.org/lkml/2017/12/13/301
>  

I trust you, that's fine.

> > Secondly, are we sure all the required memory barriers are in place?
> > I know that the IPI send contains an smp_wmb(), but when you read back
> > the value in the caller, do you have the necessary smp_wmb() on the
> > handler side and a corresponding smp_rmb() on the sending side?  I'm not
> > sure what kind of effect missing barriers for a measurement framework
> > like this would have, but it's worth making sure we're not chasing red
> > herrings here.
> 
> I don't share memory between PMUs. 

PMUs?

You do share memory between your CPUs, it's the little piece of memory
that your time variable points to.

I was concerned if the read back from your sender CPU of the value
written by the receiving CPU was properly ordered, but looking at
handle_IPI and smp_call_function_single, there are barriers pretty much
all over, and I don't think a missing barrier would result in what we
see here (given that I understand the normalization above).


>  
> > That obviously doesn't change that the overall turnaround time is
> > improved more in the v1 case than in the v3 case, which I'd like to
> > explore/bisect in any case.
> 
> So me. For any idea, let me know, I'll check it.
> 
So another thing that would be very useful (which I would do myself if I
had access to a TX2) would be to simply bisect the series and run
the benchmark and see where the regression is introduced.

In case you have time for that, I have a bisectable series with the
recent KVM/ARM fixes in the 'vhe-optimize-v3-with-fixes' branch on:
git://git.kernel.org/pub/scm/linux/kernel/git/cdall/linux.git


Thanks,
-Christoffer

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