On 2020-01-13 12:12, Will Deacon wrote:
[+PeterZ]
On Thu, Dec 26, 2019 at 09:58:27PM +0800, Zengruan Ye wrote:
This patch set aims to support the vcpu_is_preempted() functionality
under KVM/arm64, which allowing the guest to obtain the VCPU is
currently running or not. This will enhance lock performance on
overcommitted hosts (more runnable VCPUs than physical CPUs in the
system) as doing busy waits for preempted VCPUs will hurt system
performance far worse than early yielding.
We have observed some performace improvements in uninx benchmark
tests.
unix benchmark result:
host: kernel 5.5.0-rc1, HiSilicon Kunpeng920, 8 CPUs
guest: kernel 5.5.0-rc1, 16 VCPUs
test-case | after-patch |
before-patch
----------------------------------------+-------------------+------------------
Dhrystone 2 using register variables | 334600751.0 lps |
335319028.3 lps
Double-Precision Whetstone | 32856.1 MWIPS |
32849.6 MWIPS
Execl Throughput | 3662.1 lps |
2718.0 lps
File Copy 1024 bufsize 2000 maxblocks | 432906.4 KBps |
158011.8 KBps
File Copy 256 bufsize 500 maxblocks | 116023.0 KBps |
37664.0 KBps
File Copy 4096 bufsize 8000 maxblocks | 1432769.8 KBps |
441108.8 KBps
Pipe Throughput | 6405029.6 lps |
6021457.6 lps
Pipe-based Context Switching | 185872.7 lps |
184255.3 lps
Process Creation | 4025.7 lps |
3706.6 lps
Shell Scripts (1 concurrent) | 6745.6 lpm |
6436.1 lpm
Shell Scripts (8 concurrent) | 998.7 lpm |
931.1 lpm
System Call Overhead | 3913363.1 lps |
3883287.8 lps
----------------------------------------+-------------------+------------------
System Benchmarks Index Score | 1835.1 |
1327.6
Interesting, thanks for the numbers.
So it looks like there is a decent improvement to be had from targetted
vCPU
wakeup, but I really dislike the explicit PV interface and it's already
been
shown to interact badly with the WFE-based polling in
smp_cond_load_*().
Rather than expose a divergent interface, I would instead like to
explore an
improvement to smp_cond_load_*() and see how that performs before we
commit
to something more intrusive. Marc and I looked at this very briefly in
the
past, and the basic idea is to register all of the WFE sites with the
hypervisor, indicating which register contains the address being spun
on
and which register contains the "bad" value. That way, you don't bother
rescheduling a vCPU if the value at the address is still bad, because
you
know it will exit immediately.
Of course, the devil is in the details because when I say "address",
that's
a guest virtual address, so you need to play some tricks in the
hypervisor
so that you have a separate mapping for the lockword (it's enough to
keep
track of the physical address).
Our hacks are here but we basically ran out of time to work on them
beyond
an unoptimised and hacky prototype:
https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=kvm-arm64/pvcy
Marc -- how would you prefer to handle this?
Let me try and rebase this thing to a modern kernel (I doubt it applies
without
conflicts to mainline). We can then have discussion about its merit on
the list
once I post it. It'd be good to have a pointer to the benchamrks that
have been
used here.
Thanks,
M.
--
Jazz is not dead. It just smells funny...
