Re: [PATCH RFC v3 1/6] x86/paravirt: Add pv_idle_ops to paravirt ops
From: Quan Xu
Date: Tue Nov 14 2017 - 03:16:03 EST
On 2017/11/14 15:12, Wanpeng Li wrote:
2017-11-14 15:02 GMT+08:00 Quan Xu <quan.xu0@xxxxxxxxx>:
On 2017/11/13 18:53, Juergen Gross wrote:
On 13/11/17 11:06, Quan Xu wrote:
From: Quan Xu <quan.xu0@xxxxxxxxx>
So far, pv_idle_ops.poll is the only ops for pv_idle. .poll is called
in idle path which will poll for a while before we enter the real idle
state.
In virtualization, idle path includes several heavy operations
includes timer access(LAPIC timer or TSC deadline timer) which will
hurt performance especially for latency intensive workload like message
passing task. The cost is mainly from the vmexit which is a hardware
context switch between virtual machine and hypervisor. Our solution is
to poll for a while and do not enter real idle path if we can get the
schedule event during polling.
Poll may cause the CPU waste so we adopt a smart polling mechanism to
reduce the useless poll.
Signed-off-by: Yang Zhang <yang.zhang.wz@xxxxxxxxx>
Signed-off-by: Quan Xu <quan.xu0@xxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Alok Kataria <akataria@xxxxxxxxxx>
Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
Hmm, is the idle entry path really so critical to performance that a new
pvops function is necessary?
Juergen, Here is the data we get when running benchmark netperf:
1. w/o patch and disable kvm dynamic poll (halt_poll_ns=0):
29031.6 bit/s -- 76.1 %CPU
2. w/ patch and disable kvm dynamic poll (halt_poll_ns=0):
35787.7 bit/s -- 129.4 %CPU
3. w/ kvm dynamic poll:
35735.6 bit/s -- 200.0 %CPU
Actually we can reduce the CPU utilization by sleeping a period of
time as what has already been done in the poll logic of IO subsystem,
then we can improve the algorithm in kvm instead of introduing another
duplicate one in the kvm guest.
We really appreciate upstream's kvm dynamic poll mechanism, which is
really helpful for a lot of scenario..
However, as description said, in virtualization, idle path includes
several heavy operations includes timer access (LAPIC timer or TSC
deadline timer) which will hurt performance especially for latency
intensive workload like message passing task. The cost is mainly from
the vmexit which is a hardware context switch between virtual machine
and hypervisor.
for upstream's kvm dynamic poll mechanism, even you could provide a
better algorism, how could you bypass timer access (LAPIC timer or TSC
deadline timer), or a hardware context switch between virtual machine
and hypervisor. I know these is a tradeoff.
Furthermore, here is the data we get when running benchmark contextswitch
to measure the latency(lower is better):
1. w/o patch and disable kvm dynamic poll (halt_poll_ns=0):
 3402.9 ns/ctxsw -- 199.8 %CPU
2. w/ patch and disable kvm dynamic poll:
 1163.5 ns/ctxsw -- 205.5 %CPU
3. w/ kvm dynamic poll:
 2280.6 ns/ctxsw -- 199.5 %CPU
so, these tow solution are quite similar, but not duplicate..
that's also why to add a generic idle poll before enter real idle path.
When a reschedule event is pending, we can bypass the real idle path.
Quan
Alibaba Cloud
Regards,
Wanpeng Li
4. w/patch and w/ kvm dynamic poll:
42225.3 bit/s -- 198.7 %CPU
5. idle=poll
37081.7 bit/s -- 998.1 %CPU
w/ this patch, we will improve performance by 23%.. even we could improve
performance by 45.4%, if we use w/patch and w/ kvm dynamic poll. also the
cost of CPU is much lower than 'idle=poll' case..
Wouldn't a function pointer, maybe guarded
by a static key, be enough? A further advantage would be that this would
work on other architectures, too.
I assume this feature will be ported to other archs.. a new pvops makes code
clean and easy to maintain. also I tried to add it into existed pvops, but
it
doesn't match.
Quan
Alibaba Cloud
Juergen