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
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
