Re: [PATCH] pv-qspinlock: Try to re-hash the lock after spurious_wakeup

From: xinhui
Date: Wed Jun 01 2016 - 01:55:10 EST

On 2016å06æ01æ 02:13, Waiman Long wrote:
On 05/30/2016 04:53 AM, xinhui wrote:

On 2016å05æ28æ 11:41, Waiman Long wrote:
On 05/27/2016 06:32 AM, xinhui wrote:

On 2016å05æ27æ 02:31, Waiman Long wrote:
On 05/25/2016 02:09 AM, Pan Xinhui wrote:
In pv_wait_head_or_lock, if there is a spurious_wakeup, and it fails to
get the lock as there is lock stealing, then after a short spin, we need
hash the lock again and enter pv_wait to yield.

Currently after a spurious_wakeup, as l->locked is not _Q_SLOW_VAL,
pv_wait might do nothing and return directly, that is not
paravirt-friendly because pv_wait_head_or_lock will just spin on the
lock then.

Signed-off-by: Pan Xinhui<xinhui.pan@xxxxxxxxxxxxxxxxxx>
kernel/locking/qspinlock_paravirt.h | 39 +++++++++++++++++++++++++++++--------
1 file changed, 31 insertions(+), 8 deletions(-)

Is this a problem you can easily reproduce on PPC? I have not observed this issue when testing on x86.

Hi, Waiman
I notice the spurious_wakeup count is very high when I do benchmark tests and stress tests. So after a simple investigation,
I find pv_wait_head_or_lock() just keep loops.

That shouldn't happen in normal case. When testing on x86, I typically get the following stat data for an over-commited guest:


OK, here is the result after run command perf bench sched messaging -g 512


Is the high spurious wakeup caused by the way PPC schedules processor resources to vCPUs? In x86, once the vCPU voluntarily sleep, it won't get woken up until there is an explicit vCPU kick request. It may not be the case for PPC, then. That may explain the high spurious wakeup number.

yes, vPCU can run if 1) someone kick it. 2) interrupts, for example, IPI, system reset. 3) vCPU has cpu slice again.

So seems I need rework __pv_wait on ppc,
while(*ptr == val)

The queue head don't call pv_wait that often. There are a bit of spurious wakeup, but it is mostly caused by lock stealing. How long is a cpu_relax() in PPC takes?

946012160 cpu_relax loops with 10 seconds. So if SPIN_THRESHOLD is 1<<15, it costs 0.3ms to spin on the lock. How about x86?

For x86, one measurement that I got in the past is that each cpu_relax() loop took about 3ns. So the full spin will take about 0.9ms.

And only 10134976 pv_wait/pv_kick hyper-call loops within 10 seconds. so every hyper-call itself(the so-called latency) costs less than 1us.

The hypercall is much slower in x86. it is about 10-20 us for pv_kick and up to 100us for pv_kick=>pv_wait.

Okay, So for ppc, hyper-call is light-weight, It's good to enter pv_wait earlier.
thanks for your tips. :)

Here is my story, in my pv-qspinlcok patchset V1&&v2, pv_wait on ppc ignore the first two parameters of *ptr and val, that makes lock_stealing hit too much.

The pvqspinlock code does depend on pv_wait() doing a final check to see if the lock value change. The code may not work reliably without that.

agree, So pv_wait now do the check of *ptr and val.

and when I change SPIN_THRESHOLD to a small value, system is very much unstable because waiter will enter pv_wait quickly and no one will kick waiter's cpu if
we enter pv_wait twice thanks to the lock_stealing.
So what I do in my pv-qspinlcok patchset V3 is that add if (*ptr == val) in pv_wait. However as I mentioned above, then spurious_wakeup count is too high, that also means our cpu
slice is wasted.

The SPIN_THRESHOLD should be sufficiently big. A small value will cause too many waits and wake-up's which may not be good. Anyway, more testing and tuning may be needed to make the pvqspinlock code work well with PPC.

agree , but I think the SPIN_THRESHOLD (1<<15) for ppc is a little large.

I even come up with an idea that make SPIN_THRESHOLD an extern variable on ppc. But I am busy and I wonder if it's worth doing that.

The purpose of the SPIN_THRESHOLD is to make sure that the vCPU won't call pv_wait() if the vCPUs in the guest aren't over-commited. The situation may be a bit different in PPC. So you need to make a decision as to how large the SPIN_THRESHOLD should be.

thanks for explaining that. I can keep SPIN_THRESHOLD as a const value for now.