Messages in this thread | | | Date | Mon, 30 May 2016 16:53:41 +0800 | From | xinhui <> | Subject | Re: [PATCH] pv-qspinlock: Try to re-hash the lock after spurious_wakeup |
| |
On 2016年05月28日 11:41, Waiman Long wrote: > On 05/27/2016 06:32 AM, xinhui wrote: >> >> On 2016年05月27日 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@linux.vnet.ibm.com> >>>> --- >>>> 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: > > pv_lock_slowpath=9256211 > pv_lock_stealing=36398363 > pv_spurious_wakeup=311 > pv_wait_again=294 > pv_wait_early=3255605 > pv_wait_head=173 > pv_wait_node=3256280 > OK, here is the result after run command perf bench sched messaging -g 512
pv_lock_slowpath=2331407 pv_lock_stealing=192038 pv_spurious_wakeup=236319 pv_wait_again=215668 pv_wait_early=177299 pv_wait_head=9206 pv_wait_node=228781
> 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?
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.
>> 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.
> Cheers, > Longman >
| |