Re: [PATCH] locking/pvqspinlock: Hybrid PV queued/unfair locks

[Peter Zijlstra]

On Fri, Nov 03, 2017 at 11:35:31AM -0400, Waiman Long wrote:
> Currently, all the lock waiters entering the slowpath will do one
> lock stealing attempt to acquire the lock. That helps performance,
> especially in VMs with over-committed vCPUs. However, the current
> pvqspinlocks still don't perform as good as unfair locks in many cases.
> On the other hands, unfair locks do have the problem of lock starvation
> that pvqspinlocks don't have.
> 
> This patch combines the best attributes of an unfair lock and a
> pvqspinlock into a hybrid lock with 2 modes - queued mode & unfair
> mode. A lock waiter goes into the unfair mode when there are waiters
> in the wait queue but the pending bit isn't set. Otherwise, it will
> go into the queued mode waiting in the queue for its turn.
> 
> On a 2-socket 36-core E5-2699 v3 system (HT off), a kernel build
> (make -j<n>) was done in a VM with unpinned vCPUs 3 times with the
> best time selected and <n> is the number of vCPUs available. The build
> times of the original pvqspinlock, hybrid pvqspinlock and unfair lock
> with various number of vCPUs are as follows:
> 
>   vCPUs    pvqlock     hybrid pvqlock    unfair lock
>   -----    -------     --------------    -----------
>     30      342.1s         329.1s          329.1s
>     36      314.1s         305.3s          307.3s
>     45      345.0s         302.1s          306.6s
>     54      365.4s         308.6s          307.8s
>     72      358.9s         293.6s          303.9s
>    108      343.0s         285.9s          304.2s
> 
> The hybrid pvqspinlock performs better or comparable to the unfair
> lock.
> 
> By turning on QUEUED_LOCK_STAT, the table below showed the number
> of lock acquisitions in unfair mode and queue mode after a kernel
> build with various number of vCPUs.
> 
>   vCPUs    queued mode  unfair mode
>   -----    -----------  -----------
>     30      9,130,518      294,954
>     36     10,856,614      386,809
>     45      8,467,264   11,475,373
>     54      6,409,987   19,670,855
>     72      4,782,063   25,712,180
> 
> It can be seen that as the VM became more and more over-committed,
> the ratio of locks acquired in unfair mode increases. This is all
> done automatically to get the best overall performance as possible.
> 
> The table below shows the kernel build times on a smaller 2-socket
> 16-core 32-thread E5-2620 v4 system.
> 
>   vCPUs    pvqlock     hybrid pvqlock    unfair lock
>   -----    -------     --------------    -----------
>     16      436.8s         433.4s          435.6s
>     36      366.2s         364.8s          364.5s
>     48      423.6s         376.3s          370.2s
>     64      433.1s         376.6s          376.8s
> 
> Again, the performance of the hybrid pvqspinlock was comparable to
> that of the unfair lock.

You forgot to test a starvation case. And you also forgot to describe
how they cannot happen. I really cannot remember how all this is
supposed to work.