Re: [PATCH v2 4/6] locking/pvqspinlock: Allow vCPUs kick-ahead

Waiman Long
2015-07-16T02:01:10Z

On 07/15/2015 05:39 AM, Peter Zijlstra wrote:
> On Tue, Jul 14, 2015 at 10:13:35PM -0400, Waiman Long wrote:
>> Frequent CPU halting (vmexit) and CPU kicking (vmenter) lengthens
>> critical section and block forward progress. This patch implements
>> a kick-ahead mechanism where the unlocker will kick the queue head
>> vCPUs as well as up to four additional vCPUs next to the queue head
>> if they were halted. The kickings are done after exiting the critical
>> section to improve parallelism.
>>
>> The amount of kick-ahead allowed depends on the number of vCPUs
>> in the VM guest. This patch, by itself, won't do much as most of
>> the kickings are currently done at lock time. Coupled with the next
>> patch that defers lock time kicking to unlock time, it should improve
>> overall system performance in a busy overcommitted guest.
>>
>> Linux kernel builds were run in KVM guest on an 8-socket, 4
>> cores/socket Westmere-EX system and a 4-socket, 8 cores/socket
>> Haswell-EX system. Both systems are configured to have 32 physical
>> CPUs. The kernel build times before and after the patch were:
>>
>> Westmere Haswell
>> Patch 32 vCPUs 48 vCPUs 32 vCPUs 48 vCPUs
>> ----- -------- -------- -------- --------
>> Before patch 3m25.0s 10m34.1s 2m02.0s 15m35.9s
>> After patch 3m27.4s 10m32.0s 2m00.8s 14m52.5s
>>
>> There wasn't too much difference before and after the patch.
> That means either the patch isn't worth it, or as you seem to imply its
> in the wrong place in this series.

It needs to be coupled with the next patch to be effective as most of
the kicking are happening at the lock side, instead of at the unlock
side. If you look at the sample pvqspinlock stats in patch 3:

lock_kick_count=755354
unlock_kick_count=87

The number of unlock kicks is negligible compared with the lock kicks.
Patch 5 does have a dependency on patch 4 unless we make it
unconditionally defers kicking to the unlock call which was what I had
done in the v1 patch. The reason why I change this in v2 is because I
found a very slight performance degradation in doing so.

>> @@ -224,7 +233,16 @@ static unsigned int pv_lock_hash_bits __read_mostly;
>> */
>> void __init __pv_init_lock_hash(void)
>> {
>> - int pv_hash_size = ALIGN(4 * num_possible_cpus(), PV_HE_PER_LINE);
>> + int ncpus = num_possible_cpus();
>> + int pv_hash_size = ALIGN(4 * ncpus, PV_HE_PER_LINE);
>> + int i;
>> +
>> + /*
>> + * The minimum number of vCPUs required in each kick-ahead level
>> + */
>> + static const u8 kick_ahead_threshold[PV_KICK_AHEAD_MAX] = {
>> + 4, 8, 16, 32
>> + };
> You are aware we have ilog2(), right?

Right, I am not aware of ilog2(). I am going to use that instead.

>> + /*
>> + * Enable the unlock kick ahead mode according to the number of
>> + * vCPUs available.
>> + */
>> + for (i = PV_KICK_AHEAD_MAX; i> 0; i--)
>> + if (ncpus>= kick_ahead_threshold[i - 1]) {
>> + pv_kick_ahead = i;
>> + break;
>> + }
> That's missing { }.
>
>> + if (pv_kick_ahead)
>> + pr_info("PV unlock kick ahead level %d enabled\n",
>> + pv_kick_ahead);
> Idem.

Will fix the {} problems.

> That said, I still really dislike this patch, it again seems a random
> bunch of hacks.

Any suggestions to make it better suit your taste?

> You also do not offer any support for any of the magic numbers..

I chose 4 for PV_KICK_AHEAD_MAX as I didn't see much performance
difference when I did a kick-ahead of 5. Also, it may be too unfair to
the vCPU that was doing the kicking if the number is too big. Another
magic number is pv_kick_ahead number. This one is kind of arbitrary.
Right now I do a log2, but it can be divided by 4 (rshift 2) as well.

Cheers,
Longman