Re: [PATCH 3/6] locking/rwsem: Rework writer wakeup

[Waiman Long <longman@redhat.com>]

On 3/20/23 04:12, Peter Zijlstra wrote:
> On Mon, Feb 27, 2023 at 03:16:25PM -0500, Waiman Long wrote:
>> On 2/27/23 05:31, Peter Zijlstra wrote:
>>>> I do have some concern that early lock transfer to a lock owner that has not
>>>> been woken up yet may suppress writer lock stealing from optimistic spinning
>>>> causing some performance regression in some cases. Let's see if the test
>>>> robot report anything.
>>> Ah yes, I suppose that is indeed a possibility. Given this is all under
>>> wait_lock and the spinner is not, I was hoping it would still have
>>> sufficient time to win. But yes, robots will tell us.
>>>
>> I run my rwsem locking microbenchmark on a 2-socket 96-thread x86-64
>> system with lock event turned on for 15 secs.
>>
>> Before this patchset:
>>
>> Running locktest with rwsem [runtime = 15s, r% = 50%, load = 100]
>> Threads = 96, Min/Mean/Max = 74,506/91,260/112,409
>> Threads = 96, Total Rate = 584,091 op/s; Percpu Rate = 6,084 op/s
>>
>> rwsem_opt_fail=127305
>> rwsem_opt_lock=4252147
>> rwsem_opt_nospin=28920
>> rwsem_rlock=2713129
>> rwsem_rlock_fail=0
>> rwsem_rlock_fast=5
>> rwsem_rlock_handoff=280
>> rwsem_rlock_steal=1486617
>> rwsem_sleep_reader=2713085
>> rwsem_sleep_writer=4313369
>> rwsem_wake_reader=29876
>> rwsem_wake_writer=5829160
>> rwsem_wlock=127305
>> rwsem_wlock_fail=0
>> rwsem_wlock_handoff=2515
>>
>> After this patchset:
>>
>> Running locktest with rwsem [runtime = 15s, r% = 50%, load = 100]
>> Threads = 96, Min/Mean/Max = 26,573/26,749/26,833
>> Threads = 96, Total Rate = 171,184 op/s; Percpu Rate = 1,783 op/s
>>
>> rwsem_opt_fail=1265481
>> rwsem_opt_lock=17939
>> rwsem_rlock=1266157
>> rwsem_rlock_fail=0
>> rwsem_rlock_fast=0
>> rwsem_rlock_handoff=0
>> rwsem_rlock_steal=551
>> rwsem_sleep_reader=1266157
>> rwsem_sleep_writer=1265481
>> rwsem_wake_reader=26612
>> rwsem_wake_writer=0
>> rwsem_wlock=1265481
>> rwsem_wlock_ehandoff=94
>> rwsem_wlock_fail=0
>> rwsem_wlock_handoff=94
>>
>> So the locking rate is reduced to just 29.3% of the original. Looking at
>> the number of successful writer lock stealings from optimistic spinning
>> (rwsem_opt_lock), it is reduced from 4252147 to 17939. It is just about
>> 0.4% of the original.
>>
>> So for workloads that have a lot of writer contention, there will be
>> performance regressions. Do you mind if we try to keep the original
>> logic of my patchset to allow write lock acquisition in writer slow
>> path, but transfer the lock ownership in the wakeup path when handoff
>> is required. We can do this with some minor code changes on top of your
>> current patchset.
> Urgh, sorry, I seem to have lost sight of this... those results,..
> sadness :/
>
> Yeah, I suppose there's nothing for it but to have live with that mess,
> be very sure to add comments eludicating any future poor sod reading it
> as to why the code is the way it is.

OK, I will add additional patches to your series to remediate the 
performance degradation. Hopefully, I am planning to get it done either 
by the end of the week or early next week.

Thanks,
Longman