Re: [PATCH v5] powerpc/pseries/vas: Use usleep_range() to support HCALL delay

[Haren Myneni <haren@linux.ibm.com>]

On 1/11/24 9:27 AM, Nathan Lynch wrote:
> Haren Myneni <haren@linux.ibm.com> writes:
>> VAS allocate, modify and deallocate HCALLs returns
>> H_LONG_BUSY_ORDER_1_MSEC or H_LONG_BUSY_ORDER_10_MSEC for busy
>> delay and expects OS to reissue HCALL after that delay. But using
>> msleep() will often sleep at least 20 msecs even though the
>> hypervisor suggests OS reissue these HCALLs after 1 or 10msecs.
>>
>> The open and close VAS window functions hold mutex and then issue
>> these HCALLs. So these operations can take longer than the
>> necessary when multiple threads issue open or close window APIs
>> simultaneously, especially might affect the performance in the
>> case of repeat open/close APIs for each compression request.
>> On the large machine configuration which allows more simultaneous
>> open/close windows (Ex: 240 cores provides 4800 VAS credits), the
>> user can observe hung task traces in dmesg due to mutex contention
>> around open/close HCAlls.
> 
> Is this because the workload queues enough tasks on the mutex to trigger
> the hung task watchdog? With a threshold of 120 seconds, something on
> the order of ~6000 tasks each taking 20ms or more to traverse this
> critical section would cause the problem I think you're describing.
> 
> Presumably this change improves the situation, but the commit message
> isn't explicit. Have you measured the "throughput" of window open/close
> activity before and after? Anything that quantifies the improvement
> would be welcome.

Yes, tested on the large system which allows open/close 4800 windows at 
the same time (means 4800 tasks). Noticed sleep more than 20msecs for 
some tasks and getting hung traces for some tasks since the combined 
waiting timing is more then 120seconds. With this patch, the maximum 
sleep is 10msecs and did not see these traces on this system. I will add 
more description to the commit log.

Thanks
Haren