Re: [Xen-ia64-devel] Question abouttime_interpolator_get_counter() in RHEL4

[Masaki Kanno <kanno.masaki@xxxxxxxxxxxxxx>]

Hi Alex,

Thanks for your detailed information. 
You are wonderful to remember for one year or more past. 

Best regards,
 Kan

>On Mon, 2007-01-15 at 19:02 +0900, Masaki Kanno wrote:
>> Hi Alex,
>> 
>> Could you comment a this issue? 
>> 
>> When we tested RHEL4 on domVTi of SMP, we met with hanging up of the 
>> RHEL4. We examined this issue with the xenctx. As a result, the each 
>> VCPUs of domVTi seemed to be looping in the following functions. 
>>  - A VCPU is looping in time_interpolator_get_counter(). 
>>  - Other VCPUs are looping in fsys_gettimeofday(). 
>> 
>> When we were examining this issue further, we found your patch about 
>> time_interpolator_get_counter(). 
>> 
>>   [PATCH] optimize writer path in time_interpolator_get_counter()
>>   http://lkml.org/lkml/2005/8/1/134
>
>Hi Kan,
>
>   That sounds like the right scenario.  Both of these code paths are
>using seqlocks to allow multiple CPUs to try to update the last_cycle
>value via a cmpxchg.  The optimization I added allows the CPU holding
>the write lock to update the last_cycle counter and exit without
>competing with the other CPUs in the cmpxchg.  This significantly
>reduces the contention caused by the cmpxchg and maintains correctness
>because the readers (on the fsys_gettimeofday path) cannot exit while a
>write seqlock is held.  I only saw a true live-lock in this code segment
>with a prototype processor, but the possibility for it certainly exists.
>I could see the scheduling of vCPUs potentially making this problem more
>likely.  Thanks,
>
>       Alex
>
>-- 
>Alex Williamson                             HP Open Source & Linux Org.



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