Re: [PATCH 1/3] bfq: Avoid false bfq queue merging

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 




> Il giorno 11 giu 2020, alle ore 16:12, Paolo Valente <paolo.valente@xxxxxxxxxx> ha scritto:
> 
> 
> 
>> Il giorno 11 giu 2020, alle ore 10:31, Jan Kara <jack@xxxxxxx> ha scritto:
>> 
>> On Thu 11-06-20 09:13:07, Paolo Valente wrote:
>>> 
>>> 
>>>> Il giorno 5 giu 2020, alle ore 16:16, Jan Kara <jack@xxxxxxx> ha scritto:
>>>> 
>>>> bfq_setup_cooperator() uses bfqd->in_serv_last_pos so detect whether it
>>>> makes sense to merge current bfq queue with the in-service queue.
>>>> However if the in-service queue is freshly scheduled and didn't dispatch
>>>> any requests yet, bfqd->in_serv_last_pos is stale and contains value
>>>> from the previously scheduled bfq queue which can thus result in a bogus
>>>> decision that the two queues should be merged.
>>> 
>>> Good catch! 
>>> 
>>>> This bug can be observed
>>>> for example with the following fio jobfile:
>>>> 
>>>> [global]
>>>> direct=0
>>>> ioengine=sync
>>>> invalidate=1
>>>> size=1g
>>>> rw=read
>>>> 
>>>> [reader]
>>>> numjobs=4
>>>> directory=/mnt
>>>> 
>>>> where the 4 processes will end up in the one shared bfq queue although
>>>> they do IO to physically very distant files (for some reason I was able to
>>>> observe this only with slice_idle=1ms setting).
>>>> 
>>>> Fix the problem by invalidating bfqd->in_serv_last_pos when switching
>>>> in-service queue.
>>>> 
>>> 
>>> Apart from the nonexistent problem that even 0 is a valid LBA :)
>> 
>> Yes, I was also thinking about that and decided 0 is "good enough" :). But
>> I just as well just switch to (sector_t)-1 if you think it would be better.
>> 
> 
> 0 is ok :)
> 

Hi Jan,
I've finally tested this patch of yours. No regression.

Once again:
Acked-by: Paolo Valente <paolo.valente@xxxxxxxxxx>

Thanks,
Paolo

> Thanks,
> Paolo
> 
>>> Acked-by: Paolo Valente <paolo.valente@xxxxxxxxxx>
>> 
>> Thanks!
>> 
>> 								Honza
>> 
>> -- 
>> Jan Kara <jack@xxxxxxxx>
>> SUSE Labs, CR





[Index of Archives]     [Linux Kernel]     [Kernel Development Newbies]     [Linux USB Devel]     [Video for Linux]     [Linux Audio Users]     [Yosemite Hiking]     [Linux Kernel]     [Linux SCSI]

  Powered by Linux