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

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> 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 :)

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



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