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