> Oh, right, I see. But in that case, should writing the default really > stomp on all the per-station values? If I set the value of a station, I > wouldn't expect it to change just because I changed the default value > afterwards? Will persevere the value for stations with customized queue limit in the next version. > > That's indeed not right. However, if a potential aql_tx_pending > > underflow case is detected here (It should never happen), reset it to > > 0 maybe not the best remedy anyway. I think it is better just > > WARN_ONCE() and skip updating aql_tx_pending all together, so the > > retry or loop can be avoided here. What do you think? > If we don't reset the value to zero may end up with a device that is > unable to transmit. Better to reset it I think, even if this is never > supposed to happen... I mean not updating the pending airtime to prevent it from going negative when the tx_airtime is larger than aql_tx_pending. Will reset it to 0 in next version, which is simpler and cleaner. On Wed, Nov 13, 2019 at 6:02 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > > Kan Yan <kyan@xxxxxxxxxx> writes: > > > Thanks for the review. I will pick up your new patches and give it a > > try tomorrow. > > > >> Why is this setting sta and device limits to the same value? > > > > local->aql_txq_limit_low is not the per device limit, but the default > > txq_limit for all STAs. Individual stations can be configured with > > non-default value via debugfs entry > > "netdev:interface_name_x/stations/mac_addr_x/airtime". "aql_threshold" > > is the device limit for switching between the lower and higher per > > station queue limit. > > Oh, right, I see. But in that case, should writing the default really > stomp on all the per-station values? If I set the value of a station, I > wouldn't expect it to change just because I changed the default value > afterwards? > > >> Also, are you sure we won't risk write tearing when writing 32-bit > >> values without locking on some architectures? > > > > Does mac80211 ever runs in any 16-bit architectures? Even in an > > architecture that write to 32-bit value is not atomic, I don't think > > there is any side-effect for queue limit get wrong transiently in rare > > occasions. Besides, the practical value of those queue limits should > > always fit into 16 bits. > > I'm not sure about the platform characteristics of all the weird tiny > MIPS boxes that run OpenWrt; which is why I'm vary of making any > assumptions that it is safe :) > > But yeah, I suppose you're right that since we're just setting the > limit, it is not going to be a huge concern here... > > >> I don't think this is right; another thread could do atomic_inc() > >> between the atomic_read() and atomic_set() here, in which case this > >> would clobber the other value. > >> I think to get this right the logic would need to be something like > >> this: > >> retry: > >> old = atomic_read(&sta->airtime[ac].aql_tx_pending); > >> if (warn_once(tx_airtime > old)) > >> new = 0; > >> else > >> new = old - tx_airtime; > >> if (atomic_cmpxchg(&sta->airtime[ac].aql_tx_pending, old, new) != old) > >> goto retry; > >> (or use an equivalent do/while). > > > > That's indeed not right. However, if a potential aql_tx_pending > > underflow case is detected here (It should never happen), reset it to > > 0 maybe not the best remedy anyway. I think it is better just > > WARN_ONCE() and skip updating aql_tx_pending all together, so the > > retry or loop can be avoided here. What do you think? > > If we don't reset the value to zero may end up with a device that is > unable to transmit. Better to reset it I think, even if this is never > supposed to happen... > > -Toke >
