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From: Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>
Subject: Re: [PATCH v6 3/4] mac80211: Implement Airtime-based Queue Limit (AQL)
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Date: Sat, 09 Nov 2019 12:22:49 +0100
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To: Kan Yan <kyan@google.com>, Johannes Berg <johannes@sipsolutions.net>
Cc: Rajkumar Manoharan <rmanohar@codeaurora.org>, Kevin Hayes <kevinhayes@google.com>, Make-Wifi-fast <make-wifi-fast@lists.bufferbloat.net>, linux-wireless@vger.kernel.org, ath10k@lists.infradead.org, John Crispin <john@phrozen.org>, Lorenzo Bianconi <lorenzo@kernel.org>, Felix Fietkau <nbd@nbd.name>

Kan Yan <kyan@google.com> writes:

> It is most likely just insufficient locking. active_txq_lock is per
> AC, can't protect local->aql_total_pending_airtime against racing
> conditions:
> void ieee80211_sta_update_pending_airtime(...)
> {
>         spin_lock_bh(&local->active_txq_lock[ac]);
>         ...
>         local->aql_total_pending_airtime -= tx_airtime;
>         ...
>         spin_unlock_bh(&local->active_txq_lock[ac]);
> }

Ohh, right; didn't even realise those were not per-AC as well...

> After changing it to atomic_t, no more aql_total_pending_airtime
> underflow so far :). Using atomic operation should also help reduce
> CPU overhead due to lock contention, as
> ieee80211_sta_update_pending_airtime() is often called from the tx
> completion routine triggered by interrupts, often in a different core
> than where __ieee80211_schedule_txq() is running.
>
> I will post a new version a bit later if the test goes well.

Awesome! :)

-Toke


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