Re: [PATCH 1/4] mac80211: Switch to a virtual time-based airtime scheduler - Toke Høiland-Jørgensen

From: "Toke Høiland-Jørgensen" <toke@redhat.com>
To: Yibo Zhao <yiboz@codeaurora.org>, ath10k@lists.infradead.org
Cc: linux-wireless@vger.kernel.org
Subject: Re: [PATCH 1/4] mac80211: Switch to a virtual time-based airtime scheduler
Date: Tue, 17 Sep 2019 23:31:08 +0200	[thread overview]
Message-ID: <87ftkuip9f.fsf@toke.dk> (raw)
In-Reply-To: <1568639388-27291-1-git-send-email-yiboz@codeaurora.org>

Yibo Zhao <yiboz@codeaurora.org> writes:

> From: Toke Høiland-Jørgensen <toke@redhat.com>
>
> This switches the airtime scheduler in mac80211 to use a virtual time-based
> scheduler instead of the round-robin scheduler used before. This has a
> couple of advantages:
>
> - No need to sync up the round-robin scheduler in firmware/hardware with
>   the round-robin airtime scheduler.
>
> - If several stations are eligible for transmission we can schedule both of
>   them; no need to hard-block the scheduling rotation until the head of the
>   queue has used up its quantum.
>
> - The check of whether a station is eligible for transmission becomes
>   simpler (in ieee80211_txq_may_transmit()).
>
> The drawback is that scheduling becomes slightly more expensive, as we need
> to maintain an rbtree of TXQs sorted by virtual time. This means that
> ieee80211_register_airtime() becomes O(logN) in the number of currently
> scheduled TXQs. However, hopefully this number rarely grows too big (it's
> only TXQs currently backlogged, not all associated stations), so it
> shouldn't be too big of an issue.
>
> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>

I'll note that this patch still has the two issues that Felix pointed
out when I posted the RFC version. Namely:

- The use of divisions in the fast path. I guess I need to go write some
  reciprocal-calculation code, since that is also an issue with the AQL
  patches I linked to before.

- The fact that we don't count the airtime usage of multicast traffic,
  which with this series means that the vif TXQ will get priority over
  the others. I think we agreed to fix this by just adding an airtime
  v_t to the vif as well and use that for scheduling the TXQ. Does
  ath10k report airtime usage for multicast as well, or only for
  stations?

-Toke

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