mac80211: don't put null-data frames on the normal TXQ

Commit Message

Johannes Berg July 3, 2018, 12:47 p.m. UTC

From: Johannes Berg <[e-mail]>

Since (QoS) NDP frames shouldn't be put into aggregation nor are
assigned real sequence numbers, etc. it's better to treat them as
non-data packets and not put them on the normal TXQs, for example
when building A-MPDUs they need to be treated specially, and they
are more used for management (e.g. to see if the station is alive)
anyway.

Signed-off-by: Johannes Berg <[e-mail]>
---
 net/mac80211/tx.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Toke Høiland-Jørgensen July 3, 2018, 2:31 p.m. UTC | #1

Johannes Berg <[e-mail]> writes:

> From: Johannes Berg <[e-mail]>
>
> Since (QoS) NDP frames shouldn't be put into aggregation nor are
> assigned real sequence numbers, etc. it's better to treat them as
> non-data packets and not put them on the normal TXQs, for example
> when building A-MPDUs they need to be treated specially, and they
> are more used for management (e.g. to see if the station is alive)
> anyway.

No objections to this per se; but didn't we want to move towards
everything going through the TXQs? Any progress on that front? :)

-Toke

Johannes Berg July 3, 2018, 2:32 p.m. UTC | #2

On Tue, 2018-07-03 at 16:31 +0200, Toke Høiland-Jørgensen wrote:
> Johannes Berg <[e-mail]> writes:
> 
> > From: Johannes Berg <[e-mail]>
> > 
> > Since (QoS) NDP frames shouldn't be put into aggregation nor are
> > assigned real sequence numbers, etc. it's better to treat them as
> > non-data packets and not put them on the normal TXQs, for example
> > when building A-MPDUs they need to be treated specially, and they
> > are more used for management (e.g. to see if the station is alive)
> > anyway.
> 
> No objections to this per se;

:-)

> but didn't we want to move towards
> everything going through the TXQs? Any progress on that front? :)

Not really. Yes, I wanted to, but it's some massive surgery. Right now
I'm working on converting iwlwifi, perhaps I'll learn about it more and
can then do the mac80211 surgery better later.

johannes

Toke Høiland-Jørgensen July 3, 2018, 2:36 p.m. UTC | #3

Johannes Berg <[e-mail]> writes:

> On Tue, 2018-07-03 at 16:31 +0200, Toke Høiland-Jørgensen wrote:
>> Johannes Berg <[e-mail]> writes:
>> 
>> > From: Johannes Berg <[e-mail]>
>> > 
>> > Since (QoS) NDP frames shouldn't be put into aggregation nor are
>> > assigned real sequence numbers, etc. it's better to treat them as
>> > non-data packets and not put them on the normal TXQs, for example
>> > when building A-MPDUs they need to be treated specially, and they
>> > are more used for management (e.g. to see if the station is alive)
>> > anyway.
>> 
>> No objections to this per se;
>
> :-)
>
>> but didn't we want to move towards
>> everything going through the TXQs? Any progress on that front? :)
>
> Not really. Yes, I wanted to, but it's some massive surgery. Right now
> I'm working on converting iwlwifi, perhaps I'll learn about it more
> and can then do the mac80211 surgery better later.

Right, sounds good! Looking forward to the iwlwifi conversion :)

-Toke

Peter Oh July 3, 2018, 8:40 p.m. UTC | #4

On 07/03/2018 05:47 AM, Johannes Berg wrote:
> From: Johannes Berg <[e-mail]>
>
> Since (QoS) NDP frames shouldn't be put into aggregation nor are
> assigned real sequence numbers, etc. it's better to treat them as
> non-data packets and not put them on the normal TXQs, for example
> when building A-MPDUs they need to be treated specially,
To be treated specially at which layer, mac80211 or drivers?
Are you seeing any issues other than complexity of handling NDP or is it 
just improvement?

Thanks,
Peter

Johannes Berg July 3, 2018, 11:48 p.m. UTC | #5

On Tue, 2018-07-03 at 13:40 -0700, Peter Oh wrote:
> 
> On 07/03/2018 05:47 AM, Johannes Berg wrote:
> > From: Johannes Berg <[e-mail]>
> > 
> > Since (QoS) NDP frames shouldn't be put into aggregation nor are
> > assigned real sequence numbers, etc. it's better to treat them as
> > non-data packets and not put them on the normal TXQs, for example
> > when building A-MPDUs they need to be treated specially,
> 
> To be treated specially at which layer, mac80211 or drivers?

They cannot be put into an A-MPDU, so you need to skip them when
building A-MPDUs.

> Are you seeing any issues other than complexity of handling NDP or is it 
> just improvement?

I'm not actually running any hardware on my development setup that would
use TXQs today. However, I'm starting to work on converting iwlwifi to
it, and if we put the NDPs on there it means we no longer can schedule
the TXQ to a single hardware queue.

Similarly, I think for other drivers it would be a complexity reduction
and possibly performance improvement with aggregation because you no
longer need to check if the next frame is an NDP and if yes, finish the
open A-MPDU and put both frames on the HW queue.

That said, I hadn't looked much at the drivers. Seems the situation is
worse than I thought, with those not doing it so well.

ath10k appears to not do aggregation in the host, and I guess the data
isn't split over multiple queues so the firmware has to determine/buffer
it some other way. No idea how that would work.

I see no evidence of ath9k doing this correctly, so this might fix a bug
there, but I may have missed it.

mt76 also appears to behave erroneously: if the txq is marked with
aggregation it will even update the mtxq->agg_ssn to 0x10 for QoS NDPs,
because those always have seqno 0; in mt76_check_agg_ssn:
	mtxq->agg_ssn = le16_to_cpu(hdr->seq_ctrl) + 0x10;

johannes

Ben Greear July 4, 2018, 4:24 a.m. UTC | #6

On 07/03/2018 04:48 PM, Johannes Berg wrote:
> On Tue, 2018-07-03 at 13:40 -0700, Peter Oh wrote:
>>
>> On 07/03/2018 05:47 AM, Johannes Berg wrote:
>>> From: Johannes Berg <[e-mail]>
>>>
>>> Since (QoS) NDP frames shouldn't be put into aggregation nor are
>>> assigned real sequence numbers, etc. it's better to treat them as
>>> non-data packets and not put them on the normal TXQs, for example
>>> when building A-MPDUs they need to be treated specially,
>>
>> To be treated specially at which layer, mac80211 or drivers?
>
> They cannot be put into an A-MPDU, so you need to skip them when
> building A-MPDUs.
>
>> Are you seeing any issues other than complexity of handling NDP or is it
>> just improvement?
>
> I'm not actually running any hardware on my development setup that would
> use TXQs today. However, I'm starting to work on converting iwlwifi to
> it, and if we put the NDPs on there it means we no longer can schedule
> the TXQ to a single hardware queue.
>
> Similarly, I think for other drivers it would be a complexity reduction
> and possibly performance improvement with aggregation because you no
> longer need to check if the next frame is an NDP and if yes, finish the
> open A-MPDU and put both frames on the HW queue.
>
> That said, I hadn't looked much at the drivers. Seems the situation is
> worse than I thought, with those not doing it so well.
>
> ath10k appears to not do aggregation in the host, and I guess the data
> isn't split over multiple queues so the firmware has to determine/buffer
> it some other way. No idea how that would work.

This is my current understanding of ath10k..hope it helps.

ath10k firmware does handle the aggregation.  It doesn't pay much attention
to the driver's txqueues.  For wave-2 firmware, the firmware will
attempt to fetch frames for peers in a fair/optimal way, and that should
indirectly take the txqueues into account.

Wave-1 does not do any of the prefetch logic as far as I know.

Stock firmware sends mgt frames through an entirely different tx path,
while my ath10k-ct firmware can send all frames through the same 'htt'
transmit path.  Either way, the firmware has final control over what
goes to what tid and what is aggregated.

That said, there are memory use-after-free and other bugs related to
txq in ath10k.  Hopefully it will be fixed by the stop-txqueue patch
that has been in recent discussion.

Thanks,
Ben

> I see no evidence of ath9k doing this correctly, so this might fix a bug
> there, but I may have missed it.
>
> mt76 also appears to behave erroneously: if the txq is marked with
> aggregation it will even update the mtxq->agg_ssn to 0x10 for QoS NDPs,
> because those always have seqno 0; in mt76_check_agg_ssn:
> 	mtxq->agg_ssn = le16_to_cpu(hdr->seq_ctrl) + 0x10;
>
> johannes
>

Johannes Berg July 4, 2018, 7:12 a.m. UTC | #7

On Tue, 2018-07-03 at 21:24 -0700, Ben Greear wrote:

> > ath10k appears to not do aggregation in the host, and I guess the data
> > isn't split over multiple queues so the firmware has to determine/buffer
> > it some other way. No idea how that would work.
> 
> This is my current understanding of ath10k..hope it helps.

[snip]

Thanks Ben. I guess we can't really know whether or not it would be
buggy with (QoS-)NDP, but it shouldn't be since all the frames are
expected (by firmware) to go through the same path anyhow and it's
responsible for putting them together into aggregates. And, unlike Intel
firmware/hardware, it can't necessarily assume that one HW queue
consists only of packets allowed for aggregation.

johannes

Toke Høiland-Jørgensen July 4, 2018, 7:26 a.m. UTC | #8

Johannes Berg <[e-mail]> writes:

> I see no evidence of ath9k doing this correctly, so this might fix a
> bug there, but I may have missed it.

ath9k does check for this, in ath_tx_sched_aggr() and in
ath_tx_form_aggr(); it'll check for the IEEE80211_TX_CTL_AMPDU flag, and
stop building the current aggregate if the flag is not set.

-Toke

Johannes Berg July 4, 2018, 7:54 a.m. UTC | #9

On Wed, 2018-07-04 at 09:26 +0200, Toke Høiland-Jørgensen wrote:
> Johannes Berg <[e-mail]> writes:
> 
> > I see no evidence of ath9k doing this correctly, so this might fix a
> > bug there, but I may have missed it.
> 
> ath9k does check for this, in ath_tx_sched_aggr() and in
> ath_tx_form_aggr(); it'll check for the IEEE80211_TX_CTL_AMPDU flag, and
> stop building the current aggregate if the flag is not set.

Ok, thanks. Nevertheless, I guess it's more efficient to not stop the
aggregate on encountering a (QoS-)NDP :-)

johannes

Toke Høiland-Jørgensen July 4, 2018, 8:25 a.m. UTC | #10

Johannes Berg <[e-mail]> writes:

> On Wed, 2018-07-04 at 09:26 +0200, Toke Høiland-Jørgensen wrote:
>> Johannes Berg <[e-mail]> writes:
>> 
>> > I see no evidence of ath9k doing this correctly, so this might fix a
>> > bug there, but I may have missed it.
>> 
>> ath9k does check for this, in ath_tx_sched_aggr() and in
>> ath_tx_form_aggr(); it'll check for the IEEE80211_TX_CTL_AMPDU flag, and
>> stop building the current aggregate if the flag is not set.
>
> Ok, thanks. Nevertheless, I guess it's more efficient to not stop the
> aggregate on encountering a (QoS-)NDP :-)

Oh, absolutely! :)

-Toke

Patch

diff --git a/net/mac80211/tx.c b/net/mac80211/tx.c
index fa1f1e63a264..5a60832052dc 100644
--- a/net/mac80211/tx.c
+++ b/net/mac80211/tx.c
@@ -1247,7 +1247,7 @@  static struct txq_info *ieee80211_get_txq(struct ieee80211_local *local,
 	    (info->control.flags & IEEE80211_TX_CTRL_PS_RESPONSE))
 		return NULL;
 
-	if (!ieee80211_is_data(hdr->frame_control))
+	if (!ieee80211_is_data_present(hdr->frame_control))
 		return NULL;
 
 	if (sta) {