Re: [PATCH] ath10k: remove TX lock from ath10k_htt_tx_inc_pending

[Erik Stromdahl <erik.stromdahl@gmail.com>]

On 8/26/19 4:44 AM, Wen Gong wrote:
>> -----Original Message-----
>> From: ath10k <ath10k-bounces@lists.infradead.org> On Behalf Of Erik
>> Stromdahl
>> Sent: Saturday, August 24, 2019 9:49 PM
>> To: Kalle Valo <kvalo@qca.qualcomm.com>; linux-wireless@vger.kernel.org;
>> ath10k@lists.infradead.org
>> Cc: Erik Stromdahl <erik.stromdahl@gmail.com>
>> Subject: [EXT] [PATCH] ath10k: remove TX lock from
>> ath10k_htt_tx_inc_pending
>>
>> This commit removes the call to ath10k_mac_tx_lock() from
>> ath10k_htt_tx_inc_pending() in case the high water mark is reached.
>>
>> ath10k_mac_tx_lock() calls ieee80211_stop_queues() in order to stop
>> mac80211 from pushing more TX data to the driver (this is the TX lock).
>>
>> If a driver is trying to fetch an skb from a queue while the queue is
>> stopped, ieee80211_tx_dequeue() will return NULL.
>>
>> So, in ath10k_mac_tx_push_txq(), there is a risk that the call to
>> ath10k_htt_tx_inc_pending() results in a stop of the mac80211 TX queues
>> just before the skb is fetched.
>>
>> This will cause ieee80211_tx_dequeue() to return NULL and
>> ath10k_mac_tx_push_txq() to exit prematurely and return -ENOENT.
>> Before the function returns ath10k_htt_tx_dec_pending() will be called.
>> This call will re-enable the TX queues through ath10k_mac_tx_unlock().
>> When ath10k_mac_tx_push_txq() has returned, the TX queue will be
>> returned back to mac80211 with ieee80211_return_txq() without the skb
>> being properly consumed.
>>
>> Since the TX queues were re-enabled in the error exit path of
>> ath10k_mac_tx_push_txq(), mac80211 can continue pushing data to the
>> driver. If the hardware does not consume the data, the above mentioned
>> case will be repeated over and over.
>>
>> A case when the hardware is not able to transmit the data from the host
>> is when a STA has been dis-associated from an AP and has not yet been
>> able to re-associate. In this case there will be no TX_COMPL_INDs from
>> the hardware, resulting in the TX counter not be decremented.
>>
>> This phenomenon has been observed in both a real and a test setup.
>>
>> In order to fix this, the actual TX locking (the call to
>> ath10k_mac_tx_lock()) was removed from ath10k_htt_tx_inc_pending().
>> Instead, ath10k_mac_tx_lock() is called separately after the skb has
>> been fetched (after the call to ieee80211_tx_dequeue()). At this point
>> it is OK to stop the queues.
> Is this patch will impact throughput?
No, I haven't seen any performance degradation.

I can't see that it should impact the throughput even in theory (rather the opposite
actually since we reduce the likelihood of dropping packets).

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