Received-SPF: pass (google.com: best guess record for domain of linux-wireless-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
DMARC-Filter: OpenDMARC Filter v1.3.2 smtp.codeaurora.org 9B44E609F3
Content-Type: text/plain; charset="utf-8"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Subject: Re: [PATCH] ath10k: remove iteration in wake_tx_queue
From:   Kalle Valo <kvalo@codeaurora.org>
In-Reply-To: <20190327162906.6010-1-erik.stromdahl@gmail.com>
References: <20190327162906.6010-1-erik.stromdahl@gmail.com>
To:     Erik Stromdahl <erik.stromdahl@gmail.com>
Cc:     kvalo@qca.qualcomm.com, linux-wireless@vger.kernel.org,
        ath10k@lists.infradead.org, yiboz@codeaurora.org,
        Erik Stromdahl <erik.stromdahl@gmail.com>
User-Agent: pwcli/0.0.0-git (https://github.com/kvalo/pwcli/) Python/2.7.12
Message-Id: <20190925054107.935786119D@smtp.codeaurora.org>
Date:   Wed, 25 Sep 2019 05:41:07 +0000 (UTC)
Sender: linux-wireless-owner@vger.kernel.org
Precedence: bulk

Erik Stromdahl <erik.stromdahl@gmail.com> wrote:

> Iterating the TX queue and thereby dequeuing all available packets in the
> queue could result in performance penalties on some SMP systems.
> 
> The reason for this is most likely that the per-ac lock (active_txq_lock)
> in mac80211 will be held by the CPU iterating the current queue.
> 
> This will lock up other CPUs trying to push new messages on the TX
> queue.
> 
> Instead of iterating the queue we fetch just one packet at the time,
> resulting in minimal starvation of the other CPUs.
> 
> Reported-by: Yibo Zhao <yiboz@codeaurora.org>
> Signed-off-by: Erik Stromdahl <erik.stromdahl@gmail.com>

Like others, I'm not convinced about this either. To me it looks like a quick
workaround instead of properly investigating, and fixing, the root cause. To my
understanding the throughput dip was caused by this commit:

e3148cc5fecf ath10k: fix return value check in wake_tx_q op

So to me it feels like the issue has been there all along, just hidden, and the
fix above just exposed it.

-- 
https://patchwork.kernel.org/patch/10873753/

https://wireless.wiki.kernel.org/en/developers/documentation/submittingpatches