linux-kernel - Re: [PATCH 1/3] serial: qcom-geni: fix hard lockup on buffer flush

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Message-ID: <ZnraAlR9QeYhd628@hovoldconsulting.com>
Date: Tue, 25 Jun 2024 16:53:54 +0200
From: Johan Hovold <johan@...nel.org>
To: Doug Anderson <dianders@...omium.org>
Cc: Johan Hovold <johan+linaro@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>,
	Konrad Dybcio <konrad.dybcio@...aro.org>,
	Bjorn Andersson <andersson@...nel.org>,
	linux-arm-msm@...r.kernel.org, linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] serial: qcom-geni: fix hard lockup on buffer flush

On Mon, Jun 24, 2024 at 01:45:17PM -0700, Doug Anderson wrote:

> Also: if we're looking at quick/easy to land and just fix the hard
> lockup, I'd vote for this (I can send a real patch, though I'm about
> to go on vacation):
> 
> --
> 
> @@ -904,8 +904,8 @@ static void qcom_geni_serial_handle_tx_fifo(struct
> uart_port *uport,
>                 goto out_write_wakeup;
> 
>         if (!port->tx_remaining) {
> -               qcom_geni_serial_setup_tx(uport, pending);
> -               port->tx_remaining = pending;
> +               port->tx_remaining = min(avail, pending);
> +               qcom_geni_serial_setup_tx(uport, port->tx_remaining);
> 
>                 irq_en = readl(uport->membase + SE_GENI_M_IRQ_EN);
>                 if (!(irq_en & M_TX_FIFO_WATERMARK_EN))
> 
> --
> 
> That will fix the hard lockup, is short and sweet, and also doesn't
> end up outputting NUL bytes.

Yeah, this might be a good stop gap even if performance suffers.

> I measured time with that. I've been testing with a file I created
> called "alphabet.txt" that just contains the letters A-Z repeated 3
> times followed by a "\n", over and over again. I think gmail will kill
> me with word wrapping, but basically:

> head -200 /var/alphabet.txt  | wc
>     200     200   15800
> 
> Before my patch I ran `time head -200 /var/alphabet.txt` and I got:
> 
> real    0m1.386s
> 
> After my patch I ran the same thing and got:
> 
> real    0m1.409s
> 
> So it's slower, but that's not 25% slower. I get 1.7% slower:
> 
> In [6]: (1.409 - 1.386) / 1.386 * 100
> Out[6]: 1.659451659451669
> 
> IMO that seems like a fine slowdown in order to avoid printing NUL bytes.

With my 500K dmesg file test I see a similar performance drop as with
your full series even if seems to behave slightly better (e.g. 20% drop
instead of 24%). 

Johan