Re: [PATCH] block/ps3: Fix slow VRAM IO

Andrew Morton
2009-11-03T08:24:13Z

On Mon, 19 Oct 2009 12:58:27 -0700 Geoff Levand <geoffrey.levand@am.sony.com> wrote:

>
> From: Hideyuki Sasaki <Hideyuki_Sasaki@hq.scei.sony.co.jp>
>
> The current PS3 VRAM driver uses msleep() to wait for completion
> of RSX DMA transfers between system memory and VRAM. Depending
> on the system timing, the processing delay and overhead of this
> msleep() call can significantly impact VRAM driver IO.
>
> To avoid the condition, add a short duration (200 usec max)
> udelay() polling loop before entering the msleep() polling
> loop.
>

When raising a performance-based patch, please always try to include
before-and-after performance measurements in the changelog. People
want to know the magnitude of the improvement.

>
> drivers/block/ps3vram.c | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
>
> --- a/drivers/block/ps3vram.c
> +++ b/drivers/block/ps3vram.c
> @@ -123,7 +123,15 @@ static int ps3vram_notifier_wait(struct
> {
> struct ps3vram_priv *priv = ps3_system_bus_get_drvdata(dev);
> u32 *notify = ps3vram_get_notifier(priv->reports, NOTIFIER);
> - unsigned long timeout = jiffies + msecs_to_jiffies(timeout_ms);
> + unsigned long timeout;
> +
> + for (timeout = 20; timeout; timeout--) {

for (timeout = 0; timeout < 20; timeout++) {

would be simpler.

> + if (!notify[3])
> + return 0;
> + udelay(10);
> + }

You might as well do a udelay(1) here. The additional cost will be
negligible, and it will reduce latency.

> + timeout = jiffies + msecs_to_jiffies(timeout_ms);

The maximum latency is now timout_ms + 200usec.

That's OK with the current constants, but if someone later changes a
constant, the error could become significant.

Perhaps that isn't worth bothering about though.

> do {
> if (!notify[3])