linux-kernel - Re: locking changes in tty broke low latency feature

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Message-ID: <le7v07$9gn$1@ger.gmane.org>
Date:	Fri, 21 Feb 2014 16:31:03 +0000 (UTC)
From:	Grant Edwards <grant.b.edwards@...il.com>
To:	linux-kernel@...r.kernel.org
Cc:	linux-serial@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: Re: locking changes in tty broke low latency feature

On 2014-02-21, Peter Hurley <peter@...leysoftware.com> wrote:

> I think the consensus is to leave the low_latency facility in, but
> remove it's connection to the tty buffers.
>
> If the known-to-be-already-in-non-interrupt-context drivers want,
> I can add a different function for executing flush_to_ldisc()
> directly. But I don't want to do that without a use-case and test
> subject.

Three of the drivers I maintain have modes where they handle all rx
data in non-interrupt contexts, but I'm not convinced (or even
suspicious) that there would be any noticeable benefit from such a
function.  If, at some point in the future, it becomes apparent that
there is "too much latency" in certain cases then perhaps it can be
looked at again -- but I think doing it now is premature optimization.
That said, all things being equal, it would be nice to avoid anything
that would make such an addition impossible in the future.

>> First question though comes before all of this - and that is do we need
>> low_latency at all any more or is the current scheduling logic now good
>> enough to do the job anyway.
>
> Right.
>
> Based on my recent test, I think low_latency doesn't need to be a
> knob for the tty core.

I Agree: there doesn't seem to be any evidence that it's needed by the
tty/ldisc layer.

> Drivers can continue to use it to mess with their rx fifo settings
> and such like.

Excellent.  One of my serial_core drivers still has (in it's default
configuration) 10ms of latency that I can choose to eliminate on a
per-port bases (at the cost of extra CPU cycles) when the low_latency
flag is set.

> I plan on sending Greg a patch to do just that, probably this weekend.

Cool.  Thanks much for your attention to this.

-- 
Grant

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