Re: [PATCH] tty: Fix low_latency BUG

[Feng Tang]

Hi Peter,

2014-02-22 20:31 GMT+08:00 Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>:
> The user-settable knob, low_latency, has been the source of
> several BUG reports which stem from flush_to_ldisc() running
> in interrupt context. Since 3.12, which added several sleeping
> locks (termios_rwsem and buf->lock) to the input processing path,
> the frequency of these BUG reports has increased.
>
> Note that changes in 3.12 did not introduce this regression;
> sleeping locks were first added to the input processing path
> with the removal of the BKL from N_TTY in commit
> a88a69c91256418c5907c2f1f8a0ec0a36f9e6cc,
> 'n_tty: Fix loss of echoed characters and remove bkl from n_tty'
> and later in commit 38db89799bdf11625a831c5af33938dcb11908b6,
> 'tty: throttling race fix'. Since those changes, executing
> flush_to_ldisc() in interrupt_context (ie, low_latency set), is unsafe.
>
> However, since most devices do not validate if the low_latency
> setting is appropriate for the context (process or interrupt) in
> which they receive data, some reports are due to misconfiguration.
> Further, serial dma devices for which dma fails, resort to
> interrupt receiving as a backup without resetting low_latency.
>
> Historically, low_latency was used to force wake-up the reading
> process rather than wait for the next scheduler tick. The
> effect was to trim multiple milliseconds of latency from
> when the process would receive new data.
>
> Recent tests [1] have shown that the reading process now receives
> data with only 10's of microseconds latency without low_latency set.

The 10's of miscroseconds is ok for 115200 bps like device, but it may
hurt the high speed device like Bluetooth which runs at 3M/4M bps or
higher.

More and more smartphones are using uart as the Bluetooth data
interface due to its low-pin, low-power feature, and many of them
are using HZ=100 kernel, I'm afraid this added delay may cause
some problem.

Thanks,
Feng
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