how to use wait_event_interruptible_timeout with less than 1 jiffie timeout?

how to use wait_event_interruptible_timeout with less than 1 jiffie timeout?

[Daniel.]

Hi everybody,

I have some code that convert an timeout comming from userspace (in
ms) to jiffies prior passing to wait_event_interrupitible_timeout().
The code looks like this:

unsigned long tout = user.timeout * HZ / 1000; /* convert from ms to jiffie */
int status = wait_event_interruptible_timeout(wq, cond, tout);

I'm using HZ=100, the known default.
The problem is: with HZ = 100 and user.timeout = 10 I have:
10 * 100 / 1000 = 1,

This means that I can't use timeouts with less than 10ms. Is there any
way to circunvent this without changing HZ value?

Now let me explain why. I have a protocol running on linux that talk
to other embedded systems in an master-slave way. This communication
goes through some RF hardware. The communication is maded by master
transmitting a frame and receiving another frame from slave. The slave
takes about 2 ms to reply, but if the transmitting failed (because the
slave was not listening or because the frame was corrupted while
traveling through the air) the server has to transmit it again. Today
I'm using 10ms as timeout because is the smaller amount that I known
how to use, but if I can decrease this timeout to 2 or 3ms I would
have a better performance in case of failures.

Best regards,

-- 
"Do or do not. There is no try"
  Yoda Master

how to use wait_event_interruptible_timeout with less than 1 jiffie timeout?

[Anupam Kapoor]

>>>>> [2016-01-20T01:21:18+0530]: "Daniel." (Daniel):
,----[ Daniel ]
| I have some code that convert an timeout comming from userspace (in
| ms) to jiffies prior passing to wait_event_interrupitible_timeout().
| The code looks like this:
| 
| unsigned long tout = user.timeout * HZ / 1000; /* convert from ms to jiffie */
| int status = wait_event_interruptible_timeout(wq, cond, tout);
| 
| I'm using HZ=100, the known default.
| The problem is: with HZ = 100 and user.timeout = 10 I have:
| 10 * 100 / 1000 = 1,
| 
| This means that I can't use timeouts with less than 10ms. Is there any
| way to circunvent this without changing HZ value?
`----
wouldn't msleep_interruptible(...) be a better choice ? as a bonus, it
takes care of jiffies wrapping etc. 

-- 
kind regards
anupam

how to use wait_event_interruptible_timeout with less than 1 jiffie timeout?

[Nicholas Mc Guire]

On Wed, Jan 20, 2016 at 11:50:24AM +0530, Anupam Kapoor wrote:
> 
> >>>>> [2016-01-20T01:21:18+0530]: "Daniel." (Daniel):
> ,----[ Daniel ]
> | I have some code that convert an timeout comming from userspace (in
> | ms) to jiffies prior passing to wait_event_interrupitible_timeout().
> | The code looks like this:
> | 
> | unsigned long tout = user.timeout * HZ / 1000; /* convert from ms to jiffie */

that should be a  unsigned long tout = msecs_to_jiffies(user.timeout);

to ensure that all corner cases are properly handled, e.g. for HZ=100 the
above would be 0 for user.timeout < 10 which is probably not what you want.

> | int status = wait_event_interruptible_timeout(wq, cond, tout);
> | 
> | I'm using HZ=100, the known default.
> | The problem is: with HZ = 100 and user.timeout = 10 I have:
> | 10 * 100 / 1000 = 1,
> | 
> | This means that I can't use timeouts with less than 10ms. Is there any
> | way to circunvent this without changing HZ value?
> `----
> wouldn't msleep_interruptible(...) be a better choice ? as a bonus, it
> takes care of jiffies wrapping etc. 
> 

that would still have the same limitation with resepct to time granuarlity
and as Documentation/timers/

if you really need very short delays then you will need to resort to hrtimers
(and thus be to some extent config dependent) usleep_range(min, max) is the 
prefered interface in that case.

thx!
hofrat 

how to use wait_event_interruptible_timeout with less than 1 jiffie timeout?

[Anupam Kapoor]

>>>>> [2016-01-20T13:55:54+0530]: "Nicholas Mc Guire" (Nicholas):
,----[ Nicholas ]
| that would still have the same limitation with resepct to time granuarlity
| and as Documentation/timers/
`----
ah yes, that's true.

,----[ Nicholas ]
| if you really need very short delays then you will need to resort to hrtimers
| (and thus be to some extent config dependent) usleep_range(min, max) is the 
| prefered interface in that case.
`----
any specific reason why schedule_hrtimeout(...) cannot be used ?

-- 
kind regards
anupam

how to use wait_event_interruptible_timeout with less than 1 jiffie timeout?

[Nicholas Mc Guire]

On Wed, Jan 20, 2016 at 06:57:27PM +0530, Anupam Kapoor wrote:
> 
> >>>>> [2016-01-20T13:55:54+0530]: "Nicholas Mc Guire" (Nicholas):
> ,----[ Nicholas ]
> | that would still have the same limitation with resepct to time granuarlity
> | and as Documentation/timers/
> `----
> ah yes, that's true.
> 
> ,----[ Nicholas ]
> | if you really need very short delays then you will need to resort to hrtimers
> | (and thus be to some extent config dependent) usleep_range(min, max) is the 
> | prefered interface in that case.
> `----
> any specific reason why schedule_hrtimeout(...) cannot be used ?
>
It can, but same thing, unless you really need a very specific timeout
it is better to pass a range so that the kernel can consolidate the timeout 
list - so the _range is a system level optimization.

so schedule_hrtimeout_range() would be prefered

<snip>
 * The @delta argument gives the kernel the freedom to schedule the
 * actual wakeup to a time that is both power and performance friendly.
 * The kernel give the normal best effort behavior for "@expires+ at delta",
 * but may decide to fire the timer earlier, but no earlier than @expires.
<snip>

and you need to convert the timeout to ktime e.g. with ms_to_ktime()

thx!
hofrat