How long am I allowed to disable preemption? (was How to make use of SPE instructions?)

[Markus Stockhausen]

> Von: Scott Wood [scottwood at freescale.com]
> Gesendet: Dienstag, 20. Januar 2015 08:38
> An: Markus Stockhausen
> Cc: Michael Ellerman; linuxppc-dev at lists.ozlabs.org
> Betreff: Re: AW: How to make use of SPE instructions?
> ...
> > I did some tests with the tcrypt module and I get a hashing speed of
> > ~ 46MByte/s for 2K data chunks. Stock module gives 29MByte/s. In
> > other words ~22,000 hashes per second. Overhead of the tcrypt data
> > feeder of around 10% included. That are worst case 46us per hash and
> > therefore 46us inside a non preemptive task.
> ...
> It's OK if the worst case is really 46 us, but if you can find a way to
> break it up a bit without affecting throughput too much, I'd do so.

Hi Scott (and all),

with your answer I'm a bit confused about disabling preemption inside the 
kernel right now. I understand that the interval between preempt_disable()
and preempt_enable() should not take too long. But when I look at other
kernel hash crypto modules I see that nobody really cares about that. Take
sha256_ssse3_update() in sha256_ssse3_glue.c for example. It runs

  kernel_fpu_begin();
  res = __sha256_ssse3_update(desc, data, len, partial);
  kernel_fpu_end();

The kernel_fpu_xxx() enables/disables preemption like I need to do. 
Nevertheless parameter len can be any number of bytes. Just take 
reasonable parameters of 256K of input data, a 3GHz core and 13 
cycles/byte of SHA256 throughput. That will be a 1ms timeframe.

Can I rely on that implementation, or do I have to take special care 
because I'm only programming for a single core CPU? 

With your advice I would place a enable/disable preemption call after 
1K of processed data. But wil that be sufficient if I only reeanble it
for a short timeframe like this:

  do {
    disable_preemption()
    ... calc hashes for 1K of data with 16.000 CPU cycles (or 20us) ...
    enable_preemption()
  while (dataleft>0);

Markus

-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: InterScan_Disclaimer.txt
URL: <http://lists.ozlabs.org/pipermail/linuxppc-dev/attachments/20150120/2d4a9056/attachment.txt>