On Tue, 2015-01-20 at 14:53 +0000, Markus Stockhausen wrote:
> > Von: Scott Wood [scottw...@freescale.com]
> > Gesendet: Dienstag, 20. Januar 2015 08:38
> > An: Markus Stockhausen
> > Cc: Michael Ellerman; linuxppc-dev@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.
"Some people don't care about that" doesn't imply "nobody 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.
That's bad. Don't create more bad stuff. :-)
> Can I rely on that implementation, or do I have to take special care
> because I'm only programming for a single core CPU?
Not sure what you mean here.
> 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);
Yes, it's sufficient. When you enable preemption it will check to see
whether there is a pending reschedule.
-Scott
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