Re: [PATCH 11/14] Revert "crypto: caam - get rid of tasklet"

[Thomas Gleixner <tglx@linutronix.de>]

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On Wed, 9 Nov 2016, Russell King - ARM Linux wrote:
> Please include Thomas in this.

Thanks!
 
> On Wed, Nov 09, 2016 at 10:46:21AM +0200, Horia Geantă wrote:
> > This reverts commit 66d2e2028091a074aa1290d2eeda5ddb1a6c329c.
> > 
> > Quoting from Russell's findings:
> > https://www.mail-archive.com/linux-crypto@vger.kernel.org/msg21136.html
> > 
> > [quote]
> > Okay, I've re-tested, using a different way of measuring, because using
> > openssl speed is impractical for off-loaded engines.  I've decided to
> > use this way to measure the performance:
> > 
> > dd if=/dev/zero bs=1048576 count=128 | /usr/bin/time openssl dgst -md5
> > 
> > For the threaded IRQs case gives:
> > 
> > 0.05user 2.74system 0:05.30elapsed 52%CPU (0avgtext+0avgdata 2400maxresident)k
> > 0.06user 2.52system 0:05.18elapsed 49%CPU (0avgtext+0avgdata 2404maxresident)k
> > 0.12user 2.60system 0:05.61elapsed 48%CPU (0avgtext+0avgdata 2460maxresident)k
> > 	=> 5.36s => 25.0MB/s
> > 
> > and the tasklet case:
> > 
> > 0.08user 2.53system 0:04.83elapsed 54%CPU (0avgtext+0avgdata 2468maxresident)k
> > 0.09user 2.47system 0:05.16elapsed 49%CPU (0avgtext+0avgdata 2368maxresident)k
> > 0.10user 2.51system 0:04.87elapsed 53%CPU (0avgtext+0avgdata 2460maxresident)k
> > 	=> 4.95 => 27.1MB/s
> > 
> > which corresponds to an 8% slowdown for the threaded IRQ case.  So,
> > tasklets are indeed faster than threaded IRQs.

Each operation involves:

 submit job
 hard irq handler
 wakeup irq thread
 context switch
 wakeup openssl
 context switch

and that repeats over and over.

Russell has provided me traces and the extra wakeup/context switch is what
causes the slowdown vs. the tasklet case which has only one wakeup/context
switch, unless it gets outsourced to ksoftirqd which might get even slower
than the threaded handler.

You might add something like that to the changelog so it's not just a
number comparison which does not explain why this happens.

Thanks,

	tglx