On 20/09/2023 13:30, Anton Ivanov wrote:
On 20/09/2023 12:14, Anton Ivanov wrote:
Added support for kernel side fpu store/restore and real
kernel_fpu_begin/kernel_fpu_end using gcc intrinsics. Enabled PREEMPT.
It boots and seems to be alive and kicking. I do not notice any
significant effect on performance either.
I take my words back - I was testing with VOLUNTARY which seems to
provide a difference withing the margin of error.
Enabling forced preemption drops the time for
find /usr -type f -exec cat {} > /dev/null \;
from
real 19m11.207s
user 0m0.180s
sys 2m41.160s
to
real 3m13.647s
user 0m0.000s
sys 2m58.270s
And boot time from 11.5s to 6s.
It did complain on shutdown though with some traces. Nothing while
running.
This is fixed by adding a preempt_enable/disable around the interrupt
low level. We were quite obviously missing it.
The patch will hit the mailing list shortly.
I will revise the patch and address the bug report on the config.debug.
How do I test it from here onwards?
The question still stands. Can we use anything as a specific test
suite? It is not an area I am familiar with, so any pointers will be
welcome.
I am happy to throw the patches on the list as an RFC, though I would
prefer to test first :)
However, this is about as much preemption as we can enable.
Trying to do what is suggested by Peter Zijlstra in the main PREMPT mail
thread on LKM does not work for UML. At all.
His suggestion is to add preempt_schedule_irq() in the IRQ exit code. On
UML that code is mixed with host signal handling and you cannot do that,
because the irq_enable/disable in that function touch the signal mask.
This makes a hash of the signal handling. In addition to that, we have
missing preempt_enable/disable and/or guards around this part. It has
some historical level of "reinvocation" and "reentrancy" behaviour and
it gets in the way. That will need to be untangled, fixed to match
preempt semantics and made to work. I'll be honest - I have no clue how
long this will take.
My suggestion is that we take the rest with relevant fpu register
store/restore fixes and the improvements it provides and leave the
preempt_schedule_irq() part for later.
--
Anton R. Ivanov
https://www.kot-begemot.co.uk/
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