On 25/09/2023 16:20, Anton Ivanov wrote:
On 25/09/2023 15:44, Johannes Berg wrote:
On Mon, 2023-09-25 at 15:27 +0100, Anton Ivanov wrote:
On 25/09/2023 14:33, Johannes Berg wrote:
On Mon, 2023-09-25 at 14:29 +0100, Anton Ivanov wrote:
I have rebased the preempt patch on top of these series.
PREEMPT works with some performance decrease.
VOLUNTARY deadlocks early in boot around the time it starts loading modules.
non-preemptible deadlocks very early in boot.
Well I guess that means there's still some issue in here? Hmm.
Now I don't understand anything anymore, I guess.
PEBKAC. The tree got corrupted somewhere during rebase. Reapplying everything
on top of a clean master fixed it.
So it all works.
OK, whew. At least now I no longer _completely_ doubt the mental model I
have of UML VM :-)
With some performance penalties compared to the old approach, but works.
I still find this odd though, I don't see what the flush would possibly
do in a new (mm host) process that's not achievable in arch_dup_mmap()?
OK, so let's see - arch_dup_mmap() is _earlier_ than the fork_handler,
because that only happens on the very first switch into the process.
This is only when it gets scheduled. So we'd be looking for something
that copy_process() changes in the MM after copy_mm() and before it can
get scheduled?
I guess we could even move the flush it into copy_thread(), which is a
simpler patch too, but it felt a bit wrong, since that's about the
(guest!) process, not the mm.
But basically I don't see anything there - fork syscall tail-calls
kernel_clone(), which doesn't really do anything with the result of
copy_process() except wake it up, and copy_process() doesn't really do
anything either?
I am continuing to dig through that and looking for voluntary preemption points
in the process.
We have none of our own and the generic ones are not invoked - voluntary for
all practical purposes does not differ from no-preempt.
If I have some suspects, I will post their mugshots to the list :)
For the time being it is mostly negative :)
1. The performance after the mm patch is down. By 30-40% on my standard bench.
2. The preemption patches work fine on top (all 3 cases). The performance
difference stays.
3. We do not have anything of value to add in term of cond_resched() to the
drivers :(
Most drivers are fairly simplistic with no safe points to add this.
4. The upper layer resched points are not enough.
F.E. -EAGAIN from block, retries, etc should all hit paths in the upper layer
which cause a
cond_resched(). They are not noticeable. VOLUNTARY behaves nearly identical to
non-preemptive.
5. There are a couple of places where an if (condition) schedule() should
probably be replaced
with cond_resched(). F.E. interrupt_end(). While it looks more appropriate, the
performance
effect is nil.
6. Do we still need force_flush_all() in the arch_dup_mmap()? This works with a
non-forced tlb flush
using flush_tlb_mm(mm);
7. In all cases, UML is doing something silly.
The CPU usage while doing find -type f -exec cat {} > /dev/null measured from
outside in non-preemptive and
PREEMPT_VOLUNTARY stays around 8-15%. The UML takes a sabbatical for the
remaining 85 instead of actually
doing work. PREEMPT is slightly better at 60, but still far from 100%. It just
keeps going into idle and I
cannot understand why.
8. All in all I do not have an obvious suspect for the performance difference.
No change candidates
either. I also do not have any ideas why it ends up in idle instead of doing
work.
johannes
--
Anton R. Ivanov
Cambridgegreys Limited. Registered in England. Company Number 10273661
https://www.cambridgegreys.com/
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