Hello Julien,
On 20.11.18 20:47, Julien Grall wrote:
On 20/11/2018 18:10, Andrii Anisov wrote:
Hello Julien,
On 19.11.18 18:42, Julien Grall wrote:
There are no issue about processing IRQs before the syncs. It is the
same as if an IRQ was raised from ila different pCPUs.
So why do you need that?
From my understanding of gic-vgic code (old vgic), for the IRQs
targeting the `current` vcpu, it leads to a faster processing under
interrupts storm conditions. If it was all LRs set on previous switch
to a guest, a the IRQ will have a chance to go directly to LR instead
of setting on lr_pending queue. Also inflight_irqs queue have a chance
to be shorter to insert.
Do you have actual numbers?
Unfortunately, my numbers are pretty indirect. I'm referring glmark2
benchmark results. With this and the rest of my changes (not yet
published), I can cut out another percent or two of performance drop due
to XEN existence in the system. BTW, that's why I recently asked Stefano
about his approach of interrupt latency measurement.
On my board that benchmark processing causes at least 4 different HW
interrupts issuing with different frequency. Adding the reschedule IRQ
makes the system tend to not fit all IRQs into 4 LRs available in my
GIC. Moreover, the benchmark does not emit a network traffic or disk
usage during the run. So real life cases will add more concurrent IRQs.
Also to be on the same page, what is your
definition of interrupts storm?
I mean the system takes different interrupts (more IRQ sources than LRs
available) with a relatively high rate. Let's say more than 7000
interrupts per second. It's not very big number, but close to what I see
on my desk.
Bear in mind that the old vGIC will be phased out soon.
As I remember a new vgic experimental yet. Do not support GIC-v3 yet.
If you are
worried about performance, then I would recommend to try the new vGIC
and see whether it improves.
You know, we are based on XEN 4.10. Initially, when a customer said
about their dissatisfaction about performance drop in benchmark due to
XEN existence, I tried 4.12-unstable, both an old and a new VGIC. So
performance with 4.12-unstable with the old VGIC was worse than 4.10,
and the new VGIC made things even much worse. I can't remember the exact
numbers or proportions, but that was the reason we do not offer
upgrading XEN yet.
Well, if you re-enable the interrupts you give a chance for higher
priority interrupts to come up. This will not happen if you have
interrupts disabled.
I understand the theory, but can not match it with the current XEN code.
Guest interrupts prioritization within do_IRQ is pretty meaningless.
They will go through the same path. And an effect would not be seen
before exiting to a guest.
The PPI interrupts are reflected into the processing of soft IRQs or
injecting an IRQ into queues. So it does not matter much when exactly we
do read the IRQ from IAR in a gic_interrupt loop. I suppose it should be
faster to loop through gic_interrupt at once, collecting all interrupts,
without going through exception path, then switch to soft IRQs
processing in leave_hypervisor_tail.
The only thing which might get a noticeable effect here is serving
GIC_SGI_CALL_FUNCTION, which is executed right away from `gic_interrupt`.
But you seem to base your assumption on interrupts storm (yet to be
defined). If you have an interrupt storm, then you are already doomed as
your guest/Xen will not have time to do any other work.
In any case, you need to provide number to support your optimization.I'm moving all my patches to current staging and would send them as RFC
with a description of why is it done and how I measured results.
--
Sincerely,
Andrii Anisov.
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