lkml.org 
[lkml]   [2020]   [Jan]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: Kernel-managed IRQ affinity (cont)
Hi Thomas,

On Tue, Jan 14, 2020 at 02:45:00PM +0100, Thomas Gleixner wrote:
> Ming,
>
> Ming Lei <ming.lei@redhat.com> writes:
> > On Fri, Jan 10, 2020 at 08:43:14PM +0100, Thomas Gleixner wrote:
> >> Ming Lei <ming.lei@redhat.com> writes:
> >> > That is why I try to exclude isolated CPUs from interrupt effective affinity,
> >> > turns out the approach is simple and doable.
> >>
> >> Yes, it's doable. But it still is inconsistent behaviour. Assume the
> >> following configuration:
> >>
> >> 8 CPUs CPU0,1 assigned for housekeeping
> >>
> >> With 8 queues the proposed change does nothing because each queue is
> >> mapped to exactly one CPU.
> >
> > That is expected behavior for this RT case, given userspace won't submit
> > IO from isolated CPUs.
>
> What is _this_ RT case? We really don't implement policy for a specific
> use case. If the kernel implements a policy then it has to be generally
> useful and practical.

Maybe the word of 'RT case' isn't accurate, I thought isolated CPUs is only
used for realtime cases, at least that is Peter's usage, maybe I was
wrong.

But it can be generic for all isolated CPUs cases, in which users
don't want managed interrupts to disturb the isolated CPU cores.

>
> >> With 4 queues you get the following:
> >>
> >> CPU0,1 queue 0
> >> CPU2,3 queue 1
> >> CPU4,5 queue 2
> >> CPU6,7 queue 3
> >>
> >> No effect on the isolated CPUs either.
> >>
> >> With 2 queues you get the following:
> >>
> >> CPU0,1,2,3 queue 0
> >> CPU4,5,6,7 queue 1
> >>
> >> So here the isolated CPUs 2 and 3 get the isolation, but 4-7
> >> not. That's perhaps intended, but definitely not documented.
> >
> > That is intentional change, given no IO will be submitted from 4-7
> > most of times in RT case, so it is fine to select effective CPU from
> > isolated CPUs in this case. As peter mentioned, IO may just be submitted
> > from isolated CPUs during booting. Once the system is setup, no IO
> > comes from isolated CPUs, then no interrupt is delivered to isolated
> > CPUs, then meet RT's requirement.
>
> Again. This is a specific usecase. Is this generally applicable?

As mentioned above, it can be applied for all isolated CPUs, when users
don't want managed interrupts to disturb these CPU cores.

>
> > We can document this change somewhere.
>
> Yes, this needs to be documented very clearly with that command line
> parameter.

OK, will do that in formal post.

Thanks,
Ming

\
 
 \ /
  Last update: 2020-01-15 00:39    [W:0.027 / U:26.412 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site