[PATCH v4 0/3] Preventing job distribution to isolated CPUs

[Nitesh Narayan Lal]

This patch-set is originated from one of the patches that have been
posted earlier as a part of "Task_isolation" mode [1] patch series
by Alex Belits <abelits@xxxxxxxxxxx>. There are only a couple of
changes that I am proposing in this patch-set compared to what Alex
has posted earlier.
 
 
Context
=======
On a broad level, all three patches that are included in this patch
set are meant to improve the driver/library to respect isolated
CPUs by not pinning any job on it. Not doing so could impact
the latency values in RT use-cases.


Patches
=======
* Patch1:
  The first patch is meant to make cpumask_local_spread()
  aware of the isolated CPUs. It ensures that the CPUs that
  are returned by this API only includes housekeeping CPUs.

* Patch2:
  This patch ensures that a probe function that is called
  using work_on_cpu() doesn't run any task on an isolated CPU.

* Patch3:
  This patch makes store_rps_map() aware of the isolated
  CPUs so that rps don't queue any jobs on an isolated CPU. 


Proposed Changes
================
To fix the above-mentioned issues Alex has used housekeeping_cpumask().
The only changes that I am proposing here are:
- Removing the dependency on CONFIG_TASK_ISOLATION that was proposed by
  Alex. As it should be safe to rely on housekeeping_cpumask()
  even when we don't have any isolated CPUs and we want
  to fall back to using all available CPUs in any of the above scenarios.
- Using both HK_FLAG_DOMAIN and HK_FLAG_WQ in Patch2 & 3, this is
  because we would want the above fixes not only when we have isolcpus but
  also with something like systemd's CPU affinity.


Testing
=======
* Patch 1:
  Fix for cpumask_local_spread() is tested by creating VFs, loading
  iavf module and by adding a tracepoint to confirm that only housekeeping
  CPUs are picked when an appropriate profile is set up and all remaining
  CPUs when no CPU isolation is configured.

* Patch 2:
  To test the PCI fix, I hotplugged a virtio-net-pci from qemu console
  and forced its addition to a specific node to trigger the code path that
  includes the proposed fix and verified that only housekeeping CPUs
  are included via tracepoint.

* Patch 3:
  To test the fix in store_rps_map(), I tried configuring an isolated
  CPU by writing to /sys/class/net/en*/queues/rx*/rps_cpus which
  resulted in 'write error: Invalid argument' error. For the case
  where a non-isolated CPU is writing in rps_cpus the above operation
  succeeded without any error.


Changes from v3[2]:
==================
- In patch 1, replaced HK_FLAG_WQ with HK_FLAG_MANAGED_IRQ based on the
  suggestion from Frederic Weisbecker.

Changes from v2[3]:
==================
Both the following suggestions are from Peter Zijlstra.
- Patch1: Removed the extra while loop from cpumask_local_spread and fixed
  the code styling issues.
- Patch3: Change to use cpumask_empty() for verifying that the requested
  CPUs are available in the the housekeeping CPUs.

Changes from v1[4]:
==================
- Included the suggestions made by Bjorn Helgaas in the commit message.
- Included the 'Reviewed-by' and 'Acked-by' received for Patch-2.


[1] https://patchwork.ozlabs.org/project/netdev/patch/51102eebe62336c6a4e584c7a503553b9f90e01c.camel@xxxxxxxxxxx/
[2] https://patchwork.ozlabs.org/project/linux-pci/cover/20200623192331.215557-1-nitesh@xxxxxxxxxx/
[3] https://patchwork.ozlabs.org/project/linux-pci/cover/20200622234510.240834-1-nitesh@xxxxxxxxxx/
[4] https://patchwork.ozlabs.org/project/linux-pci/cover/20200610161226.424337-1-nitesh@xxxxxxxxxx/


Alex Belits (3):
  lib: Restrict cpumask_local_spread to houskeeping CPUs
  PCI: Restrict probe functions to housekeeping CPUs
  net: Restrict receive packets queuing to housekeeping CPUs

 drivers/pci/pci-driver.c |  5 ++++-
 lib/cpumask.c            | 16 +++++++++++-----
 net/core/net-sysfs.c     | 10 +++++++++-
 3 files changed, 24 insertions(+), 7 deletions(-)

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