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Subject[PATCH v5 0/8] Reduce cross CPU IPI interference
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We have lots of infrastructure in place to partition a multi-core systems
such that we have a group of CPUs that are dedicated to specific task:
cgroups, scheduler and interrupt affinity and cpuisol boot parameter.
Still, kernel code will some time interrupt all CPUs in the system via IPIs
for various needs. These IPIs are useful and cannot be avoided altogether,
but in certain cases it is possible to interrupt only specific CPUs that
have useful work to do and not the entire system.

This patch set, inspired by discussions with Peter Zijlstra and Frederic
Weisbecker when testing the nohz task patch set, is a first stab at trying
to explore doing this by locating the places where such global IPI calls
are being made and turning a global IPI into an IPI for a specific group
of CPUs. The purpose of the patch set is to get feedback if this is the
right way to go for dealing with this issue and indeed, if the issue is
even worth dealing with at all. Based on the feedback from this patch set
I plan to offer further patches that address similar issue in other code
paths.

The patch creates an on_each_cpu_mask infrastructure API (derived from
existing arch specific versions in Tile and Arm) and service wrappers
and uses them to turn several global IPI invocation to per CPU group
invocations.

This 5th iteration includes the following changes:

- Abstract away the common boilerplate as on_each_cpu_cond wrapper
function and make all the places use it.
- Move the page_alloc.c/drain_all_pages to use a static global
cpumask to avoid adding an allocation in the direct reclaim
path, based on feedback and suggestion by Mel Gorman and Chris
Metcalf.
- Add an optional patch to add vmstat counters to per-cpu page
drain request and the upside to using this patch, based on
Mel Gorman idea.
- Provide the same treatment to yet another call site - this time
the local LRU BH invalidation.

The patch was compiled for arm and boot tested on x86 in UP, SMP, with and without
CONFIG_CPUMASK_OFFSTACK and was further tested by running hackbench on x86 in
SMP mode in a 4 CPUs VM with no obvious regressions.

I also artificially exercised SLUB flush_all via the debug interface and observed
the difference in IPI count across processors with and without the patch - from
an IPI on all processors but one without the patch to a subset (and often no IPI
at all) with the patch.

I further used fault injection framework to force cpumask alloction failures for
CPUMASK_OFFSTACK=y cases and triggering the code using slub sys debug interface,
as well as running ./hackbench 400 for page_alloc, with no obvious falilures.

Gilad Ben-Yossef (8):
smp: Introduce a generic on_each_cpu_mask function
arm: Move arm over to generic on_each_cpu_mask
tile: Move tile to use generic on_each_cpu_mask
smp: Add func to IPI cpus based on parameter func
slub: Only IPI CPUs that have per cpu obj to flush
fs: only send IPI to invalidate LRU BH when needed
mm: Only IPI CPUs to drain local pages if they exist
mm: add vmstat counters for tracking PCP drains

arch/arm/kernel/smp_tlb.c | 20 ++++-------------
arch/tile/include/asm/smp.h | 7 ------
arch/tile/kernel/smp.c | 19 ----------------
fs/buffer.c | 15 ++++++++++++-
include/linux/smp.h | 32 +++++++++++++++++++++++++++
include/linux/vm_event_item.h | 1 +
kernel/smp.c | 47 +++++++++++++++++++++++++++++++++++++++++
mm/page_alloc.c | 30 +++++++++++++++++++++++++-
mm/slub.c | 10 +++++++-
mm/vmstat.c | 2 +
10 files changed, 139 insertions(+), 44 deletions(-)



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