Re: [patch 00/52] x86: Rework the vector management

[Yu Chen <yu.c.chen@intel.com>]

On Wed, Sep 13, 2017 at 11:29:02PM +0200, Thomas Gleixner wrote:
> Sorry for the large CC list, but this is a major surgery.
> 
> The vector management in x86 including the surrounding code is a
> conglomorate of ancient bits and pieces which have been subject to
> 'modernization' and featuritis over the years. The most obscure parts are
> the vector allocation mechanics, the cleanup vector handling and the cpu
> hotplug machinery. Replacing these pieces of art was on my todo list for a
> long time.
> 
> Recent attempts to 'solve' CPU offline / hibernation issues which are
> partially caused by the current vector management implementation made me
> look for real. Further information in this thread:
> 
>     http://lkml.kernel.org/r/cover.1504235838.git.yu.c.chen@intel.com
> 
> Aside of drivers allocating gazillion of interrupts, there are quite some
> things which can be addressed in the x86 vector management and in the core
> code.
> 
>   - Multi CPU affinities:
> 
>     A dubious property which is not available on all machines and causes
>     major complexity both in the allocator and the cleanup/hotplug
>     management. See:
> 
>        http://lkml.kernel.org/r/alpine.DEB.2.20.1709071045440.1827@nanos
> 
>   - Priority level spreading:
> 
>     An obscure and undocumented property which I think is sufficiently
>     argued to be not required in:
> 
>        http://lkml.kernel.org/r/alpine.DEB.2.20.1709071045440.1827@nanos
> 
>   - Allocation of vectors when interrupt descriptors are allocated.
> 
>     This is a historical implementation detail, which is not really
>     required when the vector allocation is delayed up to the point when
>     request_irq() is invoked. This might make request_irq() fail, when the
>     vector space is exhausted, but drivers should handle request_irq()
>     fails anyway.
> 
>     The upside of changing this is that the active vector space becomes
>     smaller especially on hibernation/cpu offline when drivers shut down
>     queue interrupts of outgoing CPUs.
> 
>     Some of this is already addressed with the managed interrupt facility,
>     but that was bolted on top of the existing vector management because
>     proper integration was not possible at that point. I take the blame
>     for this, but the tradeoff of not doing it would have been more
>     broken driver boiler plate code all over the place. So I went for the
>     lesser of two evils.
> 
>   - Allocation of vectors on the wrong place
> 
>     Even for managed interrupts the vector allocation at descriptor
>     allocation happens on the wrong place and gets fixed after the fact
>     with a call to set_affinity(). In case of not remapped interrupts
>     this results in at least one interrupt on the wrong CPU before it is
>     migrated to the desired target.
> 
>   - Lack of instrumentation
>  
>     All of this is a black box which allows no insight into the actual
>     vector usage.
> 
> The series addresses these points and converts the x86 vector management to
> a bitmap based allocator which provides proper reservation management for
> 'managed interrupts' and best effort reservation for regular interrupts.
> The latter allows overcommitment, which 'fixes' some of hotplug/hibernation
> problems in a clean way. It can't fix all of them depending on the driver
> involved.
> 
> This rework is no excuse for driver writers to do exhaustive vector
> allocations instead of utilizing the managed interrupt infrastructure, but
> it addresses long standing issues in this code with the side effect of
> mitigating some of the driver oddities. The proper solution for multi queue
> management are 'managed interrupts' which has been proven in the block-mq
> work as they solve issues which are worked around in other drivers in
> creative ways with lots of copied code and often enough broken attempts to
> handle interrupt affinity and CPU hotplug problems.
> 
> The new bitmap allocator and the x86 vector management code are
> instrumented with tracepoints and the irq domain debugfs files allow deep
> insight into the vector allocation and reservations.
> 
> The patches work on machines with and without interrupt remapping and
> inside of KVM guests of various flavours, though I have no idea what I
> broke on the way with other hypervisors, posted interrupts etc. So I kindly
> ask for your support in testing and review.
> 
> The series applies on top of Linus tree and is available as git branch:
> 
>    git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git WIP.x86/apic
> 
> Note, that this branch is Linus tree plus scheduler and x86 fixes which I
> required to do proper testing. They have outstanding pull requests and
> might be merged already when you read this.
> 
> Thanks,
> 
> 	tglx
> ---
Tested on top of:
commit e1b476ae32fcfa59fc6752b4b01988e759269dc3
Author: Thomas Gleixner <tglx@linutronix.de>
Date:   Thu Sep 14 09:53:10 2017 +0200

    x86/vector: Exclude IRQ0 from reservation mode

from branch WIP.x86/apic, on a platform with 16 cores,
bootup okay, cpu[1-31] offline/online okay.
Before offline:

name:   VECTOR
 size:   0
 mapped: 484
 flags:  0x00000041
Online bitmaps:       32
Global available:   6419
Global reserved:     407
Total allocated:      77
System: 41: 0-19,32,50,128,238-255
 | CPU | avl | man | act | vectors
     0   126     0    77  33-49,51-110
     1   203     0     0  
     2   203     0     0  
     3   203     0     0  
     4   203     0     0  
     5   203     0     0  
     6   203     0     0  
     7   203     0     0  
     8   203     0     0  
     9   203     0     0  
    10   203     0     0  
    11   203     0     0  
    12   203     0     0  
    13   203     0     0  
    14   203     0     0  
    15   203     0     0  
    16   203     0     0  
    17   203     0     0  
    18   203     0     0  
    19   203     0     0  
    20   203     0     0  
    21   203     0     0  
    22   203     0     0  
    23   203     0     0  
    24   203     0     0  
    25   203     0     0  
    26   203     0     0  
    27   203     0     0  
    28   203     0     0  
    29   203     0     0  
    30   203     0     0  
    31   203     0     0 

After offline:

name:   VECTOR
 size:   0
 mapped: 484
 flags:  0x00000041
Online bitmaps:        1
Global available:    126
Global reserved:     407
Total allocated:      77
System: 41: 0-19,32,50,128,238-255
 | CPU | avl | man | act | vectors
     0   126     0    77  33-49,51-110

 Thanks,
 	Yu